The essence of the laws of inheritance of traits in humans is brief. Theme

Biology grade 9. General patterns (Mamontov). Section 3. Heredity and variability of organisms. Electronic version (TRANSCRIPT). Quotes used for educational purposes.

Section 3. Heredity
and variability of organisms

Chapter 7. Regularities of traits inheritance.

Genetics - is the science of the laws of heredity and variability of living organisms.

Heredity is the ability of living organisms to transmit their characteristics, properties and developmental characteristics from generation to generation.

Variability is the ability of organisms to acquire new traits and properties in the process of individual development in comparison with other individuals of the same species.

The founder of genetics is the Czech scientist G. Mendel, who developed methods genetic research, established the basic laws of inheritance of traits and published them in 1865. These laws were confirmed by various scientists in 1900, which is considered the year of birth of genetics.

Regularities of traits inheritance. The first attempts to experimentally solve the problems associated with the transmission of traits from generation to generation were made already in the 18th century. Scientists, crossing different individuals with each other and getting offspring from them, sought to find out how parental traits are inherited. However, the wrong methodological approach - the simultaneous study of a large number of features - did not allow identifying any regularities.

14. Basic concepts of genetics

Remember! Heredity DNA variability

Genetics studies two fundamental properties of living organisms: heredity and variability.

Usually heredity is defined as the ability of parents to pass on their traits, properties and developmental characteristics to the next generation. Due to this, each species of animals or plants, fungi or microorganisms retains its characteristic features for many generations.

The cells through which the succession of generations is carried out - specialized sex cells during sexual reproduction and non-specialized cells of the body (somatic) during asexuality - do not carry the signs and properties of future organisms, but only their inclinations, called genes. A gene is a section of a molecule. DNA, which determines the possibility of developing a separate elementary trait, or the synthesis of one protein molecule.

A trait defined by any genome may not develop. The ability to manifest signs largely depends on the presence of other genes and on environmental conditions. Consequently, the study of the conditions for the manifestation of genes in the form of traits is also a subject of genetics.

In all organisms of the same species, each gene is located in the same place, or locus, of a particular chromosome. In the haploid set of chromosomes, there is only one gene responsible for the development of this trait. A diploid set of chromosomes (in somatic cells) contains two homologous chromosomes and, accordingly, two genes that determine the development of one trait. Genes located in the same loci of homologous chromosomes and responsible for the development of one trait are called allelic.

The collection of all genes of one organism is called a genotype. However, a genotype is not just the sum of genes. The possibility and form of gene expression depend, as will be shown later, on environmental conditions. The concept of the environment includes not only the conditions in which a given organism or cell exists, but also the presence of other genes. Once in the same genotype, genes can strongly influence the manifestation of the action of neighboring genes.

Organisms of the same species differ from each other. This is clearly seen in the example of the species Homo sapiens (Homo sapiens), each representative of which has its own individual characteristics... Such individual variability exists in organisms of any species of animals and plants. Thus, variability - a property of organisms, the opposite of heredity - is the ability of organisms to acquire new characteristics and properties. Variability is due to a change in the structure of hereditary inclinations - genes - and, as a consequence, a change in their manifestation in the process of development of organisms. There are different types of variability. Genetics is also studying the causes, forms of variability and its significance for evolution. At the same time, researchers are not dealing directly with genes, but with the results of their manifestation - traits or properties. Therefore, the laws of heredity and variability are studied by observing the characteristics of organisms in a series of generations.

The totality of all the characteristics of an organism is called a phenotype. This includes not only external, visible signs (skin color, hair, shape of the ear or nose, color of flowers), but also biochemical (protein structure, enzyme activity, concentration of hormones in the blood, etc.), histological (shape and size of cells , structure of tissues and organs), anatomical (structure of the body and mutual arrangement organs), etc.

What is a gene?
Do you think it would be correct to say that a gene is a section of a chromosome?
Compare the concepts of "genotype" and "phenotype".
What is a trait? What are the signs? Give examples of signs at different levels of the organization.
Based on external, visible signs, describe the phenotype of your classmate. Ask classmates to identify the phenotype by description.

15. Hybridological method for studying the inheritance of Gregor Mendel's traits

Remember! Flowering plants Self-pollination Heredity Cross-pollination

In his experiments G. Mendel used peas. He chose for experiments organisms belonging to pure lines, that is, such plants, in a series of generations of which, during self-pollination, all offspring were uniform according to the studied trait. It should also be noted that he was observing the inheritance of alternative, i.e., mutually exclusive, contrasting characters (see table). For example, one plant had purple flowers, another white, tall or short, and so on.

The essence of the method proposed by Mendel is as follows: he crossed plants that differ in one pair of mutually exclusive characteristics, and then conducted an individual analysis of the results of each crossing using mathematical statistics.

Mendel especially emphasized the average statistical nature of the patterns he discovered and the need to study a large number (thousands) of descendants to identify them. Mendel's method is called hybridological or crossing method.

The patterns of inheritance of traits, identified by Mendel, are now accepted to be formulated in the form of laws.

Review questions and assignments

Who was the discoverer of the patterns of inheritance of traits?
Why do you think G. Mendel chose peas as an experimental object?
Thanks to what techniques did G. Mendel manage to reveal the laws of inheritance of traits?
Do you know of any alternative or contrasting signs in humans? Give examples.
How to explain that the hybridological method developed by G. Mendel is not used in human genetics?
Using additional sources of information, prepare a report on the life and work of G. Mendel.

16. Mendel's first law

Remember! Sexual reproduction Homologous chromosomes Diploid set of chromosomes Haploid set of chromosomes Phenotype Genotype

The crossing of two organisms is called hybridization, the offspring from the crossing of two individuals with different heredity is called a hybrid, and an individual is called a hybrid. A monohybrid is the crossing of two organisms that differ from each other in one pair of alternative (mutually exclusive) characteristics. Therefore, with such a crossing, the patterns of inheritance of only two variants of the trait are traced, the development of which is due to a pair of allelic genes. For example, a trait is the color of the seeds, the mutually exclusive options are yellow or green. All other signs characteristic of these organisms are not taken into account.

If you cross pea plants with yellow and green seeds, then the offspring (hybrids) obtained as a result of crossing will have yellow seeds. When crossing plants that differ in the smooth and wrinkled shape of the seeds, the hybrids will have smooth seeds. Therefore, in the first generation hybrid, only one of each pair of alternative traits appears. The second symptom does not develop. G. Mendel called the predominance of the trait of one of the parents in a hybrid as dominance. The trait that manifests itself in the hybrid of the first generation and suppresses the development of another trait was called dominant (from the Latin dominus - master), and the opposite, that is, suppressed, - recessive (from the Latin recessus - retreat, withdrawal). The gene that ensures the formation of a dominant trait is usually denoted by an uppercase letter, for example, A, a recessive one - by a lowercase, a. Genes A and a are called allelic genes or alleles.

As already mentioned, G. Mendel used in experiments plants belonging to different pure lines, the descendants of which in a long series of generations were similar to their parents. Therefore, in these plants, both allelic genes are the same.

If there are two identical allelic genes in the genotype of an organism (zygote), absolutely identical in nucleotide sequence, such an organism is called homozygous for this gene. An organism can be homozygous for dominant (AA or BB) or recessive (aa or bb) genes. If allelic genes differ from each other (one of them is dominant, and the other is recessive (Aa, Bb)), such an organism is called heterozygous.

Dominance law - Mendel's first law - is also called the law of uniformity of hybrids of the first generation, since all individuals of this generation have the same trait. This law can be formulated as follows: when crossing two homozygous organisms belonging to different pure lines and differing from each other in one pair of alternative traits, the entire first generation of hybrids (F1) will be uniform and will carry the trait of one of the parents.

Consider crossing pea plants that differ in seed color (yellow and green) and shape (smooth and wrinkled).

Incomplete dominance. In the heterozygous state, the dominant gene does not always completely suppress the expression of the recessive gene. In some cases, the hybrid of the first generation Fj does not fully reproduce any of the parental traits, and the expression of the trait is intermediate. But all individuals of this generation show uniformity in this trait. So, when crossing a night beauty with red flowers (AA) with a plant whose flowers are painted white (aa), in their offspring - F1 - an intermediate, pink, flower color is formed (Aa): all descendants of F1 are uniform (Fig. 37 ).

Incomplete dominance is widespread. It was discovered when studying the inheritance of flower color in snapdragons, feather structure in birds, wool color in cattle and sheep, biochemical characteristics in humans, etc.

Review questions and assignments

What is hybridization?
What kind of crossing is called monohybrid?
What phenomenon is called dominance?
Which trait is called dominant and which is recessive?
Tell us about Mendel's experiments on monohybrid crossing of pea plants.
Which organism is called homozygous; heterozygous?
Formulate Mendel's first law. Why is this law called the law of domination?
Using additional sources of information, give examples of incomplete dominance of traits in humans.
What plants of a night beauty should be crossed with each other so that in the offspring half of the plants with pink flowers and half with white flowers are obtained?

17. Mendel's second law. Gamete Purity Law

Remember! Dominant Recessive Genotype Phenotype

Mendel's second law (the law of splitting). If the descendants of the first generation - heterozygous individuals, identical in the studied trait, are crossed with each other, then in the second generation the traits of both parents appear in a certain numerical ratio: 3/4 of individuals will have a dominant trait, 1/4 - a recessive trait.

The phenomenon in which the crossing of heterozygous individuals leads to the formation of offspring, some of which bears a dominant trait, and some are recessive, is called splitting. Therefore, splitting is the distribution of dominant and recessive traits among the offspring in a certain numerical ratio. The recessive trait in hybrids of the first generation does not disappear, but is only suppressed and manifests itself in the second hybrid generation (F2).

Thus, Mendel's second law can be formulated as follows: when two heterozygous offspring of the first generation are crossed with each other in the second generation, splitting is observed in a certain numerical ratio: according to the phenotype 3: 1, according to the genotype 1: 2: 1. This means that among the offspring, 25% of organisms will have a dominant trait and be homozygous, 50% of the offspring, also with a dominant phenotype, will be heterozygous, and the remaining 25% of individuals carrying a recessive trait will be homozygous for the recessive gene.

In case of incomplete dominance of hybrids in the offspring (F2), segregation by genotype and phenotype coincides (1: 2: 1).

The law of gamete purity. Mendel suggested that hereditary factors during the formation of hybrids do not mix, but remain unchanged. In the F1 hybrid obtained from the crossing of parents differing in alternative traits, both factors are present: dominant and recessive. The dominant hereditary factor is manifested in the form of a trait, while the recessive factor is suppressed. Communication between generations during sexual reproduction is carried out through the sex cells - gametes. Therefore, it must be assumed that each gamete contains only one factor from a pair. Then, during fertilization, the fusion of two gametes, each of which carries a recessive hereditary factor, will lead to the formation of an organism with a recessive trait that manifests itself phenotypically. The fusion of gametes carrying a dominant factor, or two gametes, one of which contains a dominant factor and the other a recessive factor, will lead to the development of an organism with a dominant trait. Thus, the appearance in the second generation (F2) of a recessive trait of one of the parents (P) can take place only if two conditions are met: 1) if the hereditary factors in hybrids remain unchanged; 2) if the germ cells contain only one hereditary factor from the allelic pair.

Mendel explained the splitting of traits in the offspring when crossing heterozygous individuals by the fact that gametes are genetically pure, that is, they carry only one gene from an allelic pair. The law of gamete purity can be formulated as follows: during the formation of sex cells, only one gene from an allelic pair gets into each gamete.

Why and how does this happen? In the process of gamete formation in a hybrid, homologous chromosomes during the first meiotic division fall into different cells:

Two varieties of gametes are formed for a given allelic pair. During fertilization, genes can randomly combine in a zygote in all possible combinations: AA, Aa, aa.

The cytological basis for the cleavage of traits in offspring during monohybrid crossing is the divergence of homologous chromosomes and the formation of haploid germ cells in meiosis.

Review questions and assignments

Formulate the second law of G. Mendel. Why is it called the law of splitting?
Explain what gamete purity is. Is the word “purity” used literally or figuratively in this term?
What phenomenon is the law of gamete purity based on?

18. Mendel's third law. Analyzing cross

Remember! Monohybrid crossing Heterozygous Homozygous Homologous chromosomes

Dihybrid crossing... Mendel's third law. Studying the inheritance of one pair of alleles allowed Mendel to establish a number of important genetic patterns. The phenomenon of cleavage allowed us to assume that gametes are genetically pure, that is, they contain only one gene from an allelic pair.

However, organisms differ from each other in many ways. It is possible to establish the patterns of inheritance of two (or more) pairs of alternative traits by means of dihybrid or polyhybrid crossing. A dihybrid or polyhybrid crossing is a crossing in which researchers observe the inheritance pattern of two or more pairs of mutually exclusive (alternative) traits.

For the dihybrid crossing, Mendel took homozygous pea plants that differ in two genes: seed color (yellow and green) and seed shape (smooth and wrinkled). Dominant features are yellow color (A) and smooth form (B) of seeds. Each plant forms one variety of gametes for the studied alleles. When these gametes merge, all offspring will be uniform.

When gametes are formed in a hybrid of the first generation, only one of each pair of allelic genes gets into the gamete, while due to the accidental divergence of the paternal and maternal chromosomes in the first division of meiosis, gene A can get into the same gamete with gene B or with gene b, just like gene a can be combined in the same gamete with gene B or with gene b.

Since many germ cells are formed in each organism, due to statistical regularities, four varieties of gametes appear in the hybrid in the same amount (25% each): AB, Ab, aB, ab. During fertilization, each of the gametes of one organism accidentally meets any of the gametes of another organism. All possible combinations of male and female gametes can be easily identified using the Pennett grid. The gametes of one parent are written out horizontally above the lattice, and the gametes of the other parent are written along the left edge of the lattice vertically. The genotypes of zygotes formed during the fusion of gametes are inscribed in the squares (Fig. 38). So, according to the phenotype, the offspring is divided into four groups in the following ratio: 9 yellow smooth: 3 yellow wrinkled: 3 green smooth: 1 green wrinkled. If we take into account the results of splitting for each pair of traits separately, it turns out that the ratio of the number of yellow seeds to the number of green seeds and the ratio of the number of smooth seeds to the number of wrinkled ones for each pair is 3: 1. Thus, in a dihybrid crossing, each pair of traits behaves in the same way as in a monohybrid crossing, that is, independently of the other pair of traits.

During fertilization, gametes are combined according to the rules of random combinations, but with equal probability for each. The independent distribution of traits in the offspring and the emergence of various combinations of genes that determine the development of these traits, with dihybrid crossing, are possible only if the pairs of allelic genes are located in different pairs of homologous chromosomes.

Now we can formulate Mendel's third law: n when two homozygous individuals are crossed, differing from each other in two (or more) pairs of alternative traits, genes and their corresponding traits are inherited independently of each other and are combined in all possible combinations.

If the parental forms differ in two pairs of traits, in the second generation splitting is observed 9: 3: 3: 1. The analysis of splitting is based on Mendel's laws and in more complex cases: when individuals differ in three, four (and more) pairs of traits.

Analyzing cross... In order to establish whether an organism that has a dominant phenotype for the gene (s) under study is homozygous or heterozygous, it is crossed with an organism that is homozygous for the recessive allele (alleles) and has a recessive phenotype.

If the dominant individual is homozygous, the offspring from such a cross will be uniform and splitting will not occur:

A different picture will occur if the organism under study is heterozygous:

Cleavage will occur in a 1: 1 phenotype ratio. This result is a direct proof of the formation of two gamete varieties in one of the parents, ie, its heterozygosity (Fig. 39).

Analyzing crossing with heterozygosity of the studied organism for two pairs of genes looks like this:

In the offspring from such a crossing, four groups of phenotypes are formed, differing from each other in the combination of the two studied traits, in a ratio of 1: 1: 1: 1.

Review questions and assignments

Formulate Mendel's third law. Why is it called the law of independent inheritance?
For which allelic pairs is Mendel's third law valid?
What is Analytical Crossing?
What will be the splitting in the analyzing crossing if the studied individual with the dominant phenotype has the AABb genotype?
How many gamete types are formed in an individual with the AaBBCcDdffEe genotype?
Discuss in class whether it can be argued that Mendel's laws are universal, that is, they are valid for all sexually reproducing organisms.

19. Linked gene inheritance

Remember! Meiosis Homologous chromosomes Non-homologous chromosomes Conjugation Crossover

G. Mendel traced the inheritance of seven pairs of traits in sweet peas. Subsequently, many researchers, studying the inheritance of traits in organisms of different species, confirmed Mendel's laws. It was recognized that these laws are universal.

However, later it turned out that in sweet pea two traits - the shape of the pollen and the color of the flowers - do not give an independent distribution in the offspring: the offspring remained similar to the parents. Gradually, more and more such exceptions from Mendel's third law accumulated. It became clear that the principle of independent distribution in offspring and free combination does not apply to all genes. Indeed, any organism has a lot of signs, and the number of chromosomes is small. Therefore, there must be many genes on each chromosome. Such genes are called linked to each other. They form a clutch group. In other words, each chromosome is nothing more than a linkage group, and since homologous chromosomes carry genes that are responsible for the development of the same traits, genetics include both paired chromosomes in it. The number of linkage groups corresponds to the number of chromosomes in a haploid (single) set. So, for example, a human has 46 chromosomes - 23 linkage groups, a Drosophila has 8 chromosomes - 4 linkage groups, a pea has 14 chromosomes - 7 linkage groups.

Genes located on the same chromosome are inherited as follows:

The phenomenon of joint inheritance of genes localized on one chromosome is called linked inheritance, and the localization of genes on one chromosome is called gene linkage.

Thus, Mendel's third law applies to the inheritance of allelic pairs that are in non-homologous chromosomes.

All genes included in one chromosome are inherited together. This pattern was first discovered by the American geneticist Thomas Morgan and was later called the law of his name: genes located on one chromosome are called linked and are inherited together.

However, when analyzing the inheritance of linked genes, it was found that in a certain percentage of cases, strictly defined for each pair of genes, the linkage can be broken.

Let's remember meiosis. In the prophase of the first meiotic division, homologous chromosomes are conjugated. At this moment, an exchange of sites can occur between them:

If, as a result of crossing-over, in some cells there is an exchange of chromosome regions between genes A and B, then the gametes Ab and aB appear and four groups of phenotypes are formed in the offspring, as in the free combination of genes. The difference lies in the fact that the numerical ratio of phenotypes does not correspond to the 1: 1: 1: 1 ratio established for the dihybrid analyzing cross.

Thus, gene linkage can be complete or incomplete. The cause of the violation of adhesion is crossing over - the crossing of chromosomes in the prophase of meiotic division. The farther from each other the genes are located on the chromosome, the higher the likelihood of an overlap between them and the greater the percentage of gametes with recombined genes, and, consequently, the greater the percentage of individuals other than parents.

Review questions and assignments

For which pairs of allelic genes is Mendel's third law valid? At what location of different pairs of allelic genes does it "not work"?
What is chained inheritance?
What are clutch groups? How many such groups does a person have?
What processes can disrupt gene linkage?
Think about how you can explain the fact that the likelihood of intersection between genes is greater, the further from each other these genes are located on the chromosome.
Do you agree with the statement that gene linkage disruption increases variability? Explain your point of view.

20. Genetics of sex. Inheritance of sex-linked traits

Remember! Primary sexual characteristics Secondary sexual characteristics Gametes Karyotype Color blindness Hemophilia

The problem of the origin of sex differences, mechanisms for determining sex and maintaining a certain sex ratio in groups of animals is very important both for theoretical biology and for practice. The ability to artificially control the sex of animals would be extremely useful for agriculture.

Sex in animals is most often determined at the time of fertilization. The most important role in this belongs to the chromosome set of the zygote. Recall that a zygote contains paired - homologous - chromosomes, the same in shape, size and set of genes in each. Figure 40 shows the chromosomes of a person - a woman and a man. In the female karyotype, all chromosomes are paired. In the male karyotype, there is one large, equal-armed unpaired chromosome that does not have a homologue, and a small rod-shaped chromosome, which is found only in the karyotype of men. Thus, the human karyotype contains 22 pairs of chromosomes, which are identical in male and female organisms, and one pair of chromosomes, according to which both sexes are distinguished.

The chromosomes by which the male and female sex differ from each other are called sex or heterochromosomes. The sex chromosomes in women are the same, they are called X chromosomes. Men have one X chromosome and one Y chromosome. When the sex cells mature, the gametes receive a haploid set of chromosomes as a result of meiosis. Moreover, all eggs have one X chromosome.

The sex that forms gametes that are the same on the sex chromosome is called homogametic and is designated as XX.

During spermatogenesis, gametes of two varieties are obtained: half carries the X chromosome, half the Y chromosome.

The sex that forms gametes that are not the same on the sex chromosome is called heterogametic and is designated as XY.

In mammals, in particular humans, some insects, such as Drosophila, and a number of other organisms, the female sex is homogametic; in butterflies, reptiles, birds - male. So, the karyotype of the rooster is designated as XX, and the karyotype of the chicken is XY.

In humans, the Y-chromosome plays a decisive role in sex determination. If an egg cell is fertilized by a sperm carrying the X chromosome, a female organism develops. Consequently, women have one X chromosome from their father and one X chromosome from their mother. If an egg is fertilized by a sperm carrying the Y chromosome, a male organism develops. A man (XY) receives the X chromosome only from his mother. This is due to the peculiarity of the inheritance of genes located in the sex chromosomes. The inheritance of traits whose genes are located on the X- or Y-chromosomes is called sex-linked inheritance. The distribution of these genes in the offspring should correspond to the distribution of sex chromosomes in meiosis and their combination during the fusion of germ cells during fertilization.

Consider the inheritance of genes located on the X chromosome. It should be borne in mind that genes that are not involved in the development of sexual characteristics may also be on the sex chromosomes. So, the X chromosome of Drosophila includes a gene on which the color of its eyes depends. The human X chromosome contains a gene that determines blood clotting (I). Its recessive allele (h) causes a serious disease characterized by decreased blood clotting - hemophilia. The same chromosome contains genes that cause blindness to red and green colors (color blindness), the shape and volume of teeth, the synthesis of a number of enzymes, etc.

When linking to sex, a recessive gene can also appear, which is present in the genotype in the singular. This happens when it is on the X chromosome of a heterogametic organism. In the XY karyotype, the recessive gene in the X chromosome manifests itself phenotypically, since the Y chromosome is not homologous to the X chromosome and does not contain a dominant allele. The inheritance of a sex-linked gene for color blindness is depicted in Figure 41.

The inheritance of hemophilia is shown in the following diagram using the example of a marriage of a woman carrying the hemophilia gene (HHXh) with a healthy man:

Half of the boys from such a marriage will suffer from hemophilia.

When a gene is localized on the Y chromosome, traits are transmitted only from father to son.

At present, the inheritance of many normal and pathological (from the Greek pathos - disease) traits in humans has been studied.

Review questions and assignments

What chromosomes are called sex chromosomes?
What sex is called homogametic and what is heterogametic?
What is gender linkage? Give examples of inheritance of a sex-linked gene.
Why are recessive genes localized in the human X chromosome manifested as a trait? Using additional sources of information, give examples of dominant and recessive traits in a person, sex-linked.
Explain why the sex of an organism is usually determined at the moment of fertilization, that is, when a sperm and an egg merge.
Solve the problem. Young chickens do not have noticeable sex differences, and yet it is economically expedient to establish different feeding regimes for future males and females. It is known that the gene that determines the color of the plumage is localized on the X chromosome, and the speckled color dominates over the white, and the difference between the colors is noticeable immediately after hatching. What kind of crossing should be done so that you can immediately divide the hatched chicks by sex?

Chapter 8. Patterns of variability

Variability is the ability of living organisms to acquire new characteristics and properties. Variability reflects the relationship of an organism with the external environment. Distinguish between hereditary (genotypic) and non-hereditary (modification, or phenotypic) variability.

21. Hereditary (genotypic) variability

Remember! Genotype Gene Crossover Karyotype Polyploidy

Hereditary variability includes such changes in the characteristics of the organism, which are determined by the genotype and persist in a number of generations. Sometimes these are large, well-marked changes, for example, short-legged in sheep (see Fig. 58), lack of plumage in chickens (Fig. 42, 43), split fingers in cats, lack of pigment (albinism), short-toed (Fig. 44) or polydactyly in humans (Fig. 45). As a result of sudden changes that are steadily inherited, a dwarf sweet pea variety, double-flowered plants and many other characteristics have arisen. More often these are small, barely noticeable deviations from the norm.

Hereditary changes in genetic material are called mutations (from the Latin mutation - change).

Darwin called hereditary variability indefinite or individual variability, thereby emphasizing its random, undirected nature and the relative rarity of its occurrence. Mutations arise from changes in the structure of a gene or chromosome and serve as a source of genetic diversity within a species. Due to the constant mutational process, various variants of genes arise, which constitute the reserve of hereditary variability. However, the diversity of living organisms and the uniqueness of each genotype are due to combinative variability - the rearrangement of chromosomes during sexual reproduction and chromosome regions during crossing over. In this case, the structure of the genes and chromosomes themselves remains the same as in the parents, but the combinations of hereditary inclinations and the nature of their interaction in the genotype change.

The nature of the manifestation of mutations. Distinguish between dominant and recessive mutations. Most of them are recessive and do not appear in heterozygous organisms. Such mutations constitute a hidden reserve of hereditary variation. The owners of harmful dominant mutations often turn out to be unviable and die on the most early stages individual development.

Place of origin of mutations. Mutations are classified as generative and somatic. A mutation that has arisen in the germ cells does not affect the manifestations of the characteristics of a given organism, but is found only in the next generation. Such mutations are called generative. If genes change in somatic cells, such mutations appear in this organism and are not transmitted to offspring during sexual reproduction. But in asexual reproduction, if an organism develops from a cell or group of cells that have a changed - mutated - gene, mutations can be passed on to offspring. Such mutations are called somatic. In plant growing, somatic mutations are used to develop new varieties of cultivated plants. An example of a somatic mutation in mammals is the occasional black spot on the background of brown coat color in astrakhan sheep.

Mutation levels. Changes caused by the replacement of one or more nucleotides within the same gene are called gene or point mutations. They entail a change in the structure of proteins. In the polypeptide chain, the amino acid sequence changes and, as a result, the normal functioning of the protein molecule is disrupted.

Changes in the structure of chromosomes are called chromosomal mutations. These mutations can result from the loss of part of a chromosome. If vital genes are included in the lost site, then such a mutation can lead the organism to death. The loss of a small part of the 21st chromosome in humans causes the development of a severe congenital disease in children - acute leukemia. In other cases, the detached section can join a non-homologous chromosome, resulting in a new combination of genes that changes the nature of their interaction.

Changes in the number of chromosomes (decrease or increase) are called genomic mutations. Due to the nondisjunction of any pair of homologous chromosomes in meiosis, one of the formed gametes contains one chromosome less, and the other one more chromosome than in the normal haploid set. Fusion with a normal haploid gamete during fertilization leads to the formation of a zygote with fewer or more chromosomes in comparison with the diploid set typical for this species. In such cases, gene imbalance is accompanied by a developmental disorder. A well-known example is Down's disease in humans, the cause of which is the presence of three chromosomes of the 21st pair in the karyotype. Down's disease is manifested by a significant decrease in vitality, insufficient mental development and a number of other disorders.

In protozoa and in plants, an increase in the number of chromosomes, a multiple of the haploid set, is often observed. This change in the chromosome set is called polyploidy. Its degree is different. In protozoa, the number of chromosomes can increase several hundred times. Polyploidy is widespread in higher plants. With an increase in the number of chromosome sets in a karyotype, the reliability of the genetic system increases, and the risk of reduced viability in case of mutation decreases. Polyploidy often increases vitality, fertility, and other vital properties. In plant growing, polyploid varieties of cultivated plants are artificially obtained, which are distinguished by high productivity (Fig. 46). In higher animals, such as mammals, polyploidy occurs only in some tissues, such as liver cells.

Properties of mutations. Mutations are hereditary, that is, they are steadily passed from generation to generation. The same mutations can occur in different organisms belonging to the same species. In their manifestation, mutations can be beneficial and harmful, dominant and recessive.

The ability to mutate is one of the properties of a gene. However, there are external factors that significantly increase the frequency of mutations. These include, for example, all types ionizing radiation, salts of heavy metals and many others.

Artificial production of mutations is also of practical importance, since it increases genetic diversity within a population or a species, "supplying" material for breeders.

Review questions and assignments

What forms of variability are you aware of?
What is mutation? How does combinative variability differ from mutational variability?
What cell structures are rearranged during mutational change?
Compare generative and somatic mutations. What do they have in common and how are they fundamentally different?
Make and fill in the table "Variety of mutations (by level of occurrence)".
What criteria are used as the basis for the classification of mutations presented in the textbook? Suggest your options for classifying mutations.
What is polyploidy? Why are there no polyploid organisms among higher animals?
How can you cause an increase in the frequency of mutations?
Explain why Charles Darwin called hereditary variation indeterminate.

22. Non-hereditary (phenotypic) variability

Remember! External environment Genotype Phenotype

Each organism develops and lives in certain conditions, experiencing the effect of various environmental factors - temperature, illumination, humidity, quantity and quality of food; in addition, he enters into relationships with other organisms of his own and other species. All these factors can change the morphological and physiological properties of organisms, that is, their phenotype.

If a Himalayan rabbit has a white coat plucked on its back and a cold bandage is applied, black wool will grow in this place (Fig. 47). If the black hair is removed and a warm bandage is applied, white hair will grow. When growing a Himalayan rabbit at a temperature of +30 ° C, all its wool will be white. The offspring of two such white rabbits, raised under normal conditions, will have a normal pigment distribution.

Many signs change during growth and development under the influence of environmental factors. Such trait changes are not inherited.

Figure: 47. Phenotypic change in the color of the wool of the Himalayan rabbit under the influence of different temperatures

In the lotus (Fig. 48) and the water nut (Fig. 49), the underwater and emergent leaves have different shapes: the lotus leaves in the water have long thin lanceolate leaves, and the water walnut has cut-out feathery leaves.

Under the influence of ultraviolet rays in all people (if they are not albinos), the skin becomes tanned due to the accumulation of melanin pigment granules in it.

Thus, each type of organism reacts specifically to the action of a certain environmental factor, and the reaction (change in a trait) is similar in all individuals of a given species.

At the same time, the variability of a trait under the influence of environmental conditions is not unlimited. The degree of variation of the trait, or, in other words, the limits of variability, is called the reaction rate. The latitude of the reaction rate is determined by the genotype and depends on the value of the trait in the vital activity of the organism. A narrow reaction rate is characteristic of such important characteristics as, for example, the size of the heart or brain. At the same time, the amount of fat in the body varies widely. The structure of the flower in plants pollinated by insects varies little, but the size of the leaves is very variable. Knowledge of the reaction rate of an organism, the limits of its modification variability is of great importance in breeding practice in the "construction" of new forms of plants, animals and microorganisms useful to humans. This is especially important for the practice of agriculture, the purpose of which is to increase the productivity of plants and animals by not only introducing new breeding forms - breeds and varieties, but also maximizing the potential of existing breeds and varieties. Knowledge of the patterns of modification variability is also necessary in medicine for the maintenance and development of the human body within the reaction norm.

Review questions and assignments

How does the environment affect the manifestation of the trait? Give examples.
Prove with examples the non-heritability of changes in a trait caused by the action of environmental conditions.
What is the reaction rate? What determines its latitude? Give examples of signs with a wide and a narrow reaction rate.
List the properties of phenotypic variability. Compare it with genotypic variation. Format the comparison results in the form of a table.
Give examples of acquired characteristics known to you from life. Explain why they are not inherited.

Chapter 9. Selection of plants, animals and microorganisms

In the process of the formation of man as a species, he had to not only defend himself from wild animals, arrange shelters, etc., but also provide himself with food. Finding edible plants and hunting are not very reliable food sources, and hunger was a constant companion of primitive people. Natural selection on intelligence and the development of social relations in the primitive human herd led to the formation for a person of an artificial habitat that reduces his dependence on natural conditions... At the same time, one of the greatest achievements was the creation of a permanent source of food through the domestication of wild animals and the cultivation of plants.

Thus, phenotypic variability is characterized by the following basic properties: 1) non-heritability; 2) the group nature of the changes; 3) dependence of changes on the action of a certain environmental factor; 4) the determination of the limits of variability by the genotype, that is, with the same direction of changes, the degree of their severity in different organisms is different.

The breeding of various breeds of animals and plant varieties became possible due to the existence of combinative hereditary variability in wild species as a result of sexual reproduction, as well as artificial selection used by humans. Animals and plants bred by humans differ sharply from their wild ancestors in a number of qualities. Cultural forms have highly developed individual traits, unnecessary or even harmful for existence in natural conditions, but useful for humans. For example, the ability of some chicken breeds to lay 300 or more eggs per year is devoid of biological meaning, since a hen cannot incubate such a number of eggs. Many similar examples can be cited, relating not only to economically useful signs, but also to decorative ones - in pigeons, fighting roosters.

The size and productivity of cultivated plants is higher than that of related wild species, but at the same time they are deprived of means of protection from adverse environmental conditions and from eating: bitter or poisonous substances, thorns, thorns.

For a more complete satisfaction of human food and technical needs, more and more new varieties of plants and animal breeds with predetermined properties are being created. The development of the theory and methods of creating and improving animal breeds and plant varieties is the subject of a special science - selection.

23. Centers of diversity and origin of cultivated plants

Remember! Wild cereals Cultivated cereals Breeding Gene pool

The gene pool of existing animal breeds or plant varieties is naturally poorer than that of the original wild species. Meanwhile, the success of breeding work depends mainly on the genetic diversity of the original group of plants or animals. Therefore, when developing new varieties of plants and animal breeds, it is very important to search for and identify useful traits in wild forms. In order to study the diversity and geographical distribution of cultivated plants, the outstanding Russian geneticist and breeder NI Vavilov in 1920-1940. organized numerous expeditions both on the territory of our country and to many foreign countries... During these expeditions, the world's plant resources were studied and huge seed material was collected, which was later used for breeding work. NI Vavilov made important generalizations that served as a major contribution to the theory of selection; he identified seven centers of origin of cultivated plants, from which they spread throughout the world. This is the South Asian tropical center - the homeland of 50% of cultivated plants, East Asian, from which 20% of cultivated plants settled around the world, Southwest Asian (14% of cultivated plants, including wheat, rye, legumes, etc.), Mediterranean (11% of cultivated plants, including cabbage, sugar beets, lentils), Abyssinian - the birthplace of barley, bananas, coffee tree, etc., Central American, where corn, cotton, pumpkin, tobacco came from, and, finally, South American - the birthplace of potatoes, pineapple, etc. ...

The history of the Vavilov collection also includes dramatic pages. In 1940, its creator was arrested on false charges and in 1943 died of starvation in a Saratov prison. The collection was kept at the All-Union Institute of Plant Industry in Leningrad. During the Nazi siege of the city, the staff of the institute, who were starving along with all the Leningraders, managed to preserve the entire collection to the last grain.

Work on the creation of seed collections of varieties of cultivated plants and their wild-growing ancestors, the beginning of which was laid by N.I. Vavilov, continues to this day. In our country, this collection includes more than 320 thousand samples. This includes wild species, relatives of cultivated plants, old local varieties, all the best and new that was created for recent times through the efforts of breeders from all countries of the world. Scientists select genetic sources of economically valuable traits from the world gene pool: yield, early maturity, resistance to diseases and pests, drought resistance, lodging resistance, etc. Modern genetic methods make it possible to achieve very large successes in plant breeding. Thus, the use of valuable genes of wild Ethiopian barley made it possible to create the Odessa-100 variety of spring barley, outstanding in terms of productivity.

Review questions and assignments

How do domesticated animals and cultivated plants differ from wild ones?
What science do you think is the theoretical basis of breeding? Explain your choice.
What is the importance of knowledge of the centers of origin of cultivated plants for breeding?
What centers of origin of cultivated plants do you know?
Determine which centers of origin are home to crops grown in your area.
Explain in class why the domestication of wild animals and cultivation of crops was a turning point in human development.
Why is it necessary to know the biological properties of the original wild species for successful breeding work?

24. Plant and animal breeding

Remember! Breed Sort. Gene pool Homozygous organisms Polyploids

The main task of breeding is to create highly productive animal breeds, plant varieties and microorganism strains that best satisfy human nutritional, aesthetic and technical needs.

A breed and a variety (pure line) is a population of organisms artificially created by humans, which is characterized by a specific gene pool, hereditarily fixed morphological and physiological characteristics, a certain level and nature of productivity.

Each breed or variety has a specific reaction rate. So, white Leghorn chickens are distinguished by high egg production. With the improvement of the conditions of keeping and feeding, the egg production of chickens increases, and their weight practically does not change. The phenotype (including productivity) is most fully manifested only under certain conditions, therefore, for each region with one or another climatic conditions, agrotechnical methods, etc., you must have your own varieties and breeds.

All these factors must be taken into account in intensive agricultural production, the goal of which is to maximize food production at a minimum cost per unit of production. Agricultural intensification has become an urgent task of our time due to acute food shortages in some regions of the world. Protein deficiency is especially important, without which normal development is impossible. This problem is solved in various ways, including the improvement of agricultural technology, the selection of animal breeds and varieties of cultivated plants that are most productive in these conditions, the production of feed protein for animals from non-traditional sources, etc. These methods include the widespread use modern methods selection.

Selection and hybridization... The main selection methods are selection and hybridization. In crop production, mass selection is often used in relation to cross-pollinated plants. With this selection, only plants with the desired qualities are retained in sowing. When sowing again, plants with certain traits are selected again. This is how rye varieties were bred (for example, the Vyatka variety). The variety obtained in this way is genetically heterogeneous, and selection has to be repeated from time to time. Individual selection is reduced to the selection of individual individuals and obtaining offspring from them. Individual selection leads to a pure line - a group of genetically homogeneous (homozygous) organisms. Many valuable varieties of cultivated plants were bred by selection (Fig. 50).

Figure: 50. A low-stem wheat variety obtained as a result of breeding work with improved gluten quality (right) and the original variety (left)

Hybridization with subsequent selection is used to introduce valuable genes into the gene pool of the created plant variety or animal breed and to obtain optimal combinations of traits. Thus, a certain variety of wheat can have a strong stem and be resistant to lodging, but at the same time it is easily affected by rust. Another variety, with a thin and weak straw, is resistant to rust. When these two wheats are crossed, various combinations are found in the offspring, including in some of the plants signs of resistance to lodging and rust are combined. Such hybrids are selected and used for sowing.

In animal husbandry, due to the small number of offspring, individual selection is widely used with careful consideration of economically useful traits and hybridization. In farm animals, either closely related crossing is carried out to transfer most of the genes of the breed to a homozygous state, or unrelated crossing between breeds or even species. Unrelated breeding aims to combine several useful traits. Such crossing, with subsequent strict selection, leads to an improvement in the properties of the breed (Fig. 51).

Figure: 51. Selection for traits useful to humans leads to a change in the original wild species. Top right - wild boar, left and bottom - purebred domesticated hog

When crossing different breeds of animals or plant varieties, as well as interspecific crosses, hybrids of the first generation are distinguished by increased viability and powerful development (Fig. 52). This phenomenon, called heterosis or hybrid strength, is explained by the transition of many genes to a heterozygous state and the interaction of favorable dominant genes.

One of the outstanding achievements of modern breeding is the development of ways to overcome the infertility of interspecific hybrids. This was done for the first time at the beginning of the 20th century. Soviet geneticist GD Karpechenko when crossing radish and cabbage. This man-made plant looked nothing like a radish or a cabbage. Its pods consisted of two halves, one of which resembled a cabbage pod, the other a radish.

Subsequently, we managed to get a hybrid of wheat with wheatgrass. On the basis of this hybrid, a new variety of wheat was developed - grain-fodder, which for three cuttings per season gives up to 300-450 centners / ha of green mass. A new grain and forage crop, a hybrid of wheat with rye, has also been obtained by means of distant hybridization. This hybrid, called triticale, successfully combines the valuable traits of wheat and rye, producing large yields of grain and green mass with high nutritional qualities.

Often, in plant growing, polyploid plants are obtained, which are distinguished by larger sizes, high productivity and more active synthesis of organic substances (Fig. 53). Polyploid varieties of clover, sugar beet, turnip, rye, buckwheat, and oil plants are widespread.

What is called a breed; variety?
What are the main breeding methods you know?
Compare mass selection and individual selection. What are their similarities and differences?
For what purpose is crossing carried out in breeding work?
What interspecific hybrids do you know?
What are the features of polyploid varieties of cultivated plants?
What is the difference between the methods of domestication used by primitive man and modern ones?
What breeds and plant varieties are typical for your area? What special features do they have?
If you have pets, prepare a message about the breed they belong to. How was this breed bred? What are its features and advantages? What conditions are necessary for keeping animals of this breed?
Explain in class why different methods are used in plant and animal breeding.
Do you agree with the statement that the source material of local origin is of great value for breeding work? Explain your point of view.

25. Selection of microorganisms

Remember! Prokaryotes Bacteria Vitamins Essential amino acids Interferon Insulin

Microorganisms are intensively used in a wide variety of technological processes. Prokaryotes and unicellular eukaryotes (mainly fungi and bacteria) are used more and more every year in various sectors of the national economy: in baking, brewing, winemaking, and the preparation of many dairy products. In this regard, industrial microbiology is developing and an intensive selection of new strains of microorganisms with increased productivity substances necessary for a person. Such strains are of great importance for the production of fodder protein, enzyme and vitamin preparations, and antibiotics (Fig. 54) used in the food industry, medicine, and animal husbandry.

Figure: 54. The graph shows the relative increase in the productivity of strains of microorganisms raised by humans, compared with the original wild forms. The left column is the productivity of the wild strain, the right one - that was bred by humans.

For example, microorganisms are used to obtain vitamins B2, B12. Yeast fungi growing on wood hydrolysates or by consuming paraffins serve as a source of fodder protein. Yeast contains up to 60% proteins. The use of these high-protein concentrates makes it possible to additionally receive up to 1 million tons of meat per year. Production is of great importance in the national economy. essential amino acids with the help of microorganisms. The lack of these compounds in food dramatically inhibits growth. In traditional animal feed, there are few essential amino acids, and for the normal nutrition of livestock, it is necessary to increase the rations. Adding 1 ton of lysine, an amino acid obtained by microbiological synthesis, saves tens of tons of forage.

The technology of obtaining products necessary for a person from living cells or with their help is called biotechnology. Biotechnology is developing extremely rapidly. Over the past decades, a number of completely new industries have emerged based on the use of various bacteria and fungi.

Microorganisms "work" in metallurgy. The usual technology for extracting metals from ores does not allow widespread use of poor or complex ores: as a result of their processing, huge accumulations of waste are formed, and poisonous gases are released into the atmosphere. Metal biotechnology is based on the ability of bacteria to oxidize minerals and convert metals into soluble compounds. When bacteria oxidize sulfide minerals, most non-ferrous metals and rare elements go into solution. In this way, for example, all over the world, hundreds of thousands of tons of copper are obtained per year, and its cost is 2-3 times lower than in the case of traditional mining. With the help of bacteria, uranium, gold and silver are extracted from the ore, and such a harmful impurity as arsenic is removed.

Microorganisms are capable of continuously synthesizing proteins under favorable conditions. Scientists have developed ways of introducing certain genes into the bacterial cell, including human genes. Such methods are called genetic engineering. A bacterial cell synthesizes a protein encoded by a gene foreign to it in large quantities. This is how they get now interferons - proteins that suppress the multiplication of viruses, and insulin, which regulates blood glucose levels.

Review questions and assignments:

What is the significance of the selection of microorganisms for the national economy?
Give examples of industrial production and use of waste products of microorganisms.
What is biotechnology?
Think and give an example showing that biotechnological industries have been used by mankind for many hundreds and even thousands of years.
Give a definition of the concept of "genetic engineering".
Which concept is broader - "biotechnology" or "genetic engineering"? Explain your point of view.
Discuss in class what perspectives are open to humankind when using microorganisms in agriculture.
Under the guidance of a teacher, together with classmates, prepare the exhibition "Microorganisms in the Service of Man".
Prepare a message "The contribution of domestic scientists (NI Vavilov, GD Karpechenko, VI Michurin, etc.) in the development of selection."

TABLE OF CONTENTS

Section of the exam: 3.5. Regularities of heredity, their cytological foundations. The patterns of inheritance established by G. Mendel, their cytological foundations (mono- and dihybrid crossing) ...

Mendel, conducting experiments on crossing various varieties of peas, established a number of laws of inheritance that laid the foundation for genetics. He developed hybrid logical method of inheritance analysis signs of organisms. This method involves crossing individuals with alternative traits; analysis of the studied characters in hybrids without taking into account the rest; quantitative accounting of hybrids.

Carrying out monohybrid crossing (crossing one pair of alternative traits), Mendel established law of uniformityfirst generation.

Basic provisions hybridological method

For crossing, organisms are taken, whose ancestors in a number of generations did not give splitting according to selected characteristics, that is, pure lines.
Organisms differ in one or two pairs of alternative traits.
An individual analysis of the offspring of each cross is carried out.
Statistical processing of results is used.

■ G. Mendel's first law

When two homozygous individuals are crossed, differing from each other by one pair of alternative characters, all offspring in the first generation are uniform both in phenotype and genotype.

■ Second lawG.Mendel

When crossing hybrids of the first generation (two heterozygous individuals) in the second, a splitting of 3: 1 occurs. Along with the dominant, a recessive trait appears.

Analyzing cross - crossing, in which an individual with an unknown genotype to be established (AA or Aa) is crossed with a recessive homozygote (aa). If all the offspring from the total crossing is monotonous, the organism under study has the AA genotype. If a 1: 1 phenotype splitting is observed in the offspring of the Sudetenland, the organism under study is heterozygous Aa.

■ ThirdlawG.Mendel

When crossing homozygous individuals differing in two pairs of alternative traits or more, each trait is inherited independently of the others, combining in all possible combinations.

In his experiments, Mendel used different methods of crossing : monohybrid, dihybrid and polyhybrid... At the last crossing, individuals differ in more than two pairs of characters. In all cases, the law of uniformity of the first generation, the law of splitting of traits in the second generation and the law of independent inheritance are respected.

Independent inheritance law:each pair of traits is inherited independently of each other. In the offspring, there is a splitting according to the phenotype 3: 1 for each pair of traits. The law of independent inheritance is valid only if the genes of the considered pairs of traits lie in different pairs of homologous chromosomes. Homologous chromosomes are similar in shape, size, and gene linkage groups.

The behavior of any pair of non-homologous chromosomes in meiosis does not depend on each other. Discrepancy: them to the poles of the cageis random. Independent inheritance is essential for evolution; since it is a source of combinative inheritance.

TABLE: all inheritance patterns

This is a synopsis in biology for grades 10-11 on the topic “Patterns of heredity. Morgan's Laws "... Choose further action:

This article briefly and clearly describes three of Mendel's laws. These laws are the basis of all genetics, having created them, Mendel actually created this science.

Here you will find the definition of each law and learn a little new about genetics and biology in general.

Before starting to read the article, it is worth understanding that the genotype is the totality of the genes of an organism, and the phenotype is its external characteristics.

Who is Mendel and what did he do

Gregor Johann Mendel is a renowned Austrian biologist who was born in 1822 in the village of Gincice. He studied well, but his family had financial difficulties. To deal with them, Johann Mendel in 1943 decided to become a monk of a Czech monastery in the city of Brno and received the name Gregor there.

Gregor Johann Mendel (1822 - 1884)

Later he studied biology at the University of Vienna, and then decided to teach physics and natural history in Brno. Then the scientist became interested in botany. He conducted experiments on crossing peas. Based on the results of these experiments, the scientist deduced three laws of heredity, to which this article is devoted.

Published in Experiments with Plant Hybrids in 1866, these laws did not receive much publicity, and the work was soon forgotten. She was remembered only after Mendel's death in 1884. You already know how many laws he brought out. Now it's time to move on to considering each.

Mendel's first law - the law of uniformity of first-generation hybrids

Consider the experiment conducted by Mendel. He took two kinds of peas. These species were distinguished by the color of the flowers. One had purple, while the other had white.

Having crossed them, the scientist saw that all the offspring had purple flowers. And the yellow and green peas gave completely yellow offspring. The biologist repeated the experiment many times, checking the inheritance of different traits, but the result was always the same.

On the basis of these experiments, the scientist derived his first law, here is his formulation: all hybrids in the first generation always inherit only one trait from their parents.

Let's designate the gene responsible for purple flowers as A, and for white flowers as a. The genotype of one parent is AA (purple), and the other is aa (white). Gene A will be inherited from the first parent, and gene a will be inherited from the second. This means that the genotype of the offspring will always be Aa. A gene with an uppercase letter is called dominant, and a lowercase letter is called recessive.

If the genotype of an organism contains two dominant or two recessive genes, then it is called homozygous, and an organism containing different genes is called heterozygous. If the organism is heterozygous, then the recessive gene, denoted by a capital letter, is suppressed by the stronger dominant, as a result of which the trait for which the dominant is responsible appears. This means that peas with genotype Aa will have purple flowers.

Crossing two heterozygous organisms with different traits is a monohybrid crossing.

Codominance and incomplete dominance

It happens that the dominant gene cannot suppress the recessive one. And then both parental traits appear in the body.

This phenomenon can be observed with the example of camellia. If in the genotype of this plant one gene is responsible for red petals, and the other for white ones, then half of the camellia petals will turn red, and the rest - white.

This phenomenon is called codominance.

Incomplete dominance is a similar phenomenon in which a third sign appears, something in between what the parents had. For example, a night beauty flower with a genotype containing both white and red petals turns pink.

Mendel's second law - the law of splitting

So, we remember that when two homozygous organisms are crossed, all offspring will take only one trait. But what if we take two heterozygous organisms from this offspring and cross them? Will the offspring be uniform?

Let's go back to the peas. Each parent is equally likely to transmit either gene A or gene a. Then the offspring will be divided as follows:

AA - purple flowers (25%);
aa - white flowers (25%);
Aa - purple flowers (50%).

It can be seen that there are three times more organisms with purple flowers. This is a splitting phenomenon. This is Gregor Mendel's second law: when heterozygous organisms are crossed, the offspring is split in a ratio of 3: 1 by phenotype and 1: 2: 1 by genotype.

However, there are so-called lethal genes. If they are present, there is a deviation from the second law. For example, the offspring of yellow mice are split in a 2: 1 ratio.

The same thing happens with platinum-colored foxes. The fact is that if both genes are dominant in the genotype of these (and some other) organisms, then they simply die. As a result, the dominant gene can only manifest itself if the organism is heterosiote.

The law of gamete purity and its cytological basis

Take yellow peas and green peas, the gene for yellow is dominant and green is recessive. The hybrid will contain both of these genes (although we will only see the manifestation of the dominant one).

It is known that genes are transferred from parent to offspring using gametes. Gamete is a reproductive cell. In the genotype of the hybrid there are two genes, it turns out that in each gamete - and there are two of them - there was one gene. Having merged, they formed a hybrid genotype.

If a recessive trait characteristic of one of the parent organisms manifested itself in the second generation, then the following conditions were met:

the hereditary factors of the hybrids did not change;
each gamete contained one gene.

The second point is the law of gamete purity. Of course, there are not two genes, there are more of them. There is a concept of allelic genes. They are responsible for the same trait. Knowing this concept, we can formulate the law as follows: one randomly selected gene from an allele enters the gamete.

The cytological basis of this rule: the cells in which there are chromosomes containing pairs of alleles with all the genetic information divide and form cells in which there is only one allele - haploid cells. In this case, these are gametes.

Mendel's third law - the law of independent inheritance

The fulfillment of the third law is possible with dihybrid crossing, when not one trait is investigated, but several. In the case of peas, this is, for example, the color and smoothness of the seeds.

The genes responsible for seed color are denoted as A (yellow) and a (green); for smoothness - B (smooth) and b (wrinkled). Let's try to carry out a dihybrid crossing of organisms with different characteristics.

The first law is not violated with such a crossing, that is, the hybrids will be the same both in genotype (AaBb) and phenotype (with yellow smooth seeds).

What will be the split in the second generation? To find out, it is necessary to find out what gametes the parent organisms can secrete. Obviously these are AB, Ab, aB and ab. After that, a scheme is built, called a Pinnet lattice.

Horizontally lists all the gametes that one organism can produce, and vertically, another. Inside the lattice, the genotype of the organism is recorded, which would appear with the given gametes.

	AB	Ab	aB	ab
AB	AABB	AABb	AaBB	AaBb
Ab	AABb	AAbb	AaBb	Aabb
aB	AaBB	AaBb	aaBB	aaBb
ab	AaBb	Aabb	aaBb	aabb

If you study the table, you can come to the conclusion that the splitting of second-generation hybrids by phenotype occurs in a ratio of 9: 3: 3: 1. Mendel also realized this after conducting several experiments.

In addition, he also came to the conclusion that which of the genes of one allele (Aa) gets into the gamete does not depend on the other allele (Bb), that is, there is only independent inheritance of traits. This is his third law, called the law of independent inheritance.

Conclusion

Mendel's three laws are the main genetic laws. Thanks to the fact that one person decided to experiment with peas, biology received a new section - genetics.

With its help, scientists from all over the world have learned a lot of things, from disease prevention to genetic engineering. Genetics is one of the most interesting and promising areas of biology.

Patterns of heredity. G. Mendel's laws, their statistical nature and cytological foundations

The main laws of heredity were established by the outstanding Czech scientist Gregor Mendel. G. Mendel began his research with a monohybrid crossing, in which the parental individuals differ in the state of one trait. The seed pea chosen by him is a self-gash plant, so the descendants of each individual are clean lines. Together, peas can be artificially cross-pollinated, which makes possible hybridization and obtaining heterozygous (hybrid) forms. Plants of the pure line with yellow seed color were taken as maternal (P) plants, and the parent (P) - with green. As a result of this crossing, the seeds of plants (hybrids of the first generation - F1) turned out to be uniform - yellow in color. That is, only dominant traits appeared in the phenotype of F1 hybrids.

The uniformity of the first hybrid generation and the identification of only a dominant trait in hybrids is called the law of dominance or And Mendel's law.

Splitting is the phenomenon of manifestation of both states of signs in the second generation of hybrids (F2), due to the difference in allelic genes that determine them.

There are self-pollinating F1 plants with yellow seeds, producing yellow and green seedlings; the recessive trait does not disappear, but is only temporarily suppressed, reappears in F2 in a ratio of 1/4 of green seeds and 3/4 of yellow seeds. That is exactly - 3: 1.

The manifestation in the phenotype of a quarter of hybrids of the second generation of a recessive trait, and three-fourths of a dominant trait, was called the law of splitting, II Mendel's law.

In the future, G. Mendel complicated the conditions in the experiments - he used plants that differed in different states of two (Dihybrid crossing) or more characters (polyhybrid crossing). When crossing pea plants with yellow smooth seeds and wrinkled green ones - all the first generation hybrids had smooth yellow seeds - a manifestation of Mendel's I law - the uniformity of the first generation hybrids. But among the F2 hybrids, there were four phenotypes.

Based on the results obtained, G. Mendel formulated the law of the independent combination of the states of characters (the law of independent inheritance of characters). This is Mendel's third law. With di- or polyhybrid crossing, the splitting of the states of each trait in the offspring occurs independently of the others. For dihybrid crosses, a phenotype of 9: 3: 3: 1 is characteristic, and groups with new combinations of traits appear.

Incomplete dominance is an intermediate nature of inheritance. There are alleles that only partially dominate over recessive ones. Then the hybrid individual has a degree of trait in the phenotype, which distinguishes it from the parent. This phenomenon is called incomplete dominance.

Methods for checking the genotype of hybrid individuals

As you know, with complete dominance, individuals with a dominant and heterozygous set of chromosomes are phenotypically the same. It is possible to determine their genotype using an analyzing crossing. It is based on the fact that individuals homozygous for a recessive trait are always phenotypically similar. This is a crossing of a recessive homozygous individual with an individual with a dominant trait, but an unknown genotype.

Upon receiving a uniform F1, each parent forms only one type of gametes. So, the dominant individual is homozygous for the genotype (AA).

If when crossing an individual with a dominant trait with an individual with a recessive homozygous trait, the resulting offspring has a splitting of 1: 1, then the studied individual with a dominant trait is heterozygous (Aa).

Features of the method of hybridological analysis. Mendel's laws.
Types of gene interaction.
Linked inheritance of traits.
Cytoplasmic inheritance.

Method hybridological analysis , which consists in crossing and subsequent accounting for splits (ratios of phenotypic and genotypic varieties of offspring), was developed by the Czech naturalist G. Mendel (1865). The peculiarities of this method include: 1) taking into account, when crossing, not the entire diverse complex of traits in parents and offspring, but the analysis of the inheritance of individual alternative traits identified by the researcher; 2) quantitative accounting in a number of successive generations of hybrid plants that differ in individual traits; 3) individual analysis of the offspring from each plant.

Working with self-pollinating plants of garden peas, G. Mendel chose varieties (pure lines) for the experiment, differing from each other by alternative manifestations of traits. Mendel processed the obtained data mathematically, as a result of which a clear pattern of inheritance of individual traits of parental forms by their descendants in a number of subsequent generations was revealed. Mendel formulated this pattern in the form of the rules of heredity, which were later named mendel's laws.

The crossing of two organisms is called hybridization. Monohybrid (monogenic) is called the crossing of two organisms, in which the inheritance of one pair of alternative manifestations of a trait is traced (the development of this trait is due to a pair of alleles of one gene). First generation hybrids are uniform in the studied trait. In F1, only one of a pair of alternative variants of the seed color trait appears, called dominant.These results illustrate Mendel's first law of uniformity for first-generation hybrids, as well as the rule of dominance.

Mendel's first law can be formulated as follows: when crossing homozygous individuals that differ in one or more pairs of alternative traits, all hybrids of the first generation will be uniform in these traits. The hybrids will show dominant parental traits.

In the second generation, splitting was found according to the studied trait

The ratio of offspring with dominant and recessive manifestations of the trait turned out to be close to ¾ to ¼. In this way, mendel's second law can be formulated as follows: during monohybrid crossing of heterozygous individuals (F1 hybrids) in the second generation, splitting according to the variants of the analyzed trait is observed in a ratio of 3: 1 for phenotype and 1: 2: 1 for genotype. To explain the distribution of traits in hybrids of successive generations, G. Mendel suggested that each hereditary trait depends on the presence in the somatic cells of two hereditary factors derived from the father and mother. To date, it has been established that Mendel's hereditary factors correspond to genes - chromosome loci.

Homozygous plants with yellow seeds (AA) form gametes of the same variety with the A allele; plants with green seeds (aa) form gametes with a. Thus, using modern terminology, the hypothesis “ purity of gametes"Can be formulated as follows:" In the process of formation of germ cells, only one gene from an allelic pair gets into each gamete, because in the process of meiosis one chromosome from a pair of homologous chromosomes gets into the gamete.

Crossing, in which inheritance is traced in two pairs of alternative traits, is called dihybrid, for several pairs of signs - polyhybrid. In Mendel's experiments, when crossing a pea cultivar that had yellow (A) and smooth (B) seeds, with a pea cultivar with green (a) and wrinkled (b) seeds, F1 hybrids had yellow and smooth seeds, i.e. dominant signs appeared (hybrids are uniform).

Hybrid seeds of the second generation (F2) were divided into four phenotypic groups in the ratio: 315 - with smooth yellow seeds, 101 - with wrinkled yellow seeds, 108 - with smooth green seeds, 32 - with green wrinkled seeds. If the number of offspring in each group is divided by the number of offspring in the smallest group, then in F2 the ratio of phenotypic classes will be approximately 9: 3: 3: 1. So according to mendel's third law, genes of different allelic pairs and their corresponding traits are passed on to offspring whateverapart, combiningin all kinds of combinations.

With the complete dominance of one allele over another, heterozygous individuals are phenotypically indistinguishable from homozygous for the dominant allele and can only be distinguished using hybridological analysis, i.e. by the offspring, which is obtained from a certain type of crossing, called analyzing... Analyzing is a type of crossing in which a test individual with a dominant trait is crossed with an individual homozygous for the recessive aplel.

If the dominant individual is homozygous, the offspring from such a cross will be uniform and no splitting will occur. In the event that an individual with a dominant trait is heterozygous, splitting will occur in a ratio of 1: 1 by phenotype and genotype.

Gene interaction

In some cases, the action of different genes is relatively independent, but, as a rule, the manifestation of signs is the result of the interaction of products of different genes. These interactions can be associated with both allelicand with non-allelic genes.

Interaction between allelic genes are carried out in three forms: complete dominance, incomplete dominance and independent manifestation (codominance).

Previously, Mendel's experiments were considered, which revealed the complete dominance of one allele and the recessiveness of the other. Incomplete dominance is observed when one gene from a pair of alleles does not provide the formation of its protein product sufficient for normal manifestation of the trait. With this form of gene interaction, all heterozygotes and homozygotes differ significantly in phenotype from each other. When co-dominance in heterozygous organisms, each of the allelic genes causes the formation of a trait controlled by it in the phenotype. An example of this form of interaction of alleles is the inheritance of human blood groups according to the ABO system, determined by gene I. There are three alleles of this gene, Io, Ia, Ib, which determine the antigens of blood groups. The inheritance of blood groups also illustrates the phenomenon plural allelism: in the gene pools of human populations, gene I exists in the form of three different alleles, which are combined in individual individuals only in pairs.

Interaction of non-allelic genes. In some cases, one trait of an organism can be influenced by two (or more) pairs of non-allelic genes. This leads to significant numerical deviations of phenotypic (but not genotypic) classes from those established by Mendel during dihybrid crossing. The interaction of non-allelic genes is subdivided into basic forms: complementarity, epistasis, and polymeria.

When complementary In interaction, the sign is manifested only in the case of the simultaneous presence of two dominant non-allelic genes in the genotype of the organism. An example of a complementary interaction is the crossing of two different varieties of sweet pea with white flower petals.

The next type of interaction of non-allelic genes is epistasis, in which the gene of one allelic pair suppresses the action of the gene of the other pair. The gene that suppresses the action of another is called epistatic genome(or suppressor).The suppressed gene is called hypostatic.Epistasis can be dominant or recessive. An example of dominant epistasis is the inheritance of the color of the plumage of chickens. Gene C in the dominant form determines the normal production of pigment, but the dominant allele of another gene I is its suppressor. As a result, chickens with a dominant allele of the color gene in their genotype turn out to be white in the presence of a suppressor. The epistatic effect of the recessive gene illustrates the inheritance of coat color in house mice. Agouti color (reddish-gray coat color) is determined by the dominant gene A. Its recessive allele and in the homozygous state causes a black color. The dominant gene of the other pair C determines the development of the pigment; homozygotes for the recessive allele c are albinos with white hair and red eyes (no pigment in the coat and iris of the eyes).

The inheritance of a trait, the transmission and development of which, as a rule, is caused by two alleles of one gene, is called monogenic... In addition, genes from different allelic pairs are known (they are called polymeric or polygenes), approximately equally influencing the sign.

The phenomenon of simultaneous action on a trait of several non-allelic genes of the same type is called polymerization. Although polymeric genes are not allelic, since they determine the development of one trait, they are usually denoted by one letter A (a), numbers indicating the number of allelic pairs. The action of polygenes is most often cumulative.

Chained inheritance

The analysis of the simultaneous inheritance of several traits in Drosophila, carried out by T. Morgan, showed that the results of the analyzing crossing of F1 hybrids sometimes differ from those expected in the case of their independent inheritance. In the offspring of such a crossing, instead of a free combination of traits of different pairs, a tendency to inherit mainly parental combinations of traits was observed. This inheritance of traits was called linked.Linked inheritance is explained by the location of the corresponding genes on the same chromosome. As part of the latter, they are transmitted from generation to generation of cells and organisms, preserving the combination of parental alleles.

The dependence of linked inheritance of traits on the localization of genes in one chromosome gives reason to consider chromosomes as separate clutch groups. Analysis of the inheritance of the trait of eye color in Drosophila in the laboratory of T. Morgan revealed some features that made it possible to distinguish traits as a separate type of inheritance sex-linked inheritance.

The dependence of the experimental results on which of the parents was the carrier of the dominant variant of the trait made it possible to suggest that the gene that determines the color of the eyes in Drosophila is located on the X chromosome and has no homologue on the Y chromosome. All the features of sex-linked inheritance are explained by the unequal dose of the corresponding genes in representatives of different - homo- and heterogametic sex. The X chromosome is present in the karyotype of each individual, therefore the traits determined by the genes of this chromosome are formed in both females and males. Individuals of the homogametic sex receive these genes from both parents and pass them on to all descendants through their gametes. Representatives of the heterogametic sex receive a single X chromosome from a homogametic parent and pass it on to their homogametic offspring. In mammals (including humans), the male sex receives X-linked genes from the mother and passes them on to daughters. At the same time, the male sex never inherits the paternal X-linked trait and does not pass it on to his sons

Actively functioning genes of the Y chromosome, which do not have alleles in the X chromosome, are present in the genotype only of the heterogametic sex, and in the hemizygous state. Therefore, they manifest themselves phenotypically and are transmitted from generation to generation only in representatives of the heterogametic sex. Thus, in humans, the sign of auricular hypertrichosis ("hairy ears") is observed exclusively in men and is inherited from father to son.

We will start with an exposition of Mendel's laws, then we will talk about Morgan, and at the end we will say why genetics is needed today, how it helps and what are its methods.

In the 1860s, the monk Mendel began researching the inheritance of traits. This was done before him, and for the first time it is said about it in the Bible. The Old Testament says that if a livestock owner wanted to get a certain breed, then he fed some sheep with peeled branches if he wanted to get offspring with white wool, and unpeeled if he wanted to get a black cattle hide. That is, how traits are inherited worried people even before the writing of the Bible. Why, before Mendel, could not find the laws of transmission of traits in generations?

The fact is that before him, researchers chose a set of features of one individual, with which it was more difficult to deal with than with one feature. Before him, the transmission of signs was often considered as a single complex (such as her grandmother's face, although there are a lot of individual signs here). And Mendel recorded the transmission of each trait separately, regardless of how other traits were passed on to descendants.

It is important that Mendel chose characteristics for the study, the registration of which was extremely simple. These are discrete and alternative signs:

discrete (discontinuous) features: this feature is either present or absent. For example, a sign of color: a pea is either green or not green.
alternative signs: one state of the sign excludes the presence of another state. For example, the state of such a sign as color: a pea is either green or yellow. Both states of a trait in one organism cannot manifest themselves.

Mendel had an approach to the analysis of descendants that had not been used before. This is a quantitative, statistical method of analysis: all offspring with a given state of the trait (for example, green peas) were combined into one group and their number was counted, which was compared with the number of offspring with a different state of the trait (yellow peas).

As a trait, Mendel chose the color of seed peas, the state of which was mutually exclusive: either yellow or green. Another feature is the shape of the seeds. Alternative states of the trait are shape or wrinkled or smooth. It turned out that these traits are stably reproduced in generations, and appear either in one state or in another. In total, Mendel investigated 7 pairs of signs, following each separately.

When crossing, Mendel examined the transmission of traits from parents to their offspring. And this is what he got. One of the parents gave only wrinkled seeds in a succession of generations during self-pollination, the other parent gave only smooth seeds.

Peas are a self-pollinator. In order to get offspring from two different parents (hybrids), he had to make sure that the plants did not self-pollinate. To do this, he removed the stamens from one parent plant, and transferred pollen from another plant to it. In this case, the resulting seeds were hybrid. All hybrid seeds in the first generation were found to be the same. They all turned out to be smooth. We call the manifested state of the trait dominant (the meaning of the root of this word is dominant). Another condition of the trait (wrinkled seeds) was not found in the hybrids. We call this state of the trait recessive (inferior).

Mendel crossed the plants of the first generation inside himself and looked at the shape of the resulting peas (this was the second generation of the descendants of the crossing). The bulk of the seeds turned out to be smooth. But the part was wrinkled, exactly the same in the original parent (if we were talking about our own family, we would say that the grandson was exactly like a grandfather, even though my father and mother did not have this sign at all). He conducted a quantitative study of what proportion of offspring belong to one class (smooth - dominant), and what proportion to another class (wrinkled - recessive). It turned out that the wrinkled seeds turned out to be about a quarter, and three-quarters - smooth.

Mendel carried out the same crosses of the first generation hybrids for all other characteristics: seed color, flower color, etc. He saw that the ratio of 3: 1 is preserved.

Mendel crossed in one direction (dad with a dominant trait, mom with a recessive trait) and in the other (dad with a recessive trait, mom with a dominant trait). At the same time, the qualitative and quantitative results of the transmission of traits in generations were the same. From this we can conclude that both the female and paternal inclinations of the trait make the same contribution to the inheritance of the trait in the offspring.

The fact that in the first generation the trait of only one parent is manifested, we call the law of uniformity of hybrids of the first generation or the law of dominance.

The fact that in the second generation the traits of both one parent (dominant) and the other (recessive) reappear allowed Mendel to assume that it is not the trait as such that is inherited, but the deposit of its development (what we now call the genome). He also suggested that every organism contains a pair of such inclinations for each trait. Only one of the two inclinations passes from parent to child. The deposit of each type (dominant or recessive) passes to the descendant with equal probability. When a descendant combines two different inclinations (dominant and recessive), only one of them appears (dominant, it is designated by a capital letter A). The recessive deposit (denoted by a small letter a) does not disappear in the hybrid, since it manifests itself as a trait in the next generation.

Since in the second generation exactly the same organism appeared as the parental one, Mendel decided that the deposit of one trait is "not blurred", when combined with another, it remains the same pure. Later, it was found that only half of its inclinations are transmitted from this organism - sex cells, they are called gametes, carry only one of two alternative signs.

In humans, there are about 5 thousand morphological and biochemical characteristics that are inherited quite clearly according to Mendel. Judging by the splitting in the second generation, the alternative inclinations of one trait were combined with each other independently. That is, the dominant trait could appear with combinations of the type Aa, aA and AA, and recessive only in combination aa.

We repeat that Mendel suggested that it is not a trait that is inherited, but the inclinations of a trait (genes) and that these inclinations do not mix, therefore this law is called the law of gamete purity. Through the study of the inheritance process, it was possible to draw conclusions about some characteristics of the inherited material, that is, that the inclinations are stable in generations, retain their properties, that the inclinations are discrete, that is, only one state of the trait is determined, that there are two of them, they are combined randomly, etc. etc.

At the time of Mendel, nothing was known about meiosis, although the cells already knew about the nuclear structure. The fact that the nucleus contains a substance called nuclein became known only a couple of years after the discovery of Mendel's laws, and this discovery had nothing to do with it.

All the conclusions of the above material can be formulated as follows:

1) Each hereditary trait is determined by a separate hereditary factor, a deposit; in the modern view, these inclinations correspond to genes;

2) Genes are preserved in their pure form in a number of generations, without losing their individuality: this was proof of the basic principle of genetics: the gene is relatively constant;

3) Both sexes are equally involved in the transfer of their hereditary properties to offspring;

4) Reduplication of an equal number of genes and their reduction in male and female germ cells; this position was a genetic prediction of the existence of meiosis;

5) Hereditary inclinations are paired, one is maternal, the other is paternal; one of them may be dominant, the other - recessive; this position corresponds to the discovery of the principle of allelism: a gene is represented by at least two alleles.

The laws of inheritance include the law of splitting hereditary traits in the offspring of a hybrid and the law of independent combination of hereditary traits. These two laws reflect the process of transmission of hereditary information in cell generations during sexual reproduction. Their discovery was the first factual proof of the existence of heredity as a phenomenon.

The laws of heredity have a different content, and they are formulated as follows:

The first law is the law of discrete (gene) hereditary determination of traits; it underlies the theory of the gene.
The second law is the law of relative constancy of the hereditary unit - the gene.
The third law is the law of the allelic state of a gene (dominance and recessiveness).

The fact that Mendel's laws are associated with the behavior of chromosomes during meiosis was discovered at the beginning of the twentieth century during the rediscovery of Mendel's laws at once by three groups of scientists, independently of each other. As you already know, the peculiarity of meiosis is that the number of chromosomes in a cell is halved, and chromosomes can change their parts during meiosis. This feature characterizes the situation with the life cycle in all eukaryotes.

In order to test the assumption about the inheritance of inclinations in this form, as we have already said, Mendel also crossed the descendants of the first generation, which have yellow seeds with the parental green (recessive). He called the crossing into a recessive organism analyzing. As a result, he got a one-to-one splitting :( Aa x aa = Aa + Aa + aa + aa). Thus, Mendel confirmed the assumption that in the body of the first generation there are makings of traits of each parent in a ratio of 1 to 1. The state when both makings of a trait are the same, Mendel called homozygous, and when different - heterozygous.

Mendel took into account the results obtained on thousands of seeds, that is, he conducted statistical studies that reflect a biological pattern. The very laws discovered by him will act on other eukaryotes, for example, mushrooms. Shown here are mushrooms in which four spores resulting from one meiosis remain in a common membrane. Analyzing crossing in such fungi leads to the fact that in one shell there are 2 spores with the trait of one parent and two with the trait of the other. Thus, the 1: 1 splitting in the analyzing cross reflects the biological regularity of the splitting of the makings of one trait in each meiosis, which will look like a statistical regularity if all the spores are mixed.

The fact that the parents had different states of the same trait suggests that the inclinations for the development of a trait can somehow change. These changes are called mutations. Mutations are neutral: hair shape, eye color, etc. Some mutations lead to changes that disrupt the normal functioning of the body. These are short-leggedness in animals (cattle, sheep, etc.), eyelessness and winglessness in insects, hairlessness in mammals, gigantism and dwarfism.

Some mutations may be harmless, such as hairlessness in humans, although all primates have hair. But sometimes there are changes in the intensity of the hairline on the body and in humans. NI Vavilov called this phenomenon the law of homologous series of hereditary variability: that is, a trait typical only for one of two related species can be detected with some frequency in individuals of a related species.

This slide shows that mutations can be quite noticeable, we see a black family in which a white black albino was born. His children are likely to be pigmented, since this mutation is recessive, and the frequency of its occurrence is low.

We talked before about the signs that appear in full. But this is not the case for all signs. For example, the phenotype of heterozygotes may be intermediate between the dominant and recessive trait of the parents. Thus, the color of the eggplant fruit in the first generation changes from dark blue to less intense purple. At the same time, in the second generation, the splitting by the presence of color remained 3: 1, but if we take into account the intensity of the color, the splitting became 1: 2: 1 (the color is dark blue - AA, purple - 2 Aa and white - aa, respectively) In this case, it can be seen that the manifestation of the trait depends on the dose of the dominant allele. Splitting by phenotype corresponds to splitting by genotype: classes AA, Aa and aa, in a ratio of 1: 2: 1.

Let us once again highlight the role of Mendel in the development of science. No one before him thought that there could be the makings of signs at all. It was believed that in each of us sits a little man, inside him - another little man, etc. Conception has something to do with his appearance, but according to the mechanism, a ready-made little man is already present from the very beginning of his growth. These were the dominant concepts, which, of course, had a drawback - according to this theory, with a large number of generations, the homunculus should have turned out to be smaller in size than an elementary particle, but then J.

How did Mendel know which trait is dominant and which is recessive? He did not know anything like that, he just took some principle of organizing the experiment. Conveniently, the traits he observed were different: height, size, flower color, bean color, etc. He did not have an a priori model of the inheritance mechanism; he derived it from the observation of the transmission of a trait in generations. Another feature of his method. He found that the proportion of individuals with a recessive trait in the second generation is a quarter of all offspring. That is, the probability that this pea is green is 1/4. Let's say we got an average of 4 peas in one pod. Will there be 1 green pea and 3 yellow peas in each pod (these are offspring from two and only two parents)? No. For example, the probability that there will be 2 green peas is 1/4 x 1/4 \u003d 1/16, and that all four are green is 1/256. That is, if you take a bunch of beans with four peas in each, then every 256th will have all the peas with recessive signs, that is, green. Mendel analyzed the offspring of many identical pairs of parents. They talked about crossing because they show that Mendel's laws appear as statistical, and are based on a biological regularity - 1: 1. That is, gametes of different types in EACH meiosis in a heterozygote are formed in an equal ratio - 1: 1, and regularities are manifested statistically, since the descendants of hundreds of meiosis are analyzed - Mendel analyzed more than 1000 offspring in crosses of each type.

Mendel first investigated the inheritance of one pair of traits. Then he wondered what would happen if you simultaneously observe two pairs of signs. Above in the figure, on the right side, such a study is illustrated by pairs of signs - the color of the peas and the shape of the peas.

Parents of one type gave yellow and round peas during self-pollination. Parents of another type gave green and wrinkled peas during self-pollination. In the first generation, he received all yellow peas, and round in shape. The resulting splitting in the second generation is conveniently viewed using the Penet lattice. Got a split at 9: 3: 3: 1 (yellow and round: yellow and wrinkled: green and round: green and wrinkled). The splitting for each pair of traits occurs independently of each other. The ratio 9zhk + 3zhm + 3zk + 1sm corresponds to an independent combination of the results of two crosses (3zh + 1h) x (3k + 1m). That is, the makings of the signs of these pairs (color and shape) are combined independently.

Let's count how many different phenotypic classes we got. We had 2 phenotypic classes: yellow and green; and on another basis 2 phenotypic classes: round and wrinkled. And in total there will be 2 * 2 \u003d 4 phenotypic classes, which we got above. If we consider three traits, then the phenotypic classes will be 2 3 \u003d 8 classes. Mendel went as far as dihybrid crosses. The makings of all traits, fortunately Mendel, were in the pea on different chromosomes, and the total chromosomes in the pea were 7 pairs. Therefore, it turned out that he took traits that combined independently in the offspring.

A person has 23 pairs of chromosomes. If we consider any one heterozygous trait for each chromosome, a person may have 2 23 ~ 8 * 10 6 phenotypic classes in the offspring of one married couple. As mentioned in the first lecture, each of us contains about 1 difference per 1000 positions between father's and mother's chromosomes, that is, only about a million differences between father's and mother's chromosomes. That is, each of us is a descendant of a million-hybrid crossing, in which the number of phenotypic classes is 2,100,000. In practice, this number of phenotypic classes in the offspring of one pair is not realized, because we have only 23 chromosomes, not a million. It turns out that 8 * 10 6 is the lower limit of the amount of possible diversity in the offspring of a given married couple. Based on this, one can understand that there cannot be two absolutely identical people. The probability of mutation of a given nucleotide in DNA in one generation is about 10 -7 - 10 -8, that is, for the entire genome (3 * 10 9) there will be about 100 de novo changes between parent and child. And the total difference in the father's half of your genome from the mother's half is about 1,000,000. This means that old mutations in your genome are much more frequent than new ones (10,000 times).

Mendel also carried out an analyzing crossing - crossing with a recessive homozygote. In the offspring of the first generation, the combination of genes looks like AaB b... If you cross it with a representative with a completely recessive set of genes ( aabb), then you get four possible classes, which will be in a ratio of 1: 1: 1: 1, in contrast to the crossing discussed above, when we got a splitting of 9: 3: 3: 1.

Some statistical criteria are shown below - which ratio of numbers should be considered as expected, say, 3: 1. For example, for 3: 1 - out of four hundred peas it is unlikely that it will turn out exactly 300 to 100. If it turns out, for example, 301 to 99, then this ratio can probably be considered equal to 3 to 1. And 350 to 50 is probably not equal to 3 to 1.

The chi-square (χ 2) statistical test is used to test the hypothesis that the observed distribution is the expected one. This Greek letter is pronounced in Russian as "chi", and in English as "chi" (chi).

The χ 2 value is calculated as the sum of the squares of the deviations of the observed values \u200b\u200bfrom the expected values, divided by the expected value. Then, according to a special table for a given value of χ 2, the value of the probability is found that such a difference between the observed and expected values \u200b\u200bis random. If the probability turns out to be less than 5%, then the deviation is considered not random (the figure of five percent was chosen by agreement).

Will there always be some hereditary predetermined trait? After all, this assumption by default underlies the interpretation of the data obtained by Mendel.

It turns out that this can depend on many reasons. There is such an inherited trait in humans - six-fingered. Although we, like all vertebrates, normally have five fingers.

The probability of manifestation of the deposit of a trait in the form of an observed trait (here - six-fingered) may be less than 100%. In the photo, a person has 6 fingers on both feet. And his twin does not necessarily show this sign. The proportion of individuals with a given genotype in whom the corresponding phenotype is manifested was called penetrance (this term was introduced by the Russian geneticist Timofeev-Resovsky).

In some cases, the sixth toe may simply be indicated by some cutaneous growth. Timofeev-Resovsky suggested calling the degree of expression of a trait in an individual expressivity.

It is especially clear that a 100% relationship between genotype and phenotype is not traced in the study of identical twins. Their genetic constitution is one to one, and their characteristics coincide to varying degrees. Below is a table that shows the coincidence of signs for identical and non-identical twins. Various diseases are taken as signs in this table.

A trait that is present in most individuals in their natural habitat is called the wild type. The most common trait is often dominant. Such a relationship can have an adaptive meaning useful to the species. In humans, the dominant features are, for example, black hair, dark eyes, curly hair. By the way, since the corresponding genes are on different chromosomes, you can get a curly black man who will be blond - nothing prohibits this.

Why is it that in the case of monohybrid crossing, three genotypic classes in the offspring of the second generation correspond in some cases to three phenotypic classes (blue-violet and white eggplants), and in other cases - to two classes (yellow or green pea)? Why is the manifestation of the dominant feature incomplete in one case, and complete in the other? An analogy can be drawn with photographic film. Depending on the amount of light, the frame can be completely transparent, gray and completely black. It's the same with genes. For example, corn has the Y gene, which determines the formation of vitamin A. When the dose of the Y gene per cell increases from one to three, the activity of the enzyme that it encodes linearly changes and, in this case, the formation of vitamin A and the color of the grain increase. (In corn, the main part of the grain is the endosperm. Each endosperm cell has three genomes - two from the mother and one from the father). That is, many traits are quantitatively dependent on the allele dose. The more copies of an allele of the desired type, the greater the value of the trait it controls. This connection is constantly used in biotechnology.

Mendel could have safely failed to open his laws. Research on peas allowed Mendel to discover his laws, because peas are a self-pollinated plant, and therefore, without compulsion, they are homozygous. With self-pollination, the proportion of heterozygotes decreases in proportion to two to the power of the generation number. This was Mendel's luck - if the proportion of heterozygotes were large, then no patterns would be observed. When he then took on cross-pollinators, the patterns were disrupted, which greatly upset Mendel because he thought he had discovered something private. It turned out not.

Above, we talked about the inheritance of qualitative traits, and usually most traits are quantitative. Their genetic control is difficult enough. Quantitative traits are described through the average value of the trait value and the range of variation, which is called the reaction rate. Both the average value and the reaction rate are species-specific indicators that depend on both the genotype and environmental conditions. For example, the life span of a person. Although the Bible says that the prophets lived for 800 years, but now it is clear that no one lives for more than 120-150 years. A mouse, for example, lives for two years, although it is also a mammal. Our height, our weight are all quantitative signs. There are no people 3-4 meters tall, although there are elephants, for example. Each species has its own average for each quantitative trait and its own range of variation.

The patterns of inheritance are discovered in the study of qualitative traits.

Most of our traits are quantitative.

The values \u200b\u200bof the values \u200b\u200bof traits in a representative sample of individuals of this species are characterized by a certain average and breadth of its variation, which is called the reaction norm and depends both on the genotype and on the conditions for the formation of the trait.

Topic 4.2 Basic patterns

heredity

Terminology 1. Alternative - contrasting signs. 2. Clean lines - Plants, in a row of which no splitting is observed during self-pollination. 3. Hybrid method - obtaining hybrid offspring and its analysis. 4. Parents - P. 5. Males – ♂. 6. Females – ♀. 7. Crossbreeding- X.8. Hybrids F 1, F 2, F n. nine. Monohybrid - crossing of individuals with one contrasting trait. The patterns of inheritance of traits The quantitative patterns of the inheritance of traits were discovered by the Czech amateur botanist G. Mendel. Having set the goal to find out the patterns of inheritance of traits, he, first of all, drew attention to the choice of the object of research. For his experiments G. Mendel chose peas - those varieties that clearly differed from each other in a number of characteristics. One of the most significant points in the whole work was the determination of the number of traits by which the crossed plants should differ. G. Mendel for the first time realized that starting with the simplest case - the differences between parents on a single sign and gradually complicating the task, one can hope to unravel the whole tangle of patterns of transmission of signs from generation to generation, i.e. their inheritance. Here the strict mathematics of his thinking was revealed. It was this approach that allowed G. Mendel to clearly plan the further complication of experiments. In this respect, Mendel stood above all contemporary biologists. Another important feature of his research was that he chose organisms belonging to pure lines for experiments, i.e. such plants, in a number of generations of which, during self-pollination, splitting according to the studied trait was not observed. It is equally important that he observed the inheritance of alternative, i.e. contrasting signs. For example, the flowers of one plant were purple and another was white, the plant was tall or short, the beans were smooth or wrinkled, etc. Comparing the results of experiments and theoretical calculations, G. Mendel especially emphasized the average statistical nature of the regularities he discovered. Thus, the method of crossing individuals differing in alternative traits, i.e. hybridization, followed by strict consideration of the distribution of parental traits in offspring, was called hybridological. The regularities of the inheritance of characters, the identification by G. Mendel and confirmation by many biologists on a variety of objects, are currently formulated in the form of laws that are of a universal nature. The law of uniformity of the first generation of hybrids Monohybrid crossing. To illustrate the law of uniformity of the first generation - Mendel's first law, let us reproduce his experiments on monohybrid crossing of pea plants. Monohybrid is the crossing of two organisms that differ from each other in one pair of alternative traits. Consequently, with such a crossing, the patterns of inheritance of only two variants of the trait are traced, the development of which is due to a pair of allelic genes. For example, the sign is the color of the seeds, the options are yellow or green. All other signs characteristic of these organisms are not taken into account. If you cross pea plants with yellow and green seeds, then all the descendants - hybrids obtained as a result of this crossing, will have yellow seeds. The same picture is observed when crossing plants that have a smooth and wrinkled seed shape - all seeds in hybrids will be smooth. Consequently, in the first generation hybrid, only one of each pair of alternative traits appears. The second symptom seems to disappear, does not appear. Mendel called the predominance of the trait of one of the parents in the hybrid domination. A trait that manifests itself in a hybrid of the first generation and suppresses the development of another trait was called dominant, the opposite, i.e. the suppressed trait is recessive. The dominant feature is usually denoted by a capital letter (A), a recessive - by a lowercase (a). Mendel used in his experiments plants belonging to different pure lines, or varieties, the descendants of which over a long series of generations were similar to their parents. Therefore, in these plants, both allelic genes are the same. Thus, if the genotype of an organism contains two identical allelic genes, i.e. two genes absolutely identical in nucleotide sequence, such an organism is called homozygous. An organism can be homozygous for dominant (AA) or recessive (aa) genes. If allelic genes differ from each other in nucleotide sequence, for example, one is dominant and the other is recessive (Aa), such an organism is called heterozygous. Mendel's first law is also called the law of dominance or uniformity, since all individuals of the first generation have the same manifestation of the trait inherent in one of the parents. It is formulated as follows:When crossing two organisms belonging to different pure lines (two homozygotes), differing from each other in a pair of alternative traits, the entire first generation of hybrids (F 1) will be uniform and will carry the trait of one parent. With regard to coloration, Mendel found that red or black will dominate over white, with pink and gray in between, of varying saturation. Mendel proposed graphic designations of signs: P - parents, ♂ - male, ♀ - female,
, - gametes, X - crossing, F 1, F 2, F n - offspring. Mendel's first law is shown in Figure 1.

Figure 1. Mendel's first law

All offspring have the same intermediate color, which does not contradict Mendel's first law.

test questions

1. Biological material of Mendel. 2. Alternative signs in Mendel's experiments. 3. Clean lines and their definition. 4. The essence of the hybridological method. 5. Monohybrid crossing. 6. Dominant and recessive traits. 7. Allelic genes. 8. Mendel's first law. The law of uniformity.

Topic 4.2.1 Incomplete gene dominance

Terminology 1. Allelic genes - genes located at the same loci of homologous chromosomes. 2. Dominant feature - suppressing the development of another. 3. Recessive trait - suppressed. 4. Homozygote - a zygote with the same genes. five. Heterozygote - a zygote with different genes. 6. Split - divergence of characters in the offspring. 7. Crossover - chromosome overlap. In the heterozygous state, the dominant gene does not always completely suppress the expression of the recessive gene. In a number of cases, the hybrid F 1 does not fully reproduce any of the parental traits and the expression of the trait is intermediate in nature with a greater or lesser deviation towards a dominant or recessive state. But all individuals of this generation show uniformity in this trait. The intermediate nature of inheritance in the previous scheme does not contradict Mendel's first law, since all descendants of F 1 are uniform. Incomplete dominance - a widespread phenomenon. It was discovered when studying the inheritance of flower color in snapdragons, the structure of bird feathers, the color of the wool of cattle and sheep, biochemical characteristics in humans, etc. Multiple allelism. So far, we have analyzed examples in which the same gene was represented by two alleles - dominant (A) and recessive (a). These two states of a gene result from mutation. A gene can mutate multiple times. As a result, several variants of allelic genes arise. The combination of these allelic genes, which determine the variety of trait variants, is called a series of allelic genes. The emergence of such a series due to repeated mutations of one gene is called multiple allelism or multiple allelomorphism. Gene A can mutate into the state a 1, a 2, a 3, and n. Gene B, located in another locus, is in the state b 1, b 2, b 3, b n. For example, in the fly Drosophila, a series of alleles for the eye color gene is known, consisting of 12 members: red, coral, cherry, apricot, etc. to white, determined by the recessive gene. Rabbits have a series of multiple alleles for coat coloration. This causes the development of a solid color or lack of pigmentation (albinism). Members of the same series of alleles can be in different dominant-recessive relationships with each other. It should be remembered that only two genes from a series of alleles can be found in the genotype of diploid organisms. The rest of the alleles of this gene in different combinations are included in pairs in the genotypes of other individuals of this species. Thus, multiple allelism characterizes the diversity of the gene pool, i.e. the set of all genes that make up the genotypes of a certain group of individuals or a whole species. In other words, multiple allelism is a specific trait, not an individual trait. Mendel's second law - the law of splitting If the descendants of the first generation, identical in the studied trait, are crossed with each other, then in the second generation the traits of both parents appear in a certain numerical ratio: 3/4 of the individuals will have a dominant trait, 1/4 - a recessive trait. By genotype, F 2 will contain 25% of individuals homozygous for dominant alleles, 50% of organisms will be heterozygous and 25% of offspring will be homozygous for recessive alleles. The phenomenon in which the crossing of heterozygous individuals leads to the formation of offspring, some of which bears a dominant trait, and some of which are recessive, is called splitting. Therefore, splitting is the distribution of dominant and recessive traits among the offspring in a certain numerical ratio. The recessive trait in hybrids of the first generation does not disappear, but is only suppressed and manifests itself in the second hybrid generation. Thus, Mendel's second law (see Fig. 2) can be formulated as follows: when two descendants of the first generation are crossed with each other (two heterozygotes) in the second generation, splitting is observed in a certain numerical ratio: by phenotype 3: 1, by genotype 1: 2: 1.

Figure 2. Mendel's second law

With incomplete dominance of F 2 hybrids in the offspring, segregation by genotype and phenotype coincides (1: 2: 1). Gamete Purity Law This law reflects the essence of the process of gamete formation in meiosis. Mendel suggested that hereditary factors (genes) during the formation of hybrids do not mix, but remain unchanged. In the body of the F hybrid, from the crossing of parents differing in alternative characteristics, both factors are present - dominant and recessive. The dominant hereditary factor is manifested in the form of a trait, while the recessive factor is suppressed. Communication between generations during sexual reproduction is carried out through the sex cells - gametes. Therefore, it must be assumed that each gamete carries only one factor from a pair. Then, during fertilization, the fusion of two gametes, each of which carries a recessive hereditary factor, will lead to the formation of an organism with a recessive trait that manifests itself phenotypically. The fusion of gametes carrying a dominant factor, or two gametes, one of which contains a dominant factor and the other a recessive factor, will lead to the development of an organism with a dominant trait. Thus, the appearance in the second generation (F 2) of a recessive trait of one of the parents (P) can take place only if two conditions are met: 1. If the hereditary factors in hybrids remain unchanged. 2. If the germ cells contain only one hereditary factor from the allelic pair. The splitting of traits in the offspring when crossing heterozygous individuals, Mendel explained by the fact that gametes are genetically pure, i.e. carry only one gene from an allelic pair. The law of gamete purity can be formulated as follows: during the formation of sex cells, only one gene from an allelic pair (from each allelic pair) gets into each gamete. The cytological proof of the law of gamete purity is the behavior of the chromosome in meiosis: in the first meiotic division, homologous chromosomes fall into different cells, and in the anaphase of the second, daughter chromosomes, which, due to crossing over, may contain different alleles of the same gene. It is known that every cell of the body has exactly the same diploid set of chromosomes. Two homologous chromosomes contain two identical allelic genes. The formation of genetically "pure" gametes is shown in the diagram in Figure 3.

Figure 3. Formation of "clean" gametes

When male and female gametes merge, a hybrid is formed that has a diploid set of chromosomes (see Fig. 4).

Figure 4. Formation of a hybrid

As can be seen from the diagram, the zygote receives half of the chromosomes from the paternal organism, and half from the maternal. In the process of gamete formation in a hybrid, homologous chromosomes during the first meiotic division also enter different cells (see Fig. 5).

Figure 5. Formation of two varieties of gametes

Two varieties of gametes are formed for a given allelic pair. Thus, the cytological basis of the law of gamete purity, as well as the cleavage of characters in offspring during monohybrid crossing, is the divergence of homologous chromosomes and the formation of haploid cells in meiosis. Analyzing cross The hybridological method for studying heredity developed by Mendel makes it possible to establish whether an organism that has a dominant phenotype for the gene under study is homozygous or heterozygous. Is the breed clean? For this, an individual with an unknown genotype and an organism homozygous for a recessive allele with a recessive phenotype are crossed. If the dominant individual is homozygous, the offspring from such a cross will be uniform and splitting will not occur (see Fig. 6).

Figure 6. Crossing of dominant individuals.

A different picture will turn out if the studied organism is heterozygous (see Fig. 7).

Figure 7. Crossing of heterozygous individuals.

Cleavage will occur in a 1: 1 phenotype ratio. Such a result when crossing is proof of the formation of two gamete varieties in one of the parents, i.e. its heterozygosity is not a pure breed (see Fig. 8).

Figure 8. Cleavage will occur in a 1: 1 phenotype ratio.

test questions

1. Incomplete dominance and its manifestation in nature. 2. The essence of multiple allelism. 3. II-Mendel's law. Splitting law. 4. The law of gamete purity. 5. Cytological evidence for the law of gamete purity. 6. Analyzing crossing, its essence and meaning.

Topic 4.2.2 III Mendel's law - the law of the independent

combining features

Terminology 1. Digibritic crossing - crossing for two contrasting characteristics. 2. Diheterozygous organisms - organisms are heterozygous for two pairs of allelic genes. 3. Pannet lattice - graphical method for calculating the results of crossing. 4. Recombination - recombination of signs. five. Crossover - the emergence of new signs when chromosomes overlap. 6. Morganida - the distance between genes. Dihybrid and polyhybrid crossing Organisms differ from each other in many ways. It is possible to establish the patterns of inheritance of two or more pairs of alternative traits by means of dihybrid or polyhybrid crossing. For the dihybrid crossing, Mendel used homozygous pea plants, which differ in two pairs of traits - seed color (yellow and green) and seed shape (smooth and wrinkled). The dominant species were yellow color (A) and smooth seed form (B). Each plant forms one variety of gametes for the studied alleles. When gametes merge, all offspring will be uniform (see Fig. 9).

Figure 9. Gamete fusion

Organisms that are heterozygous for two pairs of allelic genes are called diheterozygous. When gametes are formed in a hybrid, only one of each pair of allelic genes gets into the gamete, while due to the accidental divergence of the paternal and maternal chromosomes in the first division of meiosis, gene A can get into the same gamete with gene B or with gene b, just like gene a can combine in one gamete with gene B or with gene b (see Fig. 10).

Figure 10. Formation of gametes in a hybrid

Table 1.

Processing of the results of dihybrid crossing

AB	AABB	AABb	AaBB	AaBb
Ab	AABb	AAbb	AaBb	Aabb
aB	AaBB	AaBb	aaBB	aaBb
ab	AaBb	Aabb	aaBb	aabb

↓ → A - yellow color. a - green color. B - round shape. b - wrinkled shape. Since many germ cells are formed in each organism, due to statistical regularities, four varieties of gametes are formed in the hybrid in the same amount (25% each) AB, Ab, aB, ab. During fertilization, each of the four types of gametes of one organism accidentally meets any of the gametes of another organism. All possible combinations of male and female gametes can be easily established using the Pannett grid. The gametes of the parents are written vertically and horizontally. In squares - genotypes of zygotes formed when gametes merge. It can be seen that according to the phenotype, the offspring is divided into four groups: 9 yellow smooth, 3 yellow wrinkled, 3 green smooth, 1 yellow wrinkled. If we take into account the results of splitting for each pair of features separately, it turns out that the ratio of the number of smooth to the number of wrinkled for each pair is 3: 1. Thus, in a dihybrid crossing, each pair of traits during splitting in the offspring behaves in the same way as in a monohybrid crossing, i.e. regardless of the other pair of signs. During fertilization, gametes are combined according to the rules of random combinations, but with equal probability for each. Various combinations of genes appear in the resulting zygotes. The independent distribution of genes in the offspring and the emergence of various combinations of these genes during dihybrid crossing is possible only if the pairs of allelic genes are located in different pairs of homologous chromosomes. Mendel's third law, or the law of independent combination, can be formulated as follows: when two homozygous individuals are crossed, differing from each other in two pairs of alternative traits, genes and corresponding traits are inherited independently of each other and are combined in all possible combinations. The third law applies only to the inheritance of allelic pairs located in different pairs of homologous chromosomes. The analysis of splitting is based on Mendel's laws, and in more complex cases, when individuals are distinguished by three or more pairs of characters. If the parental individuals differ in one pair of traits, in the second generation there is a splitting of traits in a ratio of 3: 1, for a dihybrid crossing it will be (3: 1) 2 or 9: 3: 3: 1, for a trihybrid crossing (3: 1) 3 and etc. You can also calculate the number of varieties of gametes formed in hybrids using the formula 2 n, where n is the number of pairs of genes by which the parental individuals differ.

G. Mendel's laws of inheritance of traits describe the primary principles of the transfer of hereditary characteristics from parental organisms to their children; these principles underlie classical genetics. These laws were discovered by Mendel as a result of crossing organisms (in this case, plants) with different genotypes. Usually one rule and two laws are described.

First generation uniformity rule

When crossing seed peas with stable traits - purple and white flowers, Mendel noticed that the emerging hybrids were all with purple flowers, among them there was not a single white one. Mendel repeated the experiments more than once and used other signs. For example, if he crossed peas with yellow and green seeds, the offspring had yellow seeds, when crossed peas with smooth and wrinkled seeds, the offspring had smooth seeds. The offspring from tall and low plants were tall.

So, first generation hybrids always acquire one of the parental traits... One trait (stronger, dominant) always suppresses the other (weaker, recessive). This phenomenon is called complete domination.

If we apply the above rule to a person, say, using the example of brown and blue eyes, then it is explained as follows. If in one homozygous parent in the genome, both genes determine the brown eye color (we denote this genotype as AA), and in the other, also homozygous, both genes determine the blue color of the eyes (we denote this genotype as aa), then the haploid gametes produced by them will always carry either the gene ANDor and (see diagram below).

Scheme of transmission of traits when crossing homozygous organisms

Then all children will have a genotype Aabut everyone will have brown eyes, as the brown eye gene dominates the blue eye gene.

Now consider what happens if heterozygous organisms (or first generation hybrids) are crossed. In this case, there will be splitting signs in certain quantitative terms.

The law of splitting signs, or Mendel's first law

If a heterozygous the descendants of the first generation, identical in the studied trait, are crossed among themselves, then in the second generation the traits of both parents appear in a certain numerical ratio: 3/4 of individuals will have a dominant trait, 1/4 - recessive (see diagram below).

The scheme of inheritance of traits when crossing heterozygous organisms

The phenomenon in which the crossing of heterozygous individuals leads to the formation of offspring, some of which bears a dominant trait, and some of which are recessive, is called splitting... As we understand, the recessive trait in the first generation hybrids did not disappear, but was only suppressed and manifested itself in the second hybrid generation. Mendel was the first to understand that during the formation of hybrids, hereditary factors do not mix and do not "erode", but remain unchanged. In a hybrid organism, both factors (genes) are present, but only the dominant hereditary factor manifests itself as a trait.

Communication between generations during sexual reproduction is carried out through the sex cells, each gamete carries only one factor from a pair. The fusion of two gametes, each of which carries one recessive hereditary factor, will lead to an organism with a recessive trait. The fusion of gametes, each of which carries a dominant factor, or two gametes, one of which contains a dominant factor and the other a recessive factor, leads to the development of an organism with a dominant trait.

Mendel explained the cleavage during crossing of heterozygous individuals by the fact that gametes carry only one gene from an allelic pair ( gamete purity law). Indeed, this is possible only if the genes remain unchanged and the gametes contain only one gene from a pair. It is convenient to study the correlation of features using the so-called Punnett lattice:

	A (0.5)	a (0.5)
A (0.5)	AA (0.25)	Aa (0.25)
a (0.5)	Aa (0.25)	aa (0.25)

Due to the statistical probability, with a sufficiently large number of gametes in the offspring, 25% of genotypes will be homozygous dominant, 50% - heterozygous, 25% - homozygous recessive, i.e., a mathematical relationship is established 1 AA:2Aa:1aa... Accordingly, according to the phenotype, the offspring of the second generation with monohybrid crossing is distributed in a ratio of 3: 1 - 3 parts of individuals with a dominant trait, 1 part of individuals with a recessive trait.

It should not be forgotten that the distribution of genes and their entry into gametes is probabilistic. Mendel had a quantitative, statistical approach to the analysis of offspring: all offspring with a given state of the trait (for example, smooth or wrinkled peas) were combined into one group, their number was counted, which was compared with the number of offspring with a different state of the trait (wrinkled peas). This pairwise analysis ensured the success of his observations. In the case of a person, it is very difficult to observe such a distribution - it is necessary that one pair of parents have at least a dozen children, which is quite a rare phenomenon in modern society. So it may well happen that one only child is born to brown-eyed parents, and that blue-eyed one that, at first glance, violates all the laws of genetics. At the same time, if you experiment with Drosophila or laboratory mice, Mendelian laws are quite easy to observe.

It should be said that in a certain sense Mendel was lucky - from the very beginning he chose a suitable plant as an object - colored peas. If he came across, for example, such plants as night beauty or snapdragon, the result would be unpredictable. The fact is that in snapdragons, heterozygous plants obtained by crossing homozygous plants with red and white flowers have pink flowers. Moreover, none of the alleles can be called either dominant or recessive. This phenomenon can be explained by the fact that complex biochemical processes caused by different work of alleles do not necessarily lead to alternative mutually exclusive results. The result may be intermediate, depending on the characteristics of metabolism in a given organism, in which there are always many options, shunting mechanisms or parallel processes with different external manifestations.

This phenomenon is called incomplete dominance or codominance, it is quite common, including in humans. An example is the MN human blood group system (note in passing that this is only one of the systems, there are many classifications of blood groups). At one time, Landsteiner and Levin explained this phenomenon by the fact that erythrocytes can carry on their surface either one antigen (M), or another (N), or both together (MN). If in the first two cases we are dealing with homozygotes (MM and NN), then in the heterozygous state (MN) both alleles manifest themselves, while both manifest (dominate), hence the name - codominance.

The law of independent inheritance of traits, or Mendel's second law

This law describes the distribution of features for the so-called dihybrid and polyhybrid crossing, i.e. when the crossed individuals differ in two or more signs. In Mendel's experiments, plants were crossed that differed in several pairs of traits, such as: 1) white and purple flowers, and 2) yellow or green seeds. Moreover, the inheritance of each trait followed the first two laws, and the traits were combined independently of each other... As expected, the first generation after crossing had a dominant phenotype in all characteristics. The second generation followed the 9: 3: 3: 1 formula, that is, 9/16 copies were with purple flowers and yellow peas, 3/16 with white flowers and yellow peas, another 3/16 with purple flowers and green peas, and, finally, 1/16 - with white flowers and green peas. This was because Mendel had successfully selected traits whose genes were on different chromosomes of the pea. Mendel's second law is fulfilled just in cases when the analyzed pairs of genes are located on different chromosomes. According to the rule of frequency of gametes, characters are combined independently of each other, and if they are on different chromosomes, then the inheritance of characters occurs independently.

Mendel's 1st and 2nd laws are universal, but exceptions are constantly encountered from the 3rd law. The reason for this becomes clear if we remember that there are many genes on one chromosome (in humans, from several hundred to a thousand or more). If genes are on the same chromosome, then there may be chained inheritance... In this case, the signs are transmitted in pairs or in groups. Genes located on one chromosome are called in genetics clutch groups... Most often, traits determined by genes located on the chromosome close to each other are transmitted together. Such genes are called closely linked... At the same time, genes located far from each other are sometimes linked together. The reason for this different behavior genes is a special phenomenon exchange of material between chromosomes during gametogenesis, in particular, at the stage of prophase of the first division of meiosis.

This phenomenon was studied in detail by Barbara McClintock ( Nobel Prize on physiology and medicine in 1983) and received the name crossing over. Crossoveris nothing more than the exchange of homologous regions between chromosomes. It turns out that each specific chromosome during transmission from generation to generation does not remain unchanged, it can "take with itself" a homologous region from its paired chromosome, giving it, in turn, a section of its DNA.

In the case of a person, it is quite difficult to establish the linkage of genes, as well as to identify crossing over due to the impossibility of arbitrary crossings (you cannot force people to give birth in accordance with some scientific tasks!), Therefore such data were obtained mainly on plants, insects and animals ... Nevertheless, thanks to the study of large families in which several generations are present, examples of autosomal linkage (i.e., joint transfer of genes located on autosomes) are known in humans. For example, there is a close linkage between the genes that control the Rh factor (Rh) and the MNS blood group antigen system. In humans, there are more known cases of linkage of certain characters with sex, that is, in connection with sex chromosomes.

Crossing over generally enhances combinative variability, that is, it contributes to a greater diversity of human genotypes. In this regard, this process is of great importance for. Using the fact that the farther from each other genes are located on the same chromosome, the more they are susceptible to crossing over, Alfred Stertevant constructed the first maps of the chromosomes of Drosophila. Today, complete physical maps of all human chromosomes have been obtained, that is, it is known in what sequence and what genes are located on them.

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1. Hybridological method

2. Inheritance in monohybrid crossing

3. Analyzing crossing

4. Inheritance with incomplete dominance

5. Deviations from expected splitting

6. Notebook analysis, or genetic splitting

The history of modern genetics begins with the establishment of the theory of the gene in 1900, when E. Chermak, K. Corens and G. de Vries independently discovered the laws of inheritance of individual traits, without assuming that these laws were discovered by G. Mendel.

For centuries, Mendel's predecessors studied the inheritance of all the traits in hybrid offspring. G. Mendel based the study of inheritance on new principles.

The first feature of Mendel's method consisted in obtaining constant forms over several generations, which he subsequently crossed.

The second feature of Mendel's method is the analysis of the inheritance of individual pairs of traits in the offspring of crossed plants of the same pea species, differing in one, two and three pairs of contrasting, alternative traits, for example, purple and white flowers, the shape of the seeds is smooth and wrinkled, etc. Each generation was counted separately for each such pair of alternative traits, without taking into account other differences between the crossed plants.

The third feature of this method was the use of a quantitative account of hybrid plants differing in individual pairs of alternative traits in a series of successive generations.

The fourth feature of Mendel's method was the use of an individual analysis of the offspring from each hybrid plant.

The listed simple methods of research constituted a fundamentally new hybridological method for studying inheritance, which opened an entire era in the study of heredity and variability. The set of genetic methods for studying inheritance is called genetic analysis.

Monohybrid crossing... A monohybrid is a cross in which the parental forms differ in one pair of alternative, contrasting characters.

Dominance, the law of uniformity of the first generation hybrids. Splitting law.Any crossing begins with identifying a trait. A sign is a certain separate quality of an organism, according to which one of its parts different from another or one individual from another. A trait in the genetic sense can be called any feature that is revealed when describing an organism: height, weight, nose shape, eye color, leaf shape, flower color, protein molecule size or its electrophoretic mobility. Signs should be shown constantly. To be sure of them constancy, Mendel had previously tested various forms of peas for two years. Signs should be contrasting. Mendel selected 7 features, each of which had two contrasting manifestations. For example, mature seeds were either smooth or wrinkled in morphology, yellow or green in color, and white or purple flowers.

After identifying the traits, you can proceed to crosses in which genetic lines are used - related organisms that reproduce the same hereditary constant traits in a number of generations. The offspring from the crossing of two individuals with different heredities is called a hybrid, and an individual is called a hybrid.

After Mendel crossed the forms of peas, differing in 7 traits, in the hybrids, only one of the pair of parental traits appeared, or dominated. The trait of the other parent (recessive) did not appear in the first generation hybrids. Later, this phenomenon of dominance was called the first Mendel's law (the law of uniformity of hybrids of the first generation or the law of dominance).

Mendel crossed the resulting hybrids with each other. As he himself writes, "in this generation, along with the dominant traits, recessive traits also appear again in their full development and, moreover, in a clearly expressed average ratio of 3: 1, so that out of every four plants of this generation, three receive a dominant trait and one - a recessive trait" [ Mendel, 1923. S. 12]. In total, 7324 seeds were obtained in this experiment, of which 5474 were smooth, and 1850 were wrinkled, from which the ratio of 2.96: 1 is derived. The data of this experiment indicate that the recessive trait is not lost and in the next generation it appears again ) in its pure form. H. de Vries in 1900 called this phenomenon the law of splitting, and later it was called Mendel's second law.

Different classes descendants (with a dominant and recessive manifestation) Mendel again self-pollinated. It turned out that signs with a recessive manifestation remain unchanged in subsequent generations after self-pollination. If you self-pollinate plants from the dominant class, then there will be splitting again, this time in a 2: 1 ratio.

Before proceeding to the presentation of the inheritance of traits, it is necessary to inform about some of the symbols adopted in genetics.

Crossing is designated by the sign of multiplication - X. In the schemes, it is customary to put the female genotype in the first place. Sex is usually denoted by the following symbols:

female - ♀ (mirror of Venus),

male - ♂ (shield and spear of Mars).

Parental organisms taken in crossing are designated by the letter P(from Latin Parento - parents). The hybrid generation is designated by the letter F(from the Latin Filii - children) with a digital index corresponding to the ordinal number of the hybrid generation [Lobashev 1967. P. 105]. Mendel proposed to designate the dominant feature with a capital letter, and the recessive one with the same letter, but lowercase.

To facilitate the calculation of combinations of different types of gametes, the English geneticist R. Pennett proposed a notation in the form of a lattice - a table with the number of rows (columns) according to the number of types of gametes formed by crossed individuals (commonly known as the Pennett lattice), and the resulting combinations of gametes are written at the intersection. So, in crossing Aa X Aathere will be the following gametes and their combinations:

Gametes	AND	and
AND	AA	Aa
and	Aa	aa

The cross done by Mendel can be shown in the following diagram:

P AAX aa

F 1 AaX Aa

F 2 AA Aa Aa aa

with manifestation with manifestation

Dominant recessive

feature feature

IN F 2 two types of splitting can be distinguished: 3: 1 by external manifestation and 1: 2: 1 by hereditary potencies. For the "external" characteristic of the trait, V. Johansen in 1909 proposed the term "phenotype", and for the characteristic of truly hereditary inclinations - "genotype". Therefore, splitting by genotype in F 2monohybrid crossing is 1: 2: 1, and the phenotype is 3: 1.

Constant forms AA and aa, which in subsequent generations do not give splitting, W. Batson in 1902 proposed to call homozygous, and the forms Aathat give cleavage are heterozygous.

As we have seen, in hybrids F 1recessive allele and,although it does not appear, it does not mix with the dominant allele A 1,and in F 2both alleles reappear in their pure form. This phenomenon can be explained only on the assumption that the hybrid F 1 Aaforms not hybrid, but "pure gametes", while these alleles appear in different gametes. Gametes carrying alleles AND and and, are formed in equal numbers; Based on this, splitting according to the genotype 1: 2: 1. Non-mixing of alleles of each pair of alternative characters in the gametes of a hybrid organism is called the rule of gamete purity, which is based on the cytological mechanisms of meiosis.

Analyzing crossing.To test whether a given organism is homo- or heterozygous, one can, as Mendel suggested, cross it with the original homozygote for recessive alleles. This type of crossing is called analyzing.

AaX aa AAX aa

1Aa:1aa aa

If an individual was homozygous for a dominant trait, all offspring belong to the same class. If, as a result of the analyzing crossing, the splitting in both phenotype and genotype is 1: 1, this indicates that one of the parents is heterozygous.

Incomplete dominance and codominance.In addition to the complete domination described by Mendel, incomplete, or partial, domination and codominance are also found. In case of incomplete dominance, the heterozygote has a phenotype intermediate between the phenotypes of homozygotes. Moreover, Mendel's rule about the uniformity of the phenotype in F 1observed. IN F 2both by phenotype and by genotype, splitting is expressed by a ratio of 1: 2: 1. An example of incomplete dominance is the intermediate pink color of a flower in hybrids of a night beauty Mirabilis jalapa,obtained from crossing the red-flowered and white-flowered forms.

Incomplete dominance turned out to be widespread and was noted when studying the inheritance of flower color in snapdragons, plumage color in Andalusian chickens, wool in cattle and sheep, etc. [see. for more details: Lobashev, 1967].

Co-dominance is a phenomenon when both alleles make an equal contribution to the formation of the phenotype. So, if the maternal organism has blood type A, and the paternal body has B, then children have blood type AB.

Complete dominance Incomplete dominance Codominance

Dominance types of different alleles

Deviations from expected splitting. Mendel noted that "in hybrids and their offspring in subsequent generations, there should be no noticeable violation of fertility." There will be violations in the splits if the classes have different survivability. Cases of deviations from the expected 3: 1 ratio are quite numerous.

It has been known for many decades that when yellow mice are crossed with each other in the offspring, a splitting in color into yellow and black in a ratio of 2: 1 is observed. -black foxes. A detailed analysis of this phenomenon showed that platinum foxes are always heterozygous, and homozygotes for the dominant allele of this gene die on embryonic stage, homozygotes for the recessive allele are silver-black in color.

In sheep, the dominant allele giving the shirazi color (gray karakul) is lethal in homozygote, as a result of which the lambs die soon after birth, and the cleavage also shifts towards 2: 1 (shirazi - black). The dominant allele is also lethal in homozygote, which determines the linear arrangement of scales in carp [Lobashev, 1967]. Many such mutations are known in Drosophila ( N, Sb, D, Cy, Land etc.). In all cases, the split is 2: 1 instead of 3: 1. This deviation not only does not indicate the fallacy of Mendel's laws, but provides additional evidence of their validity. However, these examples show that additional work is required to identify one of the descendant classes.

Notebook analysis, or gametic cleavage.With the development of germ cells as a result of two meiotic divisions in a monohybrid Aa, i.e. an organism heterozygous for one gene, 4 cells arise from one diploid cell (cell tetrad): two cells carry alleles ANDand the other two are and. It is the meiosis mechanism that is the biological process that ensures the cleavage of the gamete types in relation to 2A: 2a or 1A: 1a... Therefore, splitting by gamete types in the case of one allelic pair will be 1: 1. Splitting 3: 1, or 1: 2: 1 is established on zygotes as a consequence of the combination of gametes during fertilization.

When considering microsporogenesis in plants, it was possible to make sure that as a result of two meiotic divisions, a cell tetrad of 4 microspores is formed, having a haploid set of chromosomes and cleavage in relation to 2A: 2a.In angiosperms, each tetrad cannot be taken into account, because mature pollen grains from the cell tetrad disintegrate and do not persist together. In such plants, splitting can be taken into account only by the totality of all pollen grains. In maize, one pair of gene alleles is known, which determines the starchy or waxy types of pollen grains. If the pollen grains of hybrid corn ( Aa) treated with iodine, the starchy ones acquire a blue color, and the waxy ones become reddish, and they can be counted. This is a 1: 1 split.

Back in the 1920s, objects (mosses) were found, in which it was possible to analyze splitting within a single tetrad. This method, which makes it possible to establish the cleavage of gametes after two divisions of maturation (meiosis), was called tetrad analysis. This method made it possible for the first time to directly prove that Mendelian splitting is the result of the regular course of meiosis, that it is not a statistical but a biological regularity. Let us give an example of tetrad analysis in the study of one allelic pair in yeast. In yeast of the genus Saccharomyces, cells are found that produce red and white colonies. These alternative traits are determined by one allelic pair of the color gene AND- White color, and - red. When haploid gametes merge, a diploid zygote is formed F 1... She soon begins meiosis, as a result of which a tetrad of haploid spores is formed in one asuke. Cutting the ask and removing each spore separately transfer them to the substrate, where they multiply. Each of the 4 haploid cells begins to divide and 4 colonies are formed. Two of them turn out to be white and two red, i.e. splitting is observed, exactly corresponding 1A: 1a.

1. What prevented Mendel's predecessors from approaching the analysis of hereditary traits? How did Mendel's genius show?

2. What are the basic laws of Mendel you know? What is their essence? Do you know about their secondary discovery?

3. Do all cases of inheritance of traits do not contradict Mendel's laws, do they supplement them? What are these additions?

4. What is a dominant and recessive trait, homo- and heterozygosity, geno- and phenotype?

5. What is the essence of the law of gamete purity?

6. What kind of inheritance is called intermediate?