The scientist was the first to investigate the basic laws of heredity. The essence of the laws of inheritance of traits in humans

Basic patterns of inheritance

1 . Karyotype

2. Genome.

3. Monohybrid crossing. Mendel's first and second laws.

4. Analyzing crossing.

Genetics is the science of heredity and variability of organisms.Heredity - the property inherent in all organisms to transmit to offspring the characteristic features of the structure, individual development, metabolism, and, consequently, the state of health and predisposition to many diseases.

The transmission of traits of previous generations to the offspring is called inheritance. The mechanism of this transmission is the process of reproduction, both during simple division of cells of protozoa and tissue cells, and during sexual reproduction, when the union of male and female reproductive cells (gametes) leads to the creation of a new organism that resembles parents and ancestors.

When studying the patterns of inheritance, individuals are usually crossed that differ from each other by alternative (mutually exclusive) traits (for example, yellow and green, smooth and wrinkled surface of peas). Genes that determine the development of alternative traits are called allelic. They are located in the same loci (places) homologous (paired) chromosomes. An alternative trait and a corresponding gene that manifests itself in first-generation hybrids. Called dominant rather than manifesting (suppressed) - recessive. Allelic genes are usually denoted by the same letters of the Latin alphabet: dominant - with a capital letter (A), and recessive - with a lowercase (a). If both homologous chromosomes contain the same allelic genes (two dominant - AA or two recessive - (aa), such an organism is called homozygous since it forms one type of gametes and does not split when crossed with an organism of the same genotype.

If different genes of the same allelic pair (Aa) are localized in homologous chromosomes, then such an organism is called heterozygous on this basis. It forms two types of gametes and, when crossed with an organism of the same genotype, splits. The totality of all the properties and characteristics of the body is called phenotype. The phenotype develops on the basis of a specific genotype as a result of interaction with environmental conditions. A separate feature is called hairdryer.

Karyotype Is a collection of metaphase chromosomes characteristic of a particular type of organism. The consistency of the karyotype is maintained by the precise mechanisms of mitosis and meiosis.

The study of karyotypes and their variability is important for public health (many genetic diseases are associated with a change in karyotype), breeding (many plant varieties differ in karyotype) and ecological biomonitoring (karyotype can change under the influence of environmental factors).

The karyotype is used as a species characteristic (there is a special section of taxonomy - karyosystematics ). Karyotypic test is one of the most important criteria for the species. The essence of this criterion lies in the fact that all individuals of a given species are characterized by a certain karyotype. The concept of "karyotype" includes the number of chromosomes, their size, morphology, features of longitudinal differentiation.

If both arms of a chromosome are equal in length, then such a chromosome is called metacentric, if unequal, then such a chromosome is called submetacentric, if one of the arms is very short, then such a chromosome is called acrocentric. The terminal sections of chromatids are called telomeres ... Some chromosomes have distant structures in the telomere region ( satellites); this is satellite chromosomes.

When special methods coloration (differential coloration) shows that chromosomes consist of alternating sections - disks: C, T, R, G, N, Q. The alternation of disks is specific for each chromosome. Thus, metaphase chromosomes have individuality.

The smallest possible set of chromosomes in a cell is called genome .

Term genome (German Genom) was proposed by the German botanist Hans Winkler in 1920 to denote the minimum set of chromosomes. This idea of \u200b\u200bthe genome is preserved in modern cytogenetics. However, it was soon proved that DNA is part of the chromosomes (Fölgen, 1924), and by the middle of the 20th century. it was found that it is DNA that is the carrier of hereditary information (O. Avery et al., 1944; J. Watson and F. Crick, 1953). Therefore, at present in molecular genetics, the term genome increasingly designate the minimum ordered set of all DNA molecules in the cell.

Genome Is a characteristic of a species, not an individual. Genomes different types denoted by Latin letters ( AND , B , C ...). Karyotypes of "pure" species include only one genome (for example, the cells of cultivated rye contain the R ). Karyotypes of hybrids and species of hybrid origin include several genomes (for example, triticale cells contain genomes A , B and R ; in durum wheat cells - genomes AND and IN (in some species AND and G)). Then the genome of a "pure" species can be called elementary, and the gene of a hybrid - complex.

The number of chromosomes in the genome is called the main chromosome number and is denoted by the symbol x ... For example, for gymnosperms x \u003d 12, and for angiosperms the main number x initially equal to 7 (although a number of angiosperms also have other main chromosomal numbers: x \u003d 12 for nightshades, x \u003d 19 for willows).

The study of genomes is important from the point of view of medicine, the theory of the breeding process and the theory of evolution.

It is more convenient to consider genome organization using the example of multicellular animals. In these organisms, two types of cells are distinguished: somatic cells from which the body is built ( catfish) organisms, and sex cells (gametes). The number of chromosomes in the germ cells of most animals corresponds to the main chromosome number and is called haploid number of chromosomes (denoted by the symbol n ), then x =n ... In a haploid set, each chromosome exists in the singular (represented by one homologue). Somatic cells contain double, or diploid chromosome set , which is denoted by the symbol 2 n ... In a diploid set, each chromosome is represented by two homologues (with the exception of the sex chromosomes in the heterogametic sex, for example, in males of most mammals X and Y –Chromosomes are not homologous).

Consider the organization of the human genome at the cytogenetic level. The number of chromosomes in the haploid set (the main number) is 23. All chromosomes are numbered and assigned to classes. Of them to class AND include chromosomes 1, 2, 3; to the class IN - chromosomes 4, 5; to the class FROM - chromosomes 6, 7, 8, 9, 10, 11, 12; to the class D - chromosomes 13, 14, 15; to the class E - chromosomes 16, 17, 18; to the class F - chromosomes 19, 20; to the class G - chromosomes 21, 22. The listed chromosomes are called autosomes, they are present in both men and women. In a diploid set ( 2 n \u003d 46) each autosome is represented by two homologues. The twenty-third chromosome is sex chromosome (gonosoma), it can be presented or X or Y –Chromosome. Sex chromosomes in women are represented by two X - chromosomes, and in men one X -Chromosome and one Y –Chromosome.

The main patterns of inheritance were studied by G. Mendel and described in his book Experiments on Plant Hybrids (1865). He carried out the crossing of pea plants, in which the parental forms were analyzed according to one pair of alternative traits. This crossing is called monohybrid. If the parental forms take into account two pairs of alternative traits, the crossing is called dihybrid, more than two signs - polyhybrid. Before carrying out experiments, G. Mendel obtained pure lines of peas with alternative traits, i.e., homozygous dominant (AA) - yellow and homozygous recessive (aa) green individuals, which were subsequently crossed with each other.

The crossing is recorded as follows: in the first line write the letter P (parents), then the genotype of the female organism, the sign of crossing X and the genotype of the male organism; in the second line write the letter G (gametes) and gametes of female and male individuals, each gamete is taken in a circle; in the third line they put the letter F (descendants) and write down the genotypes of the descendants:

When writing out gametes, you must adhere to the following principles: from each pair of allelic genes, one gene must enter the gamete; if the organism is homozygous (for example, AA), then all gametes, no matter how many of them are formed, will contain only one gene (A), that is, they will all be of the same type, and, therefore, a homozygous organism forms one type of gametes; if the organism is heterozygous (Aa), then in the process of meiosis one chromosome with gene A falls into one gamete, and the second homologous chromosome with gene a will fall into another gamete (a heterozygous organism will form two types of gametes for one pair of genes: Aa - A + a ...

When analyzing the results of crossing, it turned out that all descendants in the first generation are the same in phenotype (the dominant trait of yellow color is manifested - the law of dominance) and genotype (heterozygous), hence the name mendel's first law - the law of uniformity of the first generation hybrids. It is formulated as follows: when crossing homozygous individuals differing in one pair of alternative traits, the uniformity of the first generation hybrids is observed both in phenotype and genotype.

When crossing hybrids of the first generation with each other (i.e., heterozygous individuals), the following result is obtained:

P (F1) Aa x Aa

F2 AA Aa Aa aa

Each of the heterozygotes forms two types of gametes, that is, it is possible to obtain four of their combinations:

1) an egg with the A gene is fertilized by a sperm with the A gene - the AA genotype will be obtained;

2) an egg with the A gene is fertilized by a sperm with the a gene - the Aa genotype;

3) an egg cell with gene a is fertilized by a sperm cell with gene A - genotype Aa;

4) an egg cell with the a gene is fertilized by a sperm with the a gene - the aa genotype.

The resulting zygotes: 1AA, 2 Aa, 1 aa, the probability of their formation is equal. According to the phenotype, individuals AA and Aa are indistinguishable (yellow), therefore, a splitting is observed in the ratio of 3: 1 (three parts of the offspring with yellow seeds and one part with green seeds). By genotype, the ratio will be: 1AA (one part - yellow homozygotes): 2Aa (two parts - yellow heterozygotes): 1aa (one part - green homozygotes).

Mendel's second law - the law of splitting - is formulated as follows: when crossing hybrids of the first generation, splitting is observed in a ratio of 3: 1 by phenotype and 1: 2: 1 by genotype.

The dominant gene does not always completely suppress the action of the recessive gene. In this case, all hybrids of the first generation do not reproduce the characteristics of the parents - there is intermediate nature of inheritance. In the second generation, the dominant homo- and heterozygotes will differ phenotypically and the segregation by phenotype and genotype is the same (1: 2: 1).

For example, when homozygous plants of a night beauty are crossed with red (AA) and white (aa) flowers, the first generation is obtained with pink flowers (intermediate inheritance). In the second generation, splitting by phenotype, as well as by genotype, will be: 1 part of plants with red flowers, two parts with pink flowers and one part with white flowers.

Red White

P (F1) Aa x Aa

F2 AA Aa Aa aa

Red Pink White

Incomplete dominance is a fairly common phenomenon: it is found, for example, in the inheritance of wool color in cattle and sheep, some biochemical traits in humans (different variants of hemoglobins).

To explain the regularities of inheritance established by Mendel, Betson proposed gamete purity hypothesis ... Based on the results of monohybrid crossing, we are convinced that, although only a dominant trait is manifested in heterozygotes, the recessive gene is not only not lost, but in a heterozygous organism it does not merge with the dominant one, does not dilute, does not change, but remains in a pure allelic state. As was shown later, allelic genes are located at the same loci of homologous chromosomes and in the process of meiosis fall into different gametes. Consequently, only one of the allelic genes, which determines the development of one of the alternative traits, can be simultaneously present in a gamete, and they are "pure" for this trait. In a heterozygous organism, this process looks like this:

Diagram of the divergence of homologous chromosomes in meiosis

Briefly, the hypothesis of gamete purity can be reduced to the following two provisions:

1) in a hybrid organism, genes do not hybridize (do not mix) and are in a pure allelic state;

2) in the process of meiosis, only one gene from the allelic pair gets into the gamete.

The gamete purity hypothesis establishes that the laws of cleavage are a consequence of a random combination of gametes carrying different genes. However, the general result turns out to be not accidental, since a statistical regularity is manifested here, determined by a large number of equally probable gamete encounters. Thus, the splitting at monohybrid crossing heterozygous organisms 3: 1 in the case of complete dominance or 1: 2: 1 with incomplete dominance should be considered as a biological regularity based on statistical data.

The cytological foundations of the hypothesis of the purity of gametes and the first two laws of Mendel are the regularities of the divergence of homologous chromosomes and the formation of haploid germ cells during meiosis.

In some cases, it is necessary to establish the genotype of an individual with a dominant trait, since with complete dominance, homozygote (AA) and heterozygote (Aa) are phenotypically indistinguishable. For this use analyzing crossing, in which a given organism with an unknown genotype is crossed with a homozygous recessive one for a given allele. There are two options for crossing results:

1) P AA x aa 2) P Aa x aa

G A a G A a A

F Aa F Aa aa

If, as a result of such crossing, the uniformity of the first generation hybrids is obtained, then the analyzed organism is homozygous, and if a 1: 1 splitting occurs in F1, then the individual is heterozygous. Analyzing crossing is widely used in breeding.

Tasks to solve:

1. When crossing red-fruited and white-fruited strawberries, only pink forms were obtained. Write the genotypes of the parental and hybrid forms, if it is known that the gene for red color does not completely dominate the gene that controls white color.

2. If in wheat the gene that determines the short spike length does not completely dominate over the gene responsible for the emergence of the long spike, then how long can the spikes appear when crossing 2 plants with medium spikes.

3. In dogs, the coarse coat is dominant, the soft is recessive. Two wire-haired parents produce a wire-haired puppy. With whom should he be crossed to find out if he has a soft-haired allele in the genotype?

Genetics - the science of the laws of heredity and variability. The date of "birth" of genetics can be considered 1900, when G. De Vries in Holland, K. Correns in Germany and E. Cermak in Austria independently "rediscovered" the laws of inheritance of traits established by G. Mendel back in 1865.

Heredity - the property of organisms to transmit their characteristics from one generation to another.

Variability - the property of organisms to acquire new characteristics in comparison with their parents. In a broad sense, variability is understood as the differences between individuals of the same species.

Sign - any feature of the structure, any property of the organism. The development of a trait depends both on the presence of other genes and on environmental conditions; the formation of traits occurs during the individual development of individuals. Therefore, each individual taken separately has a set of features that are characteristic only of it.

Phenotype - the set of all external and internal signs of the body.

Gene - a functionally indivisible unit of genetic material, a section of a DNA molecule that encodes the primary structure of a polypeptide, a transport or ribosomal RNA molecule. In a broad sense, a gene is a piece of DNA that determines the possibility of developing a separate elementary trait.

Genotype - a set of genes in an organism.

Locus - the location of the gene on the chromosome.

Allelic genes - genes located at identical loci of homologous chromosomes.

Homozygote - an organism that has allelic genes of one molecular form.

Heterozygote - an organism with allelic genes of different molecular forms; in this case, one of the genes is dominant, the other is recessive.

Recessive gene - an allele that determines the development of a trait only in a homozygous state; such a sign will be called recessive.

Dominant gene - an allele that determines the development of a trait not only in a homozygous state, but also in a heterozygous state; such a feature will be called dominant.

Genetic methods

The main one is hybridological method - a system of crosses that allows tracing the patterns of inheritance of traits in a number of generations. First developed and used by G. Mendel. Distinctive features methods: 1) targeted selection of parents differing in one, two, three, etc. pairs of contrasting (alternative) stable features; 2) strict quantitative accounting of the inheritance of traits in hybrids; 3) individual assessment of offspring from each parent in a series of generations.

Crossing, in which the inheritance of one pair of alternative traits is analyzed, is called monohybrid, two pairs - dihybrid, several pairs - polyhybrid... Alternative features are understood to mean different meanings of any feature, for example, feature - color of peas, alternative features - yellow, green color of peas.

In addition to the hybridological method, genetics uses: genealogical - compilation and analysis of pedigrees; cytogenetic - study of chromosomes; twin - study of twins; population-statistical method - the study of the genetic structure of populations.

Genetic symbolism

Proposed by G. Mendel, used to record the results of crosses: P - parents; F - offspring, the number below or immediately after the letter indicates the ordinal number of the generation (F 1 - hybrids of the first generation - direct descendants of the parents, F 2 - hybrids of the second generation - result from crossing F 1 hybrids with each other); × - cross icon; G - male; E - female; A - dominant gene, and - recessive gene; AA - dominant homozygote, aa - recessive homozygote, Aa - heterozygote.

The Law of Uniformity of First Generation Hybrids, or Mendel's First Law

The success of Mendel's work was facilitated by the successful choice of an object for crossing - various varieties of peas. Pea features: 1) it is relatively easy to grow and has a short development period; 2) has numerous offspring; 3) has a large number of clearly visible alternative features (corolla color - white or red; cotyledon color - green or yellow; seed shape - wrinkled or smooth; bean color - yellow or green; bean shape - round or with constrictions; arrangement of flowers or fruits - along the entire length of the stem or at its apex; stem height - long or short); 4) is a self-pollinator, as a result of which it has a large number of clean lines that steadily retain their characteristics from generation to generation.

Mendel conducted experiments on crossing different varieties of peas for eight years, starting in 1854. February 8, 1865 G. Mendel spoke at a meeting of the Brunn Society of Naturalists with a report "Experiments on plant hybrids", where the results of his work were summarized.

Mendel's experiments were carefully thought out. If his predecessors tried to study the patterns of inheritance of many traits at once, Mendel began his research by studying the inheritance of only one pair of alternative traits.

Mendel took pea varieties with yellow and green seeds and artificially cross-pollinated them: he removed the stamens from one variety and pollinated them with pollen from another variety. The first generation hybrids had yellow seeds. A similar picture was observed in crosses in which the inheritance of other traits was studied: when crossing plants with smooth and wrinkled seed shapes, all seeds of the resulting hybrids were smooth, from crossing red-flowered plants with white-flowered plants, all obtained were red-flowered. Mendel came to the conclusion that in hybrids of the first generation only one of each pair of alternative traits appears, and the second seems to disappear. Mendel called the trait manifested in hybrids of the first generation dominant, and the suppressed trait recessive.

When monohybrid crossing of homozygous individualswith different values \u200b\u200bof alternative traits, hybrids are uniform in genotype and phenotype.

Genetic scheme of Mendel's law of uniformity

(A - yellow peas, a - green peas)

Splitting law, or Mendel's second law

G. Mendel made it possible for the first generation hybrids to self-pollinate. In the hybrids of the second generation obtained in this way, not only a dominant, but also a recessive trait appeared. The results of the experiments are shown in the table.

Signs	Dominant		Recessive		Total
	Number	%	Number	%
Seed shape	5474	74,74	1850	25,26	7324
Coloration of cotyledons	6022	75,06	2001	24,94	8023
Seed skin coloration	705	75,90	224	24,10	929
Bean shape	882	74,68	299	25,32	1181
Bean coloring	428	73,79	152	26,21	580
Flower arrangement	651	75,87	207	24,13	858
Stem height	787	73,96	277	26,04	1064
Total:	14949	74,90	5010	25,10	19959

Analysis of the table data allowed us to draw the following conclusions:

1. the uniformity of hybrids in the second generation is not observed: some hybrids carry one (dominant) trait, some carry another (recessive) trait from an alternative pair;
2. the number of hybrids carrying a dominant trait is approximately three times greater than that of hybrids carrying a recessive trait;
3. the recessive trait in hybrids of the first generation does not disappear, but is only suppressed and manifests itself in the second hybrid generation.

The phenomenon in which part of the second generation hybrids bears a dominant trait, and part - a recessive one, is called splitting... Moreover, the splitting observed in hybrids is not accidental, but obeys certain quantitative laws. On the basis of this, Mendel made another conclusion: when crossing first-generation hybrids in the offspring, traits are split in a certain numerical ratio.

When monohybrid crossing of heterozygous individuals in hybrids there is a phenotype splitting in the ratio of 3: 1, in the genotype 1: 2: 1.

Genetic scheme of Mendel's law of splitting

(A - yellow color of peas, and - green color of peas):

Gamete Purity Law

From 1854, for eight years, Mendel conducted experiments on crossing pea plants. He found that as a result of crossing different varieties of peas with each other, hybrids of the first generation have the same phenotype, and in hybrids of the second generation, there is a splitting of traits in certain proportions. To explain this phenomenon, Mendel made a number of assumptions, which are called the "hypothesis of the purity of gametes", or "the law of the purity of gametes." Mendel suggested that:

1. some discrete hereditary factors are responsible for the formation of signs;
2. organisms contain two factors that determine the development of a trait;
3. during the formation of gametes, only one of a pair of factors falls into each of them;
4. when male and female gametes merge, these hereditary factors do not mix (remain pure).

In 1909 W. Johansen will call these hereditary factors genes, and in 1912 T. Morgan will show that they are located in chromosomes.

To prove his assumptions G. Mendel used crossing, which is now called analyzing ( analyzing cross - crossing of an organism with an unknown genotype with an organism homozygous for the recessive). Probably Mendel reasoned as follows: "If my assumptions are correct, then as a result of crossing F 1 with a cultivar with a recessive trait (green peas), among the hybrids there will be half green peas and half yellow peas." As can be seen from the genetic scheme below, he really received a 1: 1 split and was convinced of the correctness of his assumptions and conclusions, but he was not understood by his contemporaries. His report "Experiments on plant hybrids", made at a meeting of the Brunn Society of Naturalists, was met with complete silence.

Cytological foundations of the first and second laws of Mendel

At the time of Mendel, the structure and development of germ cells was not studied, therefore his hypothesis of the purity of gametes is an example of a brilliant foresight, which later found scientific confirmation.

The phenomena of dominance and splitting of characters observed by Mendel are currently explained by the pairing of chromosomes, the divergence of chromosomes during meiosis and their unification during fertilization. Let's denote the gene that determines the yellow color by the letter A, and the green one by a. Since Mendel worked with pure lines, both crossed organisms are homozygous, that is, they carry two identical alleles of the seed color gene (AA and aa, respectively). During meiosis, the number of chromosomes is halved, and only one chromosome from a pair gets into each gamete. Since homologous chromosomes carry the same alleles, all gametes of one organism will contain a chromosome with the A gene, and the other with the a gene.

During fertilization, the male and female gametes merge, and their chromosomes are combined in one zygote. The hybrid resulting from crossing becomes heterozygous, since its cells will have the Aa genotype; one variant of the genotype will give one variant of the phenotype - the yellow color of the peas.

In a hybrid organism with the Aa genotype during meiosis, the chromosomes diverge into different cells and two types of gametes are formed - half of the gametes will carry the A gene, the other half will carry the a gene. Fertilization is a random and equally probable process, that is, any sperm can fertilize any egg. Since there are two types of sperm and two types of oocytes, four variants of zygotes are possible. Half of them are heterozygotes (carry genes A and a), 1/4 are homozygotes for the dominant trait (carry two genes A) and 1/4 are homozygotes for the recessive trait (carry two genes a). Dominant homozygotes and heterozygotes will produce peas yellow color (3/4), recessive homozygotes - green (1/4).

The law of independent combination (inheritance) of traits, or Mendel's third law

Organisms differ from each other in many ways. Therefore, having established the patterns of inheritance of one pair of traits, G. Mendel proceeded to study the inheritance of two (or more) pairs of alternative traits. For the dihybrid crossing, Mendel took homozygous pea plants that differ in seed color (yellow and green) and seed shape (smooth and wrinkled). Yellow color (A) and smooth shape (B) of seeds are dominant characters, green color (a) and wrinkled shape (b) are recessive characters.

By crossing a plant with yellow and smooth seeds with a plant with green and wrinkled seeds, Mendel obtained a uniform hybrid F1 generation with yellow and smooth seeds. From self-pollination of 15 hybrids of the first generation, 556 seeds were obtained, of which 315 are yellow smooth, 101 yellow wrinkled, 108 green smooth and 32 green wrinkled (splitting 9: 3: 3: 1).

Analyzing the resulting offspring, Mendel drew attention to the fact that: 1) along with the combinations of traits of the original varieties (yellow smooth and green wrinkled seeds), with dihybrid crossing new combinations of traits appear (yellow wrinkled and green smooth seeds); 2) splitting for each individual trait corresponds to splitting in monohybrid crossing. Of the 556 seeds, 423 were smooth and 133 wrinkled (ratio 3: 1), 416 seeds were yellow, and 140 were green (ratio 3: 1). Mendel came to the conclusion that splitting in one pair of traits is not associated with splitting in another pair. The seeds of hybrids are characterized not only by combinations of traits of parent plants (yellow smooth seeds and green wrinkled seeds), but also the emergence of new combinations of traits (yellow wrinkled seeds and green smooth seeds).

With a dihybrid crossing of diheterozygotes in hybrids, there is a splitting according to the phenotype in the ratio of 9: 3: 3: 1, according to the genotype in the ratio of 4: 2: 2: 2: 2: 1: 1: 1: 1, the traits are inherited independently of each other and combined in all possible combinations.

R	♀АABB yellow, smooth	×	♂aab green, wrinkled
Gamete types	AB		ab
F 1	AaBb yellow, smooth, 100%
P	♀АaBb yellow, smooth	×	♂AаBb yellow, smooth
Gamete types	AB Ab aB ab		AB Ab aB ab

Genetic scheme of the law of independent combination of traits:

Gametes:	♂	AB	Ab	aB	ab
♀
AB		AABB yellow smooth	AABb yellow smooth	AaBB yellow smooth	AaBb yellow smooth
Ab		AABb yellow smooth	AАbb yellow wrinkled	AaBb yellow smooth	Aabb yellow wrinkled
aB		AaBB yellow smooth	AaBb yellow smooth	aaBB green smooth	aaBb green smooth
ab		AaBb yellow smooth	Aabb yellow wrinkled	aaBb green smooth	aabb green wrinkled

Analysis of the results of crossing by phenotype: yellow, smooth - 9/16, yellow, wrinkled - 3/16, green, smooth - 3/16, green, wrinkled - 1/16. Cleavage phenotype 9: 3: 3: 1.

Analysis of the results of crossing by genotype: AaBb - 4/16, AABb - 2/16, AaBB - 2/16, Aabb - 2/16, aaBb - 2/16, ААBB - 1/16, Aabb - 1/16, aaBB - 1/16, aabb - 1/16. Splitting by genotype 4: 2: 2: 2: 2: 1: 1: 1: 1.

If during monohybrid crossing the parental organisms differ in one pair of traits (yellow and green seeds) and give in the second generation two phenotypes (2 1) in the ratio (3 + 1) 1, then with dihybrid crossing they differ in two pairs of traits and give in the second generation four phenotypes (2 2) in the ratio (3 + 1) 2. It is easy to calculate how many phenotypes and in what ratio will be formed in the second generation during trihybrid crossing: eight phenotypes (2 3) in a ratio (3 + 1) 3.

If the genotype splitting in F 2 with a monohybrid generation was 1: 2: 1, that is, there were three different genotypes (3 1), then with a dihybrid one 9 different genotypes are formed - 3 2, with a trihybrid crossing 3 3 - 27 different genotypes are formed.

Mendel's third law is valid only for those cases when the genes of the analyzed characters are in different pairs of homologous chromosomes.

Cytological foundations of Mendel's third law

Let A be the gene that determines the development of yellow seed color, a - green color, B - smooth seed form, b - wrinkled. Hybrids of the first generation with the AaBb genotype are crossed. When gametes are formed from each pair of allelic genes, only one gets into the gamete, while as a result of a random divergence of chromosomes in the first division of meiosis, gene A can get into the same gamete with gene B or with gene b, and gene a - with gene B or with gene b. Thus, each organism forms four varieties of gametes in the same amount (25% each): AB, Ab, aB, ab. During fertilization, each of the four types of sperm can fertilize any of the four types of eggs. As a result of fertilization, nine genotypic classes may appear, which will give four phenotypic classes.

Go to lectures No. 16 "Ontogenesis of sexually reproducing multicellular animals"

Go to lectures number 18 "Chained inheritance"

The main patterns of inheritance were discovered by G. Mendel on peas. He carried out intraspecific crosses of forms differing in a single number of characters and having alternative (contrasting) manifestations. Among the traits he used were the color of seeds, flowers and beans, the shape of seeds and beans, the arrangement of flowers, the height of plants. Initially, a hybridological analysis of pea forms that differed in one characteristic was carried out. Crosses in which parental forms are involved that differ in the manifestations of one trait are called monohybrid.

When two original forms belonging to pure lines are crossed, in the first daughter generation, as a rule, the appearance of offspring of the same phenotype is observed. This pattern is known as the first generation law of uniformity. F 1 hybrids can have a manifestation of the trait of one of the parents, and an expression intermediate between the original forms. In this case, if the differences in parental forms are determined by one gene (monogenic), the record of crossing is as follows: P AA x aa → F 1 Aa. This means that gene A is responsible for the manifestation of this trait, which exists in two different states - A and a. These alternative gene states are called alleles.

Analyzing the results of monohybrid crosses, G. Mendel established a rule (sometimes called the law) of gamete purity. It implies that any gamete of any organism carries one allele of each gene, the alleles in them are not mixed. This means that in individuals of the AA genotype, gametes of the same species are formed - A, in individuals of the aa genotype - also of the same type - a. Such individuals, which form gametes of only one variety (at least for the gene that is in the focus of attention), are homozygous (or homozygous). Thus, it is not difficult to make sure that pure lines consist of homozygous individuals. Px hybrids of genotype Aa form gametes of two varieties - A and a, each of which is "pure" with respect to allele A or a. Such individuals (or genotypes) that form gametes of several species are called heterozygous (or heterozygotes). The law of uniformity of the first generation hybrids is based on the mechanism of chromosome divergence in meiosis. Each of the alleles lies in its own chromosome (or chromatid), and when chromosomes diverge (in the first division of meiosis), and then chromatids (in the second division of meiosis), along with them, one of the corresponding alleles leaves the haploid cells. Thus, the law of uniformity of the first generation hybrids is a consequence of the fundamental rule of gamete purity, which determines other laws of inheritance.

Alleles of a single gene interact with each other in different ways. If the heterozygote Aa exhibits a phenotypic expression of the trait, which is the same as in individuals of the AA genotype, then allele A completely dominates over a, then AA individuals have a dominant manifestation of the trait, and homozygotes for a are recessive. This is another rule of Mendelism - the rule of domination. If the heterozygote has a manifestation of a trait intermediate between the two parental forms (for example, when crossing night beauty plants with red and white flowers, hybrids with a pink corolla are formed), then we are talking about incomplete dominance.

Sometimes heterozygotes show signs of both parents - this is the absence of dominance, or co-dominance.

Splitting law in monohybrid crossing

A monohybrid crossing is a cross in which the original forms differ in one trait. When crossing hybrids of the first generation, obtained from crossing homozygous forms, a split into 3/4 individuals with a dominant manifestation of the trait and 1/4 with a recessive manifestation of a trait is found.

In the second generation, obtained by crossing P1 hybrids with each other, two phenotypic classes appear in a strictly defined ratio. This is splitting, which is understood as the presence of several phenotypes in the offspring in specific numerical ratios.

First generation hybrids can interbreed not only with their own kind. If a heterozygous P1 individual is crossed with an organism homozygous for the recessive allele of the gene in question, then a splitting is obtained: Aa x aa → 1/2 Aa: 1/2 aa.

This crossing is called the analyzing one. In the analyzing crossing, it is not difficult to establish the types of gametes formed by a heterozygous individual and their numerical ratio, it is easy to determine which organisms are heterozygous and which are homozygous for the trait of interest to us.

The law of splitting in monohybrid crossing is also read in the reverse order: if, when crossing two individuals, one of the splits discussed above is obtained (in P2 - 3: 1, 1: 2: 1, 2: 1, and in analyzing crossing - 1: 1), then the original parental forms differ in the alleles of one gene, that is, there is a difference between them in one gene (monogenic difference in the original forms).

Independent Inheritance Law in Dihybrid Crossing

A dihybrid crossing is a crossing in which the original forms differ in two ways. For each of the traits, the parental forms differ in one gene (according to trait A - according to gene A, according to trait B - according to gene B). When crossing F 1 hybrids obtained from a dihybrid crossing, a splitting according to the phenotype is observed: 9/16 A-B-: 3/16 A-bb: 3/16 aaB-: 3/16 aabb.

In this case, the characters are inherited independently of each other, and for each of them there is a 3/4: 1/4 split.

This splitting is easily obtained as a combined one, combining two monohybrid ones (in the second generation of each of which a 3: 1 splitting is observed), with one gene responsible for each trait:

(3/4 A- + 1/4 aa) x (3/4 B- + 1/4 bb) \u003d 9/16 A-B- + 3/16 A-bb + 3/16 aaB- + 1/16 aabb.

In the analyzing cross, a split of 1: 1: 1: 1 is obtained similarly.

The implementation of this law is determined by the independent nature of the divergence of chromosomes of non-homologous pairs in meiosis, as well as by the fact that genes A and B are located in different (non-homologous) chromosomes. Independent divergence of chromosomes in meiosis leads to the emergence of new combinations of genes and traits that were not present in parental organisms - recombinants appear in the offspring (individuals carrying recombined combinations of traits).

Splits are also obtained in polyhybrid crosses (crosses in which the parental forms differ in several or many characteristics).

All the laws of inheritance by G. Mendel illustrate his postulated point of view about the discrete nature of inheritance: it is not the trait itself that is inherited, but the material factors that determine it. These factors are genes.

Gene interaction

Some traits are determined not by one gene, but by the simultaneous action of several. In such cases, there is undoubtedly a change and complication of the splitting formulas and methods of analysis. Genes that affect the development of one trait are called interacting. There are several types of such interaction of genes: complementary, epistatic, polymeric.

The dominant alleles of both genes lead to the formation of a new manifestation of the trait, mutually complementing each other (complementing). If the genotype contains only recessive alleles of both genes, then the trait does not appear. Biochemical analysis complements this scheme. The color of the eyes in Drosophila is caused by two pigments (bright red and brown), each of which is formed in a separate biosynthetic chain. The recessive allele "b" in homozygotes interrupts the synthesis of a bright red pigment - in such individuals the eyes are brown, allele "a" disrupts the synthesis of brown pigment - in homozygotes aa, the eyes have a bright red color, in individuals "A-B-" both pigments, causing the dark red color of the eyes, and the homozygotes for both genes "aabb" have no dyes in the eyes at all - the eyes are colorless (white).

Gene interactions (or non-allelic gene interactions) result in digen-type cleavages. In addition to the case discussed above, splits can be observed in the second generation: 9: 7, 9: 6: 1, 9: 3: 4, 12: 3: 1, 13: 3, 15: 1.

Conditions for the implementation of laws of inheritance

The patterns of inheritance of traits discussed above are fulfilled only if certain conditions are met. It is necessary that all types of gametes are formed with equal probability, have the same viability and participate in fertilization with the same efficiency, forming all types of zygotes with the same frequency, while zygotes should be characterized by equal viability. The severity of the trait should also be unchanged. Failure to meet at least one of these conditions leads to distortion of the splittings.

For example, if in a monohybrid crossing, in which there is a splitting in F 2 1/4 AA: 2/4 Aa: 1/4 aa, selective death of zygotes of the AA genotype is observed, then the phenotypic splitting will look like 2/3 Aa: 1/3 aa.

It should be noted that even if the above conditions are met, the actual splitting does not always exactly match the theoretically calculated one. The fact is that the laws of inheritance, discovered by Mendel, are manifested in a fairly large amount of statistical material. For their accurate implementation, it is necessary to analyze a sample of a certain size. Thus, the patterns of inheritance are biological in nature, but have a statistical character of manifestation.

1. Features of the method of hybridological analysis. Mendel's laws.
2. Types of gene interaction.
3. Linked inheritance of traits.
4. 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 series 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 for the experiment varieties (clean lines), 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 differing 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 the corresponding traits are passed on to the 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 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 the normal manifestation of a 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 the same 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 passed 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.

People have always been interested in the patterns of inheritance of traits. Why are children like their parents? Is there a risk of transmission of hereditary diseases? These and many other questions remained under a veil of secrecy until the 19th century. It was then that Mendel managed to accumulate all the accumulated knowledge on this topic, and also through complex analytical experiments to establish specific patterns.

Mendel's contribution to the development of genetics

The main patterns of inheritance of traits are the principles according to which certain characteristics are transmitted from parental organisms to offspring. Their discovery and clear formulation is the merit of Gregor Mendel, who conducted numerous experiments on this issue.

The main achievement of the scientist is the proof of the discrete nature of hereditary factors. In other words, a specific gene is responsible for each trait. The first maps were built for maize and fruit flies. The latter is a classic object for carrying out genetic experiments.

Mendel's merits can hardly be overestimated, as Russian scientists speak about. Thus, the famous geneticist Timofeev-Resovsky noted that Mendel was the first to conduct fundamental experiments and give an accurate description of the phenomena that previously existed at the level of hypotheses. Thus, he can be considered a pioneer of mathematical thinking in the fields of biology and genetics.

Predecessors

It should be noted that the regularities of the inheritance of traits according to Mendel were not formulated out of nowhere. His research was based on the research of his predecessors. The following scientists should be especially noted:

• J. Goss carried out experiments on peas, crossing plants with fruits of different colors. It was thanks to these studies that the laws of uniformity of the first generation of hybrids, as well as incomplete dominance, were discovered. Mendel only specified and confirmed this hypothesis.
• Augustin Sarget is a plant breeder who chose pumpkin crops for his experiments. He was the first to study hereditary traits not in aggregate, but separately. He owns the statement that when transferring certain characteristics, they do not mix with each other. Thus, heredity is constant.
• Noden conducted research on various species of the plant such as datura. After analyzing the results obtained, he considered it necessary to talk about the presence of dominant features, which in most cases will prevail.

Thus, by the 19th century, such phenomena as dominance, uniformity of the first generation, as well as combinatorics of traits in subsequent hybrids were known. Nevertheless, no general laws have been worked out. It is the analysis of the available information and the development of reliable research methods that are Mendel's main merit.

Mendel's method of work

The regularities of the inheritance of traits according to Mendel were formulated as a result of fundamental research. The scientist's activities were carried out as follows:

• were not considered in aggregate, but separately;
• only alternative characters were selected for analysis, which represent a significant difference between varieties (this is what made it possible to most clearly explain the patterns of the inheritance process);
• the research was fundamental (Mendel studied a large number of pea varieties, which were both pure and hybrid, and then crossed "offspring"), which made it possible to speak about the objectivity of the results;
• the use of accurate quantitative methods in the analysis of the data obtained (using knowledge in the field of probability theory, Mendel reduced the indicator of random deviations).

The law of uniformity of hybrids

Considering the patterns of inheritance of traits, it is worth paying special attention to the uniformity of the first generation hybrids. It was discovered by means of an experiment during which parental forms were crossed with one contrasting feature (shape, color, etc.).

Mendel decided to conduct an experiment on two varieties of peas - with red and white flowers. As a result, the first generation hybrids got purple inflorescences. Thus, there was a reason to talk about the presence of dominant and recessive traits.

It should be noted that Mendel's experience was not the only one. He used plants with different shades of inflorescences, with different shapes of fruits, different stem heights and other options for experiments. Empirically, he was able to prove that all first-order hybrids are uniform and characterized by a dominant trait.

Incomplete dominance

In the course of studying such a question as the regularities of the inheritance of traits, experiments were carried out both on plants and on living organisms. Thus, it was possible to establish that the signs are not always in relationships and suppression. So, for example, when crossing black and white chickens, it was possible to get gray offspring. It was the same with some plants, when varieties with purple and white flowers produced pink hues at the output. Thus, it is possible to correct the first principle, indicating that the first generation of hybrids will have the same traits, while they may be intermediate.

Splitting traits

Continuing to study the patterns of inheritance of traits, Mendel considered it necessary to interbreed two descendants of the first generation (heterozygous). As a result, offspring were obtained, some of which bore and the other - recessive. From this we can conclude that the secondary trait in the first generation of hybrids does not disappear at all, but is only suppressed and may well manifest itself in the subsequent offspring.

Independent inheritance

The patterns of inheritance of traits raise many questions. Mendel's experiments also affected individuals that differ from each other in several ways. For each separately, the previous regularities were observed. But now, considering the totality of features, it was not possible to identify any regularities between their combinations. Thus, there is reason to talk about the independence of inheritance.

Gamete Purity Law

Some of the patterns of inheritance of traits established by Mendel were purely hypothetical. We are talking about the law of the purity of gametes, which consists in the fact that only one allele from the pair contained in the gene of the parent individual gets into them.

At the time of Mendel, there was no technical means to confirm this hypothesis. Nevertheless, the scientist managed to formulate a general statement. Its essence lies in the fact that in the process of the formation of hybrids, hereditary traits are preserved unchanged, and not mixed.

Essential conditions

Genetics is the science that studies the patterns of inheritance of traits. Mendel made a significant contribution to its development, developing fundamental provisions on this issue. However, in order for them to be fulfilled, the following essential conditions must be met:

• the original forms must be homozygous;
• alternativeness of signs;
• the same probability of the formation of different alleles in a hybrid;
• equal viability of gametes;
• during fertilization, the gametes are combined in a random way;
• zygotes with different combinations of genes are equally viable;
• the number of individuals of the second generation should be sufficient to consider the results obtained as natural;
• the manifestation of signs should not be dependent on the influence of external conditions.

It should be noted that most living organisms, including humans, correspond to these characteristics.

Patterns of inheritance of traits in humans

Despite the fact that initially genetic principles were studied on the example of plants, they are also valid for animals and humans. It is worth noting the following types of inheritance:

• Autosomal dominant - inheritance of dominant traits that are localized through autosomes. In this case, the phenotype can be either strongly expressed or barely noticeable. With this type of inheritance, the probability of a child receiving a pathological allele from a parent is 50%.
• Autosomal recessive - inheritance of minor traits connected to autosomes. Diseases are manifested through homozygotes, with both alleles affected.
• The dominant X-linked type implies the transmission of dominant traits by deterministic genes. At the same time, diseases in women are 2 times more common than in men.
• Recessive X-linked type - inheritance occurs according to a weaker trait. The disease or its individual signs always appear in male offspring, and in women - only in a homozygous state.

Basic concepts

In order to understand how the patterns of inheritance of Mendel's traits and other genetic processes work, it is worth familiarizing yourself with the basic definitions and concepts. These include the following:

• A dominant trait is a predominant characteristic that acts as a determining state of a gene and suppresses the development of recessive ones.
• A recessive trait is a characteristic that is inherited, but does not act as a determining one.
• A homozygote is a diploid individual or cell, the chromosomes of which contain the same cells of the specified gene.
• Heterozygote - a diploid individual or cell that cleaves and has different alleles within the same gene.
• An allele is one of the alternative forms of a gene that is located at a specific place on the chromosome and is characterized by a unique nucleotide sequence.
• An allele is a pair of genes that are located in the same areas and control the development of certain traits.
• are located on different parts of chromosomes and are responsible for the manifestation of various signs.

Conclusion

Mendel formulated and in practice proved the basic laws of the inheritance of traits. Their description is given on the example of plants and is slightly simplified. But in practice, it is true for all living organisms.