Crossbreeding:
1. Monohybrid. Observation is carried out only on one basis, i.e. alleles of one gene are tracked.
2. Dihybrid. The observation is carried out on two grounds, i.e. the alleles of two genes are tracked.
Genetic designations:
P - parents; F - offspring, the number indicates the ordinal number of the generation, F1, F2.
X - crossing icon, males, females; A, a, B, b, C, c - separately taken hereditary traits. A, B, C - dominant alleles of the gene, and, b, c - recessive alleles of the gene. Aa -, heterozygote; aa - recessive homozygote, AA - dominant homozygote.
Monohybrid crossing.
A classic example of a monohybrid crossing is a cross between varieties with yellow and green seeds: all offspring had yellow seeds. Mendel came to the conclusion that in the hybrid of the first generation, out of each pair of alternative traits, only one appears - dominant, and the second - recessive - does not develop, as if disappears.
R AA * aa - parents (clean lines)
A, a - parents
Aa - the first generation of hybrids
This pattern was called the law of uniformity of first-generation hybrids or the law of dominance. This is Mendel's first law: when crossing two organisms belonging to different pure lines (two organisms), 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.
Mendel's second law
The seeds of the first generation hybrids were used by Mendel to obtain the second generation. When crossing, there is a splitting of traits in a certain numerical ratio. Some of the hybrids are dominant, some are recessive.
F1 Aa * Aa A, a, A, a F2 AA (0.25); Aa (0.25); Aa (0.25); aa (0.25)
In the offspring, a splitting of traits occurs in a ratio of 3: 1.
To explain the phenomena of dominance and splitting, Mendel proposed a hypothesis of gamete purity: hereditary factors during the formation of hybrids do not mix, but remain unchanged.
Mendel's second law can be formulated: when two descendants of the first generation are crossed with each other (two heterozygous individuals) in the second generation, splitting is observed in a certain numerical ratio: by phenotype 3: 1, by - 1: 2: 1.
Mendel's third law: with dihybrid crossing in hybrids of the second generation, each pair of contrasting traits is inherited independently of the others and gives different combinations with them. The law is valid only in those cases when the analyzed features are not linked to each other, i.e. are located in non-homologous chromosomes.
Consider Mendel's experiment in which he studied the independent inheritance of traits in peas. One of the plants crossed had smooth, yellow seeds, while the other was wrinkled and green. In the first generation of hybrids, the plants had smooth and yellow seeds. In the second generation, the phenotype 9: 3: 3: 1 was split.
Mendel's third law is formulated as follows: splitting for each pair of genes occurs independently of other pairs of genes.
This law states that crossing individuals that differ in this trait (homozygous for different alleles) gives genetically homogeneous offspring (generation F1), all individuals of which are heterozygous. All F 1 hybrids can have either the phenotype of one of the parents (complete dominance), as in Mendel's experiments, or, as was discovered later, an intermediate phenotype (incomplete dominance). Later it turned out that hybrids of the first generation F1, can show signs of both parents (codominance). This law is based on the fact that when two forms homozygous for different alleles (AA and aa) are crossed, all their descendants are the same in genotype (heterozygous - Aa), and therefore in phenotype.
2.3 Splitting Law (Mendel's second law)
This law is called the law of (independent) splitting. Its essence is as follows. When sex cells - gametes - are formed in an organism heterozygous for the trait under study, then one half of them carries one allele of a given gene, and the other half carries another. Therefore, when such F 1 hybrids are crossed among themselves, among the second generation F2 hybrids, individuals with phenotypes of both the original parental forms and F 1 appear in certain ratios.
This law is based on the regular behavior of a pair of homologous chromosomes (with alleles A and a), which ensures the formation of gametes of two types in F1 hybrids, as a result of which individuals of three possible genotypes are identified among F2 hybrids in the ratio 1AA: 2 Aa: 1aa. In other words, the "grandchildren" of the original forms - two homozygotes phenotypically different from each other - split by phenotype in accordance with Mendel's second law.
However, this ratio can change depending on the type of inheritance. So, in the case of complete dominance, 75% of individuals with a dominant and 25% with a recessive trait stand out, i.e. two phenotypes in a ratio of 3: 1. With incomplete dominance and codominance, 50% of the second generation hybrids (F2) have the phenotype of the first generation hybrids and 25% each have the phenotypes of the original parental forms, i.e. a splitting of 1: 2: 1 is observed.
2.4. The law of independent combination (inheritance) of traits (Mendel's third law)
This law says that each pair of alternative traits behaves independently of each other in a series of generations, as a result of which among the descendants of the first generation (i.e., in the F2 generation), individuals with new ones appear in a certain ratio (in comparison with the parental ones) combinations of features. For example, in the case of complete dominance during crossing of the original forms differing in two characteristics, in the next generation (F2) individuals with four phenotypes in a ratio of 9: 3: 3: 1 are revealed. Moreover, two phenotypes have “parental” combinations of traits, and the remaining two are new. This law is based on the independent behavior (splitting) of several pairs of homologous chromosomes. So, with a dihybrid crossing, this leads to the formation of 4 types of gametes in the first generation (F1) hybrids (AB, AB, aB, AB), and after the formation of zygotes - to a regular splitting by genotype and, accordingly, by phenotype in the next generation ( F2).
Paradoxically, in modern science, much attention is paid not so much to the very third Mendel's law in its original formulation, but to exceptions from it. The law of independent combination is not observed if the genes that control the studied traits are linked, i.e. are located adjacent to each other on the same chromosome and are inherited as a connected pair of elements, and not as separate elements. Mendel's scientific intuition told him which features should be chosen for his dihybrid experiments - he chose unlinked features. If he randomly chose traits controlled by linked genes, then his results would be different, since linked traits are not inherited independently of each other.
What is the reason for the importance of exceptions to Mendel's law of independent combination? The fact is that it is these exceptions that make it possible to determine the chromosomal coordinates of genes (the so-called locus).
In cases where the heritability of a particular pair of genes does not obey Mendel's third law, most likely these genes are inherited together and, therefore, are located on the chromosome in close proximity to each other. Dependent gene inheritance is called linkage, and the statistical method used to analyze this inheritance is called linkage. However, under certain conditions, the patterns of inheritance of linked genes are violated. The main reason for these disorders is the phenomenon of crossing over, leading to recombination (recombination) of genes. The biological basis of recombination lies in the fact that in the process of gamete formation, homologous chromosomes, before separating, exchange their regions.
Crossing over is a probabilistic process, and the probability of whether or not a chromosome break occurs in a given specific area is determined by a number of factors, in particular, the physical distance between two loci of the same chromosome. Crossing over can also occur between neighboring loci, but its probability is much less than the probability of rupture (leading to the exchange of regions) between loci with a large distance between them.
This regularity is used when compiling genetic maps of chromosomes (mapping). The distance between two loci is estimated by counting the number of recombinations per 100 gametes. This distance is considered a unit of measurement of the length of a gene and is called Sentimorgan in honor of the geneticist T. Morgan, who first described the groups of linked genes in the fruit fly Drosophila, a favorite object of geneticists. If two loci are located at a considerable distance from each other, then the gap between them will occur as often as when these loci are located on different chromosomes.
Using the patterns of reorganization of genetic material during recombination, scientists have developed a statistical method of analysis called linkage analysis.
Mendel's laws in their classical form are subject to certain conditions. These include:
1) homozygosity of the original crossed forms;
2) the formation of gametes of hybrids of all possible types in equal proportions (ensured by the correct course of meiosis; the same viability of gametes of all types; equal probability of meeting any gametes during fertilization);
3) the same viability of all types of zygotes.
Violation of these conditions can lead either to the absence of splitting in the second generation, or to splitting in the first generation; or to distortion of the ratio of different genotypes and phenotypes. Mendel's laws are universal for all sexually reproducing diploid organisms. In general, they are valid for autosomal genes with full penetrance (i.e. 100% frequency of manifestation of the analyzed trait; 100% penetrance implies that the trait is expressed in all carriers of the allele that determines the development of this trait) and constant expressiveness (i.e. constant the severity of the sign); constant expressiveness implies that the phenotypic severity of a trait is the same or approximately the same in all carriers of the allele that determines the development of this trait.
Knowledge and application of Mendel's laws is of great importance in medical and genetic counseling and determination of the genotype of phenotypically "healthy" people whose relatives suffered from hereditary diseases, as well as in determining the degree of risk of developing these diseases in relatives of patients.
Question 1. Formulate Mendel's third law. Why is it called the law of independent inheritance?
The law of independent inheritance (Mendel's third law) - when crossing two homozygous individuals that differ 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 (as in monohybrid crossing) .
Question 2. For which allelic pairs is Mendel's third law valid?
According to Mendel's third law, it follows that the genes that determine the traits must be in different pairs of chromosomes.
Question 3. What is analyzing cross?
Analyzing crossing - crossing of a hybrid individual with an individual homozygous for recessive alleles, that is, an "analyzer". The meaning of the analyzing crossing lies in the fact that the descendants from the analyzing crossing necessarily carry one recessive allele from the "analyzer", against which the alleles obtained from the analyzed organism should appear. For analyzing crosses (excluding cases of gene interaction), phenotypic cleavage coincides with genotype cleavage among the offspring. Thus, the analyzing crossing makes it possible to determine the genotype and the ratio of gametes of different types formed by the analyzed individual.
Question 4. What will be the splitting in the analyzing cross, if the studied individual with a dominant phenotype has the AABb genotype?
This cross can be represented as follows:
offspring: AaBb Aabb
Splitting 1: 1
Question 5. How many types of gametes are formed in an individual with the genotype AaBBCcDdffEe?
The number of gametes depends on the number of heterozygous alleles in the parent (0-1; 1-2; 2-4; 3-8).
There are four heterozygotes in the AaBBCcDdffEe genotype of alleles, which means there will be 16 species of gametes.
Question 6. Discuss in class whether it can be argued that Mendel's laws are universal, that is, they are valid for all sexually reproducing organisms.
The laws discovered by Gregor Mendel are not always applicable in genetics. There are many conditions for compliance with Mendel's laws. For such cases, there are other laws (for example: Morgan's law), or explanations.
Let's formulate the basic conditions for compliance with the laws of inheritance.
To comply with the law of uniformity of first generation hybrids, it is necessary that:
The parental organisms were homozygous;
The genes of different alleles were located in different chromosomes, and not in one (otherwise, the phenomenon of "linked inheritance" may occur).
The law of splitting will be observed if the hereditary factors in hybrids remain unchanged.
The law of independent distribution of genes in offspring and the emergence of various combinations of these genes during dihybrid crossing is possible only if pairs of allelic genes are located in different pairs of homologous chromosomes.
Violation of these conditions can lead either to the absence of splitting in the second generation, or to splitting in the first generation; or to distortion of the ratio of different genotypes and phenotypes. Mendel's laws are universal for all sexually reproducing diploid organisms. In general, they are valid for autosomal genes with full penetrance (100% frequency of manifestation of the analyzed trait; 100% penetrance implies that the trait is expressed in all carriers of the allele that determines the development of this trait) and constant expressivity; constant expressiveness implies that the phenotypic severity of a trait is the same or approximately the same in all carriers of the allele that determines the development of this trait.
Mendel's laws - these are the principles of the transmission of hereditary traits from parents to descendants, named after their discoverer. Explanations of scientific terms - in.
Mendel's laws are valid only for monogenic traits, that is, traits, each of which is determined by one gene. Those traits that are influenced by two or more genes are inherited according to more complex rules.
Law of uniformity for first generation hybrids (first Mendel's law) (another name is the law of dominance of traits): when crossing two homozygous organisms, one of which is homozygous for the dominant allele of a given gene, and the other for the recessive, all individuals of the first generation of hybrids (F1) will be the same according to the trait determined by this gene, and identical the parent that carries the dominant allele. All individuals of the first generation from such a crossing will be heterozygous.
Suppose we crossed a black cat and a brown cat. Black and brown are determined by alleles of the same gene, the black B allele dominates over the brown b allele. Crossbreeding can be written as BB (cat) x bb (cat). All kittens from this cross will be black and have the Bb genotype (Figure 1).
Note that the recessive trait (brown color) has not really disappeared anywhere, it is masked by the dominant trait and, as we will now see, will manifest itself in subsequent generations.
Splitting law (Mendel's second law): when two heterozygous offspring of the first generation are crossed with each other in the second generation (F2), the number of offspring identical in this trait to the dominant parent will be 3 times greater than the number of offspring identical to the recessive parent. In other words, the phenotypic cleavage in the second generation will be 3: 1 (3 phenotypically dominant: 1 phenotypically recessive). (splitting is the distribution of dominant and recessive traits among the offspring in a certain numerical ratio). By genotype, the cleavage will be 1: 2: 1 (1 homozygote for the dominant allele: 2 heterozygotes: 1 homozygote for the recessive allele).
This splitting occurs thanks to the principle that is called gamete purity law... The law of gamete purity states: only one allele from a pair of alleles of a given gene of a parent individual gets into each gamete (a reproductive cell - an egg or sperm). When gametes fuse during fertilization, they accidentally join, which leads to this splitting.
Returning to our example with cats, suppose your black kittens grew up, you did not track them, and two of them produced offspring - four kittens.
Both the cat and the cat are heterozygous for the color gene, they have the Bb genotype. Each of them, according to the law of gamete purity, produces gametes of two types - B and b. Their offspring will have 3 black kittens (BB and Bb) and 1 brown kitten (bb) (Fig. 2) (In fact, this pattern is statistical, therefore, splitting is performed on average, and such accuracy may not be observed in a real case).
For clarity, the results of the crossing in the figure are shown in the table corresponding to the so-called Pennett lattice (a diagram that allows you to quickly and clearly describe a specific crossing, which is often used by geneticists).
Independent inheritance law (Mendel's third law) - when crossing two homozygous individuals 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. crossing). The law of independent cleavage is fulfilled only for genes located in non-homologous chromosomes (for unlinked genes).
The key point here is that different genes (if they are not on the same chromosome) are inherited independently of each other. Let's continue our example from the life of cats. Coat length (gene L) and color (gene B) are inherited independently of each other (located on different chromosomes). Short coat (L allele) dominates long coat (l) and black (B) over brown b. Suppose we cross a short haired black cat (BB LL) with a long haired brown cat (bb ll).
In the first generation (F1) all kittens will be black and short-haired, and their genotype will be Bb Ll. However, the brown color and long-haired hair have not gone anywhere - the alleles that control them are simply “hidden” in the genotype of heterozygous animals! By crossing a cat and a cat from these offspring, in the second generation (F2) we will observe a split of 9: 3: 3: 1 (9 short-haired black, 3 long-haired black, 3 short-haired brown and 1 long-haired brown). Why this happens and what genotypes in these offspring are shown in the table.
In conclusion, we recall once again that segregation according to Mendel's laws is a statistical phenomenon and is observed only in the case of a sufficiently large number of animals and in the case when the alleles of the studied genes do not affect the viability of the offspring. If these conditions are not met, deviations from Mendelian ratios will be observed in the offspring.
Mendel's third law (independent inheritance of traits) - when crossing two homozygous individuals differing from each other in two or more pairs of alternative traits, genes and their corresponding traits are inherited independently of each other and combined in all possible combinations.
The law manifests itself, as a rule, for those pairs of traits whose genes are located outside the homologous chromosomes. If we denote by a letter the number of allelic pairs in non-homologous chromosomes, then the number of phenotypic classes will be determined by the formula 2n, and the number of genotypic classes - 3n. In case of incomplete dominance, the number of phenotypic and genotypic classes coincides.
Conditions for independent inheritance and combination of non-allelic genes.
Studying cleavage during dihybrid crossing, Mendel found that traits are inherited independently of each other. This pattern, known as the rule of independent combination of features, is formulated as follows: when crossing homozygous individuals differing in two (or more) pairs of alternative traits in the second generationF 2 ) there is an independent inheritance and combination of traits if the genes that determine them are located on different homologous chromosomes.This is possible, since during meiosis, the distribution (combination) of chromosomes in germ cells during their maturation proceeds independently, which can lead to the appearance of offspring carrying characters in combinations that are not characteristic of parental and grandparent individuals. Digheterozygotes marry for eye color and ability to better control the right hand (AaBb). During the formation of gametes, the allele ANDmay be in the same gamete as with an allele IN,so with allele b. Similarly, allele andcan fall into the same gamete or with an allele IN,either with the allele b. Therefore, in a diheterozygous individual, four possible combinations of genes in gametes are formed: AB, Ab, aB, ab. All types of gametes will be equally divided (25% each).
This is easy to explain by the behavior of chromosomes during meiosis. Non-homologous chromosomes in meiosis can be combined in any combination, therefore the chromosome carrying the allele AND,equally likely to move into the gamete as with the chromosome carrying the allele INand with the chromosome carrying the allele b. Likewise, the chromosome carrying the allele and,can be combined with both the chromosome carrying the allele IN,and with a chromosome carrying the b allele. So, a diheterozygous individual forms 4 types of gametes. Naturally, when these heterozygous individuals are crossed, any of the four types of gametes from one parent can be fertilized by any of the four types of gametes formed by the other parent, that is, 16 combinations are possible. The same number of combinations should be expected according to the laws of combinatorics.
When calculating the phenotypes recorded on the Punnett lattice, it turns out that out of 16 possible combinations in the second generation in 9, two dominant traits are realized (AB,in our example - brown-eyed right-handers), in 3, the first sign is dominant, the second is recessive (ANDb, in our example - brown-eyed left-handers), in 3 more - the first sign is recessive, the second is dominant (aB,i.e. blue-eyed right-handers), and in one - both signs are recessive (andb, in this case, a blue-eyed left-hander). Phenotypic cleavage occurred in a ratio of 9: 3: 3: 1.
If, during dignbride crossing in the second generation, one sequentially counts the obtained individuals for each trait separately until the result is the same as with monoghbride crossing, i.e. 3: 1.
In our example, when split according to the color of the eyes, the ratio is obtained: brown-eyed 12/16, blue-eyed 4/16, according to another criterion - right-handed 12/16, left-handed 4/16, that is, the known ratio of 3: 1.
The diheterozygote forms four types of gametes, therefore, when crossed with a recessive homozygote, four types of offspring are observed; at the same time, splitting both by phenotype and by genotype occurs in a ratio of 1: 1: 1: 1.
When calculating the phenotypes obtained in this case, a splitting is observed in the ratio of 27: 9: 9: 9:: 3: 3: 3: 1. This is a consequence of the fact that the signs we took into account: the ability to better control the right hand, eye color and the Rh factor is controlled by genes located on different chromosomes, and the likelihood of meeting the chromosome carrying the gene AND,with the chromosome carrying the gene INor R, depends entirely on chance, since the same chromosome with the gene ANDcould equally meet with the chromosome carrying the b gene or r .
In more general form, for any crosses, phenotype splitting occurs according to the formula (3 + 1) n, where p- the number of pairs of traits taken into account when crossing.
Cytological foundations and universality of Mendel's laws.
1) pairings of chromosomes (pairings of genes that determine the possibility of the development of any trait)
2) the peculiarities of meiosis (the processes occurring in meiosis, which provide an independent divergence of chromosomes with genes on them to different pluses of the cell, and then to different gametes)
3) features of the fertilization process (random combination of chromosomes carrying one gene from each allelic pair)
Mendelian signs of a person.
| Dominant traits Recessive traits | |
| Hair: dark curly not red | Hair: blonde straight red |
| Eyes: brown large | Eyes: small |
| Myopia | Normal vision |
| Long eyelashes | Short eyelashes |
| Aquiline nose | Straight nose |
| Loose earlobe | Adherent earlobe |
| Wide gap between incisors | Narrow gap between incisors or lack thereof |
| Full lips | Thin lips |
| Freckles | Lack of freckles |
| Six-fingered | Normal limb structure |
| Better right hand control | Better left hand control |
| The presence of pigment | Albinism |
| Rh factor positive | Rh factor negative |