Mendel's third law (independent inheritance of characters)– when crossing two homozygous individuals that differ from each other in two or more pairs of alternative traits, the genes and corresponding traits are inherited independently of each other and are 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 the number of allelic pairs in non-homologous chromosomes with a letter, then the number of phenotypic classes will be determined by the formula 2n, and the number of genotypic classes - 3n. With incomplete dominance, the number of phenotypic and genotypic classes coincides.

Conditions for independent inheritance and combination of non-allelic genes.

While studying segregation in dihybrid crosses, Mendel discovered that characters 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 characters in the second generationF 2 ) independent inheritance and combination of traits is observed 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 occurs independently, which can lead to the appearance of descendants carrying characteristics in combinations that are not characteristic of the parent and grandparent individuals. Diheterozygotes marry based on eye color and ability to better use their right hand (AaBb). During the formation of gametes, the allele A may appear in the same gamete as with an allele IN, same with the allele b. In the same way the allele A can end up in one gamete or with an allele IN, or with an allele b. Consequently, a diheterozygous individual produces four possible combinations of genes in gametes: AB, Ab, aB, ab. There will be an equal share of all types of gametes (25% each).

This is easy to explain by the behavior of chromosomes during meiosis. Non-homologous chromosomes during meiosis can be combined in any combination, so the chromosome carrying the allele A, can equally likely go into a gamete as with the chromosome carrying the allele IN and with the chromosome carrying the allele b. Likewise, the chromosome carrying the allele A, can be combined with both the chromosome carrying the allele IN, and with the chromosome carrying allele b. So, a diheterozygous individual produces 4 types of gametes. Naturally, when crossing these heterozygous individuals, any of the four types of gametes of one parent can be fertilized by any of the four types of gametes formed by the other parent, i.e. 16 combinations are possible. The same number of combinations should be expected according to the laws of combinatorics.

When counting the phenotypes recorded on the Punnett lattice, it turns out that out of 16 possible combinations in the second generation, 9 have two dominant traits (AB, in our example - brown-eyed right-handers), in 3 - the first sign is dominant, the second is recessive (Ab, in our example - brown-eyed left-handers), in another 3 - the first sign is recessive, the second is dominant (aB, i.e. blue-eyed right-handers), and in one - both traits are recessive (Ab, in this case - a blue-eyed left-hander). Phenotypic cleavage occurred in a ratio of 9:3:3:1.

If, during a single-bred crossing in the second generation, we sequentially count the resulting individuals for each characteristic separately until the result is the same as with a single-bred crossing, i.e. 3:1.

In our example, when splitting according to eye color, the ratio is obtained: brown-eyed 12/16, blue-eyed 4/16, according to another criterion - right-handed 12/16, left-handed 4/16, i.e. the well-known ratio of 3:1.

A diheterozygote produces four types of gametes, so when crossed with a recessive homozygote, four types of offspring are observed; in this case, splitting both by phenotype and 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 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 coloring and Rh factor is controlled by genes localized on different chromosomes, and the probability of meeting a chromosome carrying the gene A, with a chromosome carrying the gene IN or R, depends entirely on chance, since the same chromosome with a gene A could equally encounter a chromosome carrying the b gene or r .

In a more general form, for any crosses, phenotypic splitting occurs according to the formula (3 + 1) n, where P- the number of pairs of characteristics taken into account when crossing.

Cytological foundations and universality of Mendel's laws.

1) pairing of chromosomes (pairing of genes that determine the possibility of developing any trait)

2) features of meiosis (processes occurring in meiosis, which ensure the independent divergence of chromosomes with the genes located on them to different parts of the cell, and then into different gametes)

3) features of the fertilization process (random combination of chromosomes carrying one gene from each allelic pair)

Mendelian characteristics of man.

Dominant traits Recessive traits
Hair: dark curly not red	Hair: light straight red
Eyes: large brown	Eyes: small
Myopia	Normal vision
Eyelashes are long	Short eyelashes
Aquiline nose	Straight nose
Loose earlobe	fused earlobe
Wide gap between incisors	Narrow gap between the incisors or its absence
Full lips	Thin lips
Presence of freckles	No freckles
Six-fingered	Normal limb structure
Better right hand control	Better left hand control
Presence of pigment	Albinism
Positive Rh factor	Negative Rh factor

Czech explorer Gregor Mendel(1822-1884) considered founder of genetics, since he was the first, even before this science took shape, to formulate the basic laws of inheritance. Many scientists before Mendel, including the outstanding German hybridizer of the 18th century. I. Kelreuter noted that when crossing plants belonging to different varieties, great variability is observed in the hybrid offspring. However, no one was able to explain the complex splitting and, moreover, reduce it to precise formulas due to the lack of a scientific method of hybridological analysis.

It was thanks to the development of the hybridological method that Mendel managed to avoid the difficulties that had confused earlier researchers. G. Mendel reported on the results of his work in 1865 at a meeting of the Society of Natural Scientists in Brünn. The work itself, entitled “Experiments on Plant Hybrids,” was later published in the “Proceedings” of this society, but did not receive proper assessment from contemporaries and remained forgotten for 35 years.

As a monk, G. Mendel conducted his classical experiments on crossing different varieties of peas in the monastery garden in Brünn. He selected 22 pea varieties that had clear alternative differences in seven characteristics: seeds yellow and green, smooth and wrinkled, flowers red and white, plants tall and short, etc. An important condition of the hybridological method was the mandatory use of pure ones as parents, i.e. forms that do not split according to the studied characteristics.

A successful choice of object played a major role in the success of Mendel's research. Peas are self-pollinators. To obtain first-generation hybrids, Mendel castrated the flowers of the mother plant (removed the anthers) and artificially pollinated the pistils with the pollen of the male parent. When obtaining second-generation hybrids, this procedure was no longer necessary: ​​he simply left the F 1 hybrids to self-pollinate, which made the experiment less labor-intensive. Pea plants reproduced exclusively sexually, so that no deviations could distort the results of the experiment. And finally, in peas, Mendel discovered a sufficient number of pairs of brightly contrasting (alternative) and easily distinguishable pairs of characters for analysis.

Mendel began his analysis with the simplest type of crossing - monohybrid, in which the parent individuals differ in one pair of traits. The first pattern of inheritance discovered by Mendel was that all first-generation hybrids had the same phenotype and inherited the trait of one of the parents. Mendel called this trait dominant. An alternative trait of the other parent, which did not appear in hybrids, was called recessive. The discovered pattern was named I of Mendel's law, or the law of uniformity of hybrids of the 1st generation. During the analysis of the second generation, a second pattern was established: the splitting of hybrids into two phenotypic classes (with a dominant trait and with a recessive trait) in certain numerical ratios. By counting the number of individuals in each phenotypic class, Mendel established that splitting in a monohybrid cross corresponds to the formula 3: 1 (three plants with a dominant trait, one with a recessive trait). This pattern is called Mendel's II law, or law of segregation. Open patterns emerged in the analysis of all seven pairs of characteristics, on the basis of which the author came to the conclusion about their universality. When self-pollinating F 2 hybrids, Mendel obtained the following results. Plants with white flowers produced offspring with only white flowers. Plants with red flowers behaved differently. Only a third of them gave uniform offspring with red flowers. The offspring of the rest were split in the ratio of red and white colors in a ratio of 3: 1.

Below is a diagram of the inheritance of pea flower color, illustrating Mendel's I and II laws.

In an attempt to explain the cytological basis of open patterns, Mendel formulated the idea of ​​discrete hereditary inclinations contained in gametes and determining the development of paired alternative characters. Each gamete carries one hereditary deposit, i.e. is “pure”. After fertilization, the zygote receives two hereditary deposits (one from the mother, the other from the father), which do not mix and later, when gametes are formed by the hybrid, they also end up in different gametes. This hypothesis of Mendel was called the rule of “purity of gametes.” The combination of hereditary inclinations in the zygote determines what character the hybrid will have. Mendel denoted the inclination that determines the development of a dominant trait with a capital letter ( A), and recessive is capitalized ( A). Combination AA And Ahh in the zygote determines the development of a dominant trait in the hybrid. A recessive trait appears only when combined ahh.

In 1902, V. Betson proposed to designate the phenomenon of paired characters by the term “allelomorphism”, and the characters themselves, accordingly, “allelomorphic”. According to his proposal, organisms containing the same hereditary inclinations began to be called homozygous, and those containing different inclinations - heterozygous. Later, the term “allelomorphism” was replaced by the shorter term “allelism” (Johansen, 1926), and the hereditary inclinations (genes) responsible for the development of alternative traits were called “allelic”.

Hybridological analysis involves reciprocal crossing of parental forms, i.e. using the same individual first as the maternal parent (forward crossing) and then as the paternal parent (backcrossing). If both crosses produce the same results, corresponding to Mendel’s laws, then this indicates that the analyzed trait is determined by an autosomal gene. Otherwise, the trait is linked to sex, due to the localization of the gene on the sex chromosome.

Letter designations: P - parental individual, F - hybrid individual, ♀ and ♂ - female or male individual (or gamete),
capital letter (A) is a dominant hereditary disposition (gene), lowercase letter (a) is a recessive gene.

Among the second generation hybrids with yellow seed color there are both dominant homozygotes and heterozygotes. To determine the specific genotype of a hybrid, Mendel proposed crossing the hybrid with a homozygous recessive form. It is called analyzing. When crossing a heterozygote ( Ahh) with the analyzer line (aa), splitting is observed both by genotype and phenotype in a 1: 1 ratio.

If one of the parents is a homozygous recessive form, then the analyzing cross simultaneously becomes a backcross - a return crossing of the hybrid with the parent form. The offspring from such a cross are designated Fb.

The patterns Mendel discovered in his analysis of monohybrid crosses also appeared in dihybrid crosses in which the parents differed in two pairs of alternative traits (for example, yellow and green seed color, smooth and wrinkled shape). However, the number of phenotypic classes in F 2 doubled, and the phenotypic splitting formula was 9: 3: 3: 1 (for 9 individuals with two dominant traits, three individuals each with one dominant and one recessive trait, and one individual with two recessive traits ).

To facilitate the analysis of splitting in F 2, the English geneticist R. Punnett proposed a graphical representation of it in the form of a lattice, which began to be called after his name ( Punnett grid). On the left, vertically, it contains the female gametes of the F1 hybrid, and on the right - the male ones. The inner squares of the lattice contain the combinations of genes that arise when they merge, and the phenotype corresponding to each genotype. If the gametes are placed in a lattice in the sequence shown in the diagram, then in the lattice you can notice the order in the arrangement of genotypes: all homozygotes are located along one diagonal, and heterozygotes for two genes (diheterozygotes) are located along the other. All other cells are occupied by monoheterozygotes (heterozygotes for one gene).

The cleavage in F 2 can be represented using phenotypic radicals, i.e. indicating not the entire genotype, but only the genes that determine the phenotype. This entry looks like this:

The dashes in the radicals mean that the second allelic genes can be either dominant or recessive, and the phenotype will be the same.

Dihybrid crossing scheme
(Punnet grid)

	AB	Ab	aB	ab
AB	AABB yellow Ch.	AABb yellow Ch.	AaBB yellow Ch.	AaBb yellow Ch.
Ab	AABb yellow Ch.	AAbb yellow wrinkle	AaBb yellow Ch.	Aabb yellow wrinkle
aB	AaBB yellow Ch.	AaBb yellow Ch.	aaBB green Ch.	aaBb green Ch.
ab	AaBb yellow Ch.	Aabb yellow wrinkle	aaBb green Ch.	aabb green wrinkle

The total number of F2 genotypes in the Punnett lattice is 16, but there are 9 different ones, since some genotypes are repeated. The frequency of different genotypes is described by the rule:

In an F2 dihybrid cross, all homozygotes occur once, monoheterozygotes occur twice, and diheterozygotes occur four times. The Punnett grid contains 4 homozygotes, 8 monoheterozygotes and 4 diheterozygotes.

Segregation by genotype corresponds to the following formula:

1AABB: 2AABBb: 1AAbb: 2AaBB: 4AaBBb: 2Aabb: 1aaBB: 2aaBBb: 1aabb.

Abbreviated as 1:2:1:2:4:2:1:2:1.

Among the F 2 hybrids, only two genotypes repeat the genotypes of the parental forms: AABB And aabb; in the rest, recombination of parental genes occurred. It led to the emergence of two new phenotypic classes: yellow wrinkled seeds and green smooth ones.

Having analyzed the results of dihybrid crossing for each pair of characters separately, Mendel established the third pattern: the independent nature of inheritance of different pairs of characters ( Mendel's III law). Independence is expressed in the fact that splitting for each pair of characteristics corresponds to the monohybrid crossing formula 3: 1. Thus, a dihybrid crossing can be represented as two simultaneously occurring monohybrid ones.

As was established later, the independent type of inheritance is due to the localization of genes in different pairs of homologous chromosomes. The cytological basis of Mendelian segregation is the behavior of chromosomes during cell division and the subsequent fusion of gametes during fertilization. In prophase I of the reduction division of meiosis, homologous chromosomes conjugate, and then in anaphase I they diverge to different poles, due to which allelic genes cannot enter the same gamete. When they diverge, non-homologous chromosomes freely combine with each other and move to the poles in different combinations. This determines the genetic heterogeneity of germ cells, and after their fusion during the process of fertilization, the genetic heterogeneity of zygotes, and as a consequence, the genotypic and phenotypic diversity of the offspring.

Independent inheritance of different pairs of traits makes it easy to calculate segregation formulas in di- and polyhybrid crosses, since they are based on simple monohybrid cross formulas. When calculating, the law of probability is used (the probability of the occurrence of two or more phenomena at the same time is equal to the product of their probabilities). A dihybrid cross can be decomposed into two, and a trihybrid cross into three independent monohybrid crosses, in each of which the probability of the manifestation of two different traits in F 2 is equal to 3: 1. Therefore, the formula for splitting the phenotype in F 2 dihybrid cross will be:

(3: 1) 2 = 9: 3: 3: 1,

trihybrid (3: 1) 3 = 27: 9: 9: 9: 3: 3: 3: 1, etc.

The number of phenotypes in an F2 polyhybrid cross is equal to 2 n, where n is the number of pairs of characteristics in which the parent individuals differ.

Formulas for calculating other characteristics of hybrids are presented in Table 1.

Table 1. Quantitative patterns of segregation in hybrid offspring
for various types of crossings

Quantitative characteristics	Type of crossing
	monohybrid	dihybrid	polyhybrid
Number of gamete types formed by hybrid F 1	2	2 2	2n
Number of gamete combinations during the formation of F 2	4	4 2	4n
Number of phenotypes F 2	2	2 2	2n
Number of genotypes F 2	3	3 2	3
Phenotype splitting in F 2	3: 1	(3: 1) 2	(3:1)n
Segregation by genotype in F 2	1: 2: 1	(1: 2: 1) 2	(1:2:1)n

The manifestation of the patterns of inheritance discovered by Mendel is possible only under certain conditions (independent of the experimenter). They are:

1. Equally probable formation by hybridomas of all varieties of gametes.
2. All possible combinations of gametes during the process of fertilization.
3. Equal viability of all varieties of zygotes.

If these conditions are not met, then the nature of segregation in the hybrid offspring changes.

The first condition may be violated due to the non-viability of one or another type of gamete, possibly due to various reasons, for example, the negative effect of another gene manifested at the gametic level.

The second condition is violated in the case of selective fertilization, in which there is a preferential fusion of certain types of gametes. Moreover, a gamete with the same gene can behave differently during the process of fertilization, depending on whether it is female or male.

The third condition is usually violated if the dominant gene has a lethal effect in the homozygous state. In this case, in F 2 monohybrid crossing as a result of the death of dominant homozygotes AA instead of a 3:1 split, a 2:1 split is observed. Examples of such genes are: the gene for platinum fur color in foxes, the gene for gray coat color in Shirazi sheep. (More details in the next lecture.)

The reason for deviation from Mendelian segregation formulas can also be incomplete manifestation of the trait. The degree of manifestation of the action of genes in the phenotype is denoted by the term expressivity. For some genes it is unstable and highly dependent on external conditions. An example is the recessive gene for black body color in Drosophila (mutation ebony), the expressivity of which depends on temperature, as a result of which individuals heterozygous for this gene can have a dark color.

Mendel's discovery of the laws of inheritance was more than three decades ahead of the development of genetics. The work “Experience with Plant Hybrids” published by the author was not understood and appreciated by his contemporaries, including Charles Darwin. The main reason for this is that at the time of the publication of Mendel’s work, chromosomes had not yet been discovered and the process of cell division, which, as mentioned above, constituted the cytological basis of Mendelian patterns, had not yet been described. In addition, Mendel himself doubted the universality of the patterns he discovered when, on the advice of K. Nägeli, he began to check the results obtained on another object - the hawkweed. Not knowing that the hawksbill reproduces parthenogenetically and, therefore, it is impossible to obtain hybrids from it, Mendel was completely discouraged by the results of the experiments, which did not fit into the framework of his laws. Under the influence of failure, he abandoned his research.

Recognition came to Mendel at the very beginning of the twentieth century, when in 1900 three researchers - G. de Vries, K. Correns and E. Cermak - independently published the results of their studies, reproducing Mendel's experiments, and confirmed the correctness of his conclusions . Since by this time mitosis, almost completely meiosis (its full description was completed in 1905), as well as the process of fertilization, had been completely described, scientists were able to connect the behavior of Mendelian hereditary factors with the behavior of chromosomes during cell division. The rediscovery of Mendel's laws became the starting point for the development of genetics.

The first decade of the twentieth century. became the period of the triumphal march of Mendelism. The patterns discovered by Mendel were confirmed in the study of various characteristics in both plant and animal objects. The idea of ​​the universality of Mendel's laws arose. At the same time, facts began to accumulate that did not fit within the framework of these laws. But it was the hybridological method that made it possible to clarify the nature of these deviations and confirm the correctness of Mendel’s conclusions.

All pairs of characters that were used by Mendel were inherited according to the type of complete dominance. In this case, the recessive gene in the heterozygote has no effect, and the phenotype of the heterozygote is determined solely by the dominant gene. However, a large number of traits in plants and animals are inherited according to the type of incomplete dominance. In this case, the F 1 hybrid does not completely reproduce the trait of one or the other parent. The expression of the trait is intermediate, with a greater or lesser deviation in one direction or the other.

An example of incomplete dominance can be the intermediate pink color of flowers in night beauty hybrids obtained by crossing plants with a dominant red and recessive white color (see diagram).

Scheme of incomplete dominance in the inheritance of flower color in the night beauty

As can be seen from the diagram, the law of uniformity of first-generation hybrids applies in crossing. All hybrids have the same color - pink - as a result of incomplete dominance of the gene A. In the second generation, different genotypes have the same frequency as in Mendel’s experiment, and only the phenotypic segregation formula changes. It coincides with the formula for segregation by genotype - 1: 2: 1, since each genotype has its own characteristic. This circumstance facilitates the analysis, since there is no need for analytical crossing.

There is another type of behavior of allelic genes in a heterozygote. It is called codominance and is described in the study of the inheritance of blood groups in humans and a number of domestic animals. In this case, a hybrid whose genotype contains both allelic genes exhibits both alternative traits equally. Codominance is observed when inheriting blood groups of the A, B, 0 system in humans. People with a group AB(IV group) there are two different antigens in the blood, the synthesis of which is controlled by two allelic genes.

Mendel's laws

Rediscovery Mendel's laws Hugo de Vries in Holland, Karl Correns in Germany and Erich Chermak in Austria occurred only in 1900 year. At the same time, archives were opened and Mendel's old works were found.

At this time, the scientific world was already ready to accept genetics. Her triumphal march began. They checked the validity of the laws of inheritance according to Mendel (Mendelization) on more and more new plants and animals and received constant confirmation. All exceptions to the rules quickly developed into new phenomena of the general theory of heredity.

Currently, the three fundamental laws of genetics, Mendel's three laws, are formulated as follows.

Mendel's first law. Uniformity of first generation hybrids. All characteristics of an organism can be in their dominant or recessive manifestation, which depends on the alleles of a given gene present. Each organism has two alleles of each gene (2n chromosomes). For manifestation dominant allele one copy of it is enough to manifest recessive- we need two at once. So, genotypes AA And Ahh peas produce red flowers, and only the genotype ahh gives white. So when we cross red peas with white peas:

AA x aa Aa

As a result of crossing, we get all the first generation offspring with red flowers. However, not all so simple. Some genes in some organisms may not be dominant or recessive, but codominant. As a result of such crossing, for example, in petunia and cosmos, we will get the entire first generation with pink flowers - an intermediate manifestation of the red and white alleles.

Mendel's second law. Splitting of characters in the second generation in a ratio of 3:1. When heterozygous hybrids of the first generation, carrying dominant and recessive alleles, self-pollinate, in the second generation the characters are split in a ratio of 3:1.

Mendelian crosses can be shown in the following diagram:

P: AA x aa F1: Aa x Aa F2: AA + Aa + Aa + aa

That is, one F 2 plant carries a homozygous dominant genotype, two have a heterozygous genotype (but the dominant allele appears in the phenotype!), and one plant is homozygous for a recessive allele. This results in a phenotypic splitting of the trait in a ratio of 3:1, although the genotypic splitting is actually 1:2:1. In the case of a codominant trait, such a split is observed, for example, in the color of flowers in petunia: one plant with red flowers, two with pink and one with white.

Mendel's third law. Law of independent inheritance of different characteristics

For dihybrid crossing, Mendel took homozygous pea plants that differed in two genes - seed color (yellow, green) and seed shape (smooth, wrinkled). Dominant characteristics - yellow color (I) and smooth shape (R) seeds Each plant produces one variety of gametes according to the alleles studied. When gametes merge, all offspring will be uniform: II Rr.

When gametes are formed in a hybrid, from each pair of allelic genes, only one gets into the gamete, and due to the randomness of the divergence of the paternal and maternal chromosomes in the first division of meiosis, the gene I can get into the same gamete with the gene R or with a gene r. Likewise, the gene i may be in the same gamete with the gene R or with a gene r. Therefore, the hybrid produces four types of gametes: IR, Ir, iR, ir. During fertilization, each of the four types of gametes from one organism encounters randomly any of the gametes from another organism. All possible combinations of male and female gametes can be easily established using Punnett gratings, in which the gametes of one parent are written out horizontally, and the gametes of the other parent vertically. The genotypes of zygotes formed during the fusion of gametes are entered into the squares.

It is easy to calculate that according to the phenotype, the offspring are divided into 4 groups: 9 yellow smooth, 3 yellow wrinkled, 3 green smooth, 1 yellow wrinkled, that is, a splitting ratio of 9: 3: 3: 1 is observed. If we take into account the results of splitting for each pair of characters separately, it turns out that the ratio of the number of yellow seeds to the number of green ones and the ratio of smooth seeds to wrinkled ones for each pair is equal to 3:1. Thus, with a dihybrid crossing, each pair of characters, when split in the offspring, behaves in the same way as with a monohybrid crossing, i.e., independently of the other pair of characters.

During fertilization, gametes are combined according to the rules of random combinations, but with equal probability for each. In the resulting zygotes, various combinations of genes arise.

Independent distribution of genes in the offspring and the occurrence 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.

Thus, Mendel's third law is formulated as follows: When crossing two homozygous individuals that differ from each other in two or more pairs of alternative traits, the genes and their corresponding traits are inherited independently of each other.

Recessive flew. Mendel obtained identical numerical ratios when splitting the alleles of many pairs of traits. This in particular implied equal survival of individuals of all genotypes, but this may not be the case. It happens that a homozygote for some trait does not survive. For example, yellow coloration in mice may be due to heterozygosity for Aguti yellow. When crossing such heterozygotes with each other, one would expect segregation for this trait in a ratio of 3:1. However, a 2:1 split is observed, that is, 2 yellow to 1 white (recessive homozygote).

A y a x A y a 1aa + 2A y a + 1A y A y -- the last genotype does not survive.

It has been shown that the dominant (by color) homozygote does not survive even at the embryonic stage. This allele is simultaneously recessive lethality(that is, a recessive mutation leading to the death of the organism).

Half-flying. Mendelian segregation disorder often occurs because some genes are semi-flying-- the viability of gametes or zygotes with such alleles is reduced by 10-50%, which leads to a violation of 3:1 cleavage.

Influence of the external environment. The expression of some genes may be highly dependent on environmental conditions. For example, some alleles appear phenotypically only at a certain temperature during a certain phase of the organism's development. This can also lead to violations of Mendelian segregation.

Modifier genes and polygenes. Except main gene, which controls this trait, there may be several more in the genotype modifier genes, modifying the expression of the main gene. Some traits may be determined not by one gene, but by a whole complex of genes, each of which contributes to the manifestation of the trait. This sign is usually called polygenic. All this also disrupts the 3:1 split.

heredity hybrid crossing mendel

Mendel's first law. Law of Uniformity of First Generation Hybrids

When crossing homozygous individuals that differ in one pair of alternative (mutually exclusive) characters, all offspring in first generation uniform in both phenotype and genotype.

Pea plants with yellow (dominant trait) and green (recessive trait) seeds were crossed. The formation of gametes is accompanied by meiosis. Each plant produces one type of gamete. From each homologous pair of chromosomes, one chromosome with one of the allelic genes (A or a) goes into gametes. After fertilization, the pairing of homologous chromosomes is restored and hybrids are formed. All plants will have only yellow seeds (phenotype), heterozygous for the Aa genotype. This happens when complete dominance.

Hybrid Aa has one gene A from one parent, and the second gene - a - from the other parent (Fig. 73).

Haploid gametes (G), unlike diploid organisms, are circled.

As a result of crossing, first generation hybrids are obtained, designated F 1.

To record crosses, a special table is used, proposed by the English geneticist Punnett and called the Punnett grid.

The gametes of the paternal individual are written out horizontally, and the gametes of the maternal individual vertically. Genotyping is recorded at intersections.

Rice. 73.Inheritance in monohybrid crosses.

I - crossing two varieties of peas with yellow and green seeds (P); II

Cytological foundations of Mendel's I and II laws.

F 1 - heterozygotes (Aa), F 2 - segregation according to genotype 1 AA: 2 Aa: 1 aa.

py descendants. In the table, the number of cells depends on the number of gamete types produced by the individuals being crossed.

Mendel's II law. The law of splitting of first generation hybrids

When hybrids of the first generation are crossed with each other, individuals with both dominant and recessive traits appear in the second generation and splitting occurs by phenotype in a ratio of 3:1 (three dominant phenotypes and one recessive) and 1:2:1 by genotype (see. Fig. 73). Such splitting is possible when complete dominance.

Hypothesis of "purity" of gametes

The law of splitting can be explained by the hypothesis of the “purity” of gametes.

Mendel called the phenomenon of non-mixing of alleles of alternative characters in the gametes of a heterozygous organism (hybrid) the hypothesis of “purity” of gametes. Two allelic genes (Aa) are responsible for each trait. When hybrids are formed, allelic genes are not mixed, but remain unchanged.

As a result of meiosis, Aa hybrids form two types of gametes. Each gamete contains one of a pair of homologous chromosomes with allelic gene A or allelic gene a. Gametes are pure from another allelic gene. During fertilization, the homology of chromosomes and allelicity of genes are restored, and a recessive trait (the green color of peas) appears, the gene of which did not show its effect in the hybrid organism. Traits develop through the interaction of genes.

Incomplete dominance

At incomplete dominance heterozygous individuals have their own phenotype, and the trait is intermediate.

When crossing night beauty plants with red and white flowers, pink-colored individuals appear in the first generation. When crossing first-generation hybrids (pink flowers), the cleavage in the offspring by genotype and phenotype coincides (Fig. 74).

Rice. 74.Inheritance with incomplete dominance in the night beauty plant.

The gene that causes sickle cell anemia in humans has the property of incomplete dominance.

Analysis cross

The recessive trait (green peas) appears only in the homozygous state. Homozygous (yellow peas) and heterozygous (yellow peas) individuals with dominant traits do not differ from each other in phenotype, but have different genotypes. Their genotypes can be determined by crossing with individuals with a known genotype. Such an individual may be green peas, which have a homozygous recessive trait. This cross is called an analyzed cross. If, as a result of crossing, all the offspring are uniform, then the individual under study is homozygous.

If splitting occurs, then the individual is heterozygous. The offspring of a heterozygous individual produces cleavage in a 1:1 ratio.

Mendel's III law. Law of independent combination of characteristics (Fig. 75). Organisms differ from each other in several ways.

The crossing of individuals that differ in two characteristics is called dihybrid, and in many respects - polyhybrid.

When crossing homozygous individuals that differ in two pairs of alternative characters, in the second generation occurs independent combination of features.

As a result of dihybrid crossing, the entire first generation is uniform. In the second generation, phenotypic cleavage occurs in a ratio of 9:3:3:1.

For example, if you cross a pea with yellow seeds and a smooth surface (dominant trait) with a pea with green seeds and a wrinkled surface (recessive trait), the entire first generation will be uniform (yellow and smooth seeds).

When hybrids were crossed with each other in the second generation, individuals appeared with characteristics that were not present in the original forms (yellow wrinkled and green smooth seeds). These traits are inherited regardless from each other.

A diheterozygous individual produced 4 types of gametes

For the convenience of counting individuals resulting in the second generation after crossing hybrids, the Punnett grid is used.

Rice. 75.Independent distribution of traits in dihybrid crosses. A, B, a, b - dominant and recessive alleles that control the development of two traits. G - germ cells of the parents; F 1 - first generation hybrids; F 2 - second generation hybrids.

As a result of meiosis, one of the allelic genes from a homologous pair of chromosomes will be transferred to each gamete.

4 types of gametes are formed. Cleavage after crossing in the ratio 9:3:3:1 (9 individuals with two dominant traits, 1 individual with two recessive traits, 3 individuals with one dominant and the other recessive traits, 3 individuals with dominant and recessive traits).

The appearance of individuals with dominant and recessive traits is possible because the genes responsible for the color and shape of peas are located on various non-homologous chromosomes.

Each pair of allelic genes is distributed independently of the other pair, and therefore genes can be combined independently.

A heterozygous individual for “n” pairs of characteristics forms 2 n types of gametes.

Questions for self-control

1. How is Mendel’s first law formulated?

2. What seeds did Mendel cross with peas?

3. Plants with what seeds resulted from crossing?

4. How is Mendel’s II law formulated?

5. Plants with what characteristics were obtained as a result of crossing first generation hybrids?

6. In what numerical ratio does splitting occur?

7. How can the law of splitting be explained?

8. How to explain the hypothesis of “purity” of gametes?

9. How to explain the incomplete dominance of traits? 10.What kind of cleavage by phenotype and genotype occurs

after crossing first generation hybrids?

11.When is an analytical cross carried out?

12. How is an analytical cross carried out?

13.What kind of cross is called dihybrid?

14. On which chromosomes are the genes responsible for the color and shape of peas located?

15. How is Mendel’s III law formulated?

16. What phenotypic cleavage occurs in the first generation?

17. What kind of phenotypic cleavage occurs in the second generation?

18.What is used for the convenience of counting individuals resulting from crossing hybrids?

19.How can we explain the appearance of individuals with characteristics that were not there before?

Keywords of the topic “Mendel’s Laws”

allelicity anemia

interaction

gametes

gene

genotype

heterozygote

hybrid

hypothesis of "purity" of gametes

homozygote

homology

peas

pea

action

dihybrid

dominance

uniformity

law

meiosis

education coloring

fertilization

individual

pairing

surface

count

generation

polyhybrid

offspring

appearance

sign

plant

split

Punnett grid

parents

property

seeds

crossing

merger

ratio

variety

convenience

phenotype

form

character

color

flowers

Multiple allelism

Allelic genes may include not two, but a greater number of genes. These are multiple alleles. They arise as a result of mutation (replacement or loss of a nucleotide in a DNA molecule). An example of multiple alleles can be the genes responsible for human blood groups: I A, I B, I 0. Genes I A and I B are dominant to the I 0 gene. Only two genes from a series of alleles are always present in a genotype. Genes I 0 I 0 determine blood group I, genes I A I A, I A I O - group II, I B I B, I B I 0 - group III, I A I B - group IV.

Gene interaction

There is a complex relationship between a gene and a trait. One gene can be responsible for the development of one trait.

Genes are responsible for the synthesis of proteins that catalyze certain biochemical reactions, resulting in certain characteristics.

One gene can be responsible for the development of several traits, exhibiting pleiotropic effect. The severity of the pleiotropic effect of a gene depends on the biochemical reaction catalyzed by the enzyme synthesized under the control of this gene.

Several genes may be responsible for the development of one trait - this polymer gene action.

The manifestation of symptoms is the result of the interaction of various biochemical reactions. These interactions can be associated with allelic and non-allelic genes.

Interaction of allelic genes.

The interaction of genes located in the same allelic pair occurs as follows:

. complete dominance;

. incomplete dominance;

. co-dominance;

. overdominance.

At complete In dominance, the action of one (dominant) gene completely suppresses the action of another (recessive). When crossing, a dominant trait appears in the first generation (for example, the yellow color of peas).

At incomplete dominance occurs when the effect of a dominant allele is weakened in the presence of a recessive one. Heterozygous individuals obtained as a result of crossing have their own genotype. For example, when crossing night beauty plants with red and white flowers, pink flowers appear.

At co-dominance The effect of both genes is manifested when they are present simultaneously. As a result, a new symptom appears.

For example, blood group IV (I A I B) in humans is formed by the interaction of genes I A and I B. Separately, the I A gene determines the II blood group, and the I B gene determines the III blood group.

At overdominance the dominant allele in the heterozygous state has a stronger manifestation of the trait than in the homozygous state.

Interaction of nonallelic genes

One trait of an organism can often be influenced by several pairs of non-allelic genes.

The interaction of non-allelic genes occurs as follows:

. complementarity;

. epistasis;

. polymers.

Complementary the effect manifests itself with the simultaneous presence of two dominant non-allelic genes in the genotype of organisms. Each of the dominant genes can manifest itself independently if the other is in a recessive state, but their joint presence in a dominant state in the zygote determines a new state of the trait.

Example. Two varieties of sweet peas with white flowers were crossed. All first generation hybrids had red flowers. Flower color depends on two interacting genes A and B.

Proteins (enzymes) synthesized on the basis of genes A and B catalyze biochemical reactions that lead to the manifestation of the trait (red color of flowers).

Epistasis- an interaction in which one of the dominant or recessive non-allelic genes suppresses the action of another non-allelic gene. A gene that suppresses the action of another is called an epistatic gene, or suppressor. The suppressed gene is called hypostatic. Epistasis can be dominant or recessive.

Dominant epistasis. An example of dominant epistasis would be the inheritance of plumage color in chickens. The dominant gene C is responsible for plumage color. The dominant non-allelic gene I suppresses the development of plumage color. As a result of this, chickens that have the C gene in the genotype, in the presence of the I gene, have white plumage: IICC; IICC; IiCc; Iicc. Hens with the iicc genotype will also be white because these genes are in a recessive state. The plumage of chickens with the iiCC, iiCc genotype will be colored. The white color of the plumage is due to the presence of a recessive allele of the i gene or the presence of the color suppressor gene I. The interaction of genes is based on biochemical connections between enzyme proteins, which are encoded by epistatic genes.

Recessive epistasis. Recessive epistasis explains the Bombay phenomenon - the unusual inheritance of antigens of the ABO blood group system. There are 4 known blood groups.

In the family of a woman with blood group I (I 0 I 0), a man with blood group II (I A I A) gave birth to a child with blood group IV (I A I B), which is impossible. It turned out that the woman inherited the I B gene from her mother and the I 0 gene from her father. Only the I 0 gene showed an effect, therefore

it was believed that the woman had blood type I. Gene I B was suppressed by the recessive gene x, which was in a homozygous state - xx.

In the child of this woman, the suppressed I B gene showed its effect. The child had IV blood group I A I B.

PolymerThe effect of genes is due to the fact that several non-allelic genes can be responsible for the same trait, enhancing its manifestation. Traits that depend on polymer genes are classified as quantitative. Genes responsible for the development of quantitative traits have a cumulative effect. For example, polymeric non-allelic genes S 1 and S 2 are responsible for skin pigmentation in humans. In the presence of dominant alleles of these genes, a lot of pigment is synthesized, in the presence of recessive ones - little. The intensity of skin color depends on the amount of pigment, which is determined by the number of dominant genes.

From a marriage between mulattoes S 1 s 1 S 2 s 2, children are born with skin pigmentation from light to dark, but the probability of having a child with white and black skin color is 1/16.

Many traits are inherited according to the polymeric principle.

Questions for self-control

1. What are multiple alleles?

2. What genes are responsible for human blood types?

3. What blood types does a person have?

4. What connections exist between a gene and a trait?

5. How do allelic genes interact?

6. How do non-allelic genes interact?

7. How can the complementary action of a gene be explained?

8. How can epistasis be explained?

9. How can the polymeric action of a gene be explained?

Keywords of the topic “Multiple alleles and gene interaction”

allelism allele antigens marriage

interaction

genotype

hybrid

peas

peas

blood type

action

children

dominance

woman

replacement

codominance

co-dominance

leather

chickens

mother

molecule

mulatto

mutation

Availability

inheritance

nucleotides

coloring

plumage

the basis

attitude

pigment

pigmentation

pleiotropy

suppressor

generation

polymerism

sign

example

presence

manifestation

development

reactions

child

result

overdominance connection

protein synthesis system

crossing

state

degree

loss

phenomenon

enzymes

color

flowers

Human

Mendel's laws- principles of transmission of hereditary characteristics from parent organisms to their descendants, resulting from the experiments of Gregor Mendel. These principles formed the basis for classical genetics and were subsequently explained as a consequence of the molecular mechanisms of heredity. Although three laws are usually described in Russian-language textbooks, the “first law” was not discovered by Mendel. Of particular importance among the patterns discovered by Mendel is the “hypothesis of gamete purity.”

Encyclopedic YouTube

1 / 5

✪ Mendel's first and second laws. Science 3.2

✪ Mendel's third law. Science 3.3

✪ Biology lesson No. 20. Gregor Mendel and his First Law.

✪ Mendel's first and second laws are super clear

✪ 1st Mendel's law. The law of dominance. Preparation for the Unified State Exam and Unified State Exam in biology

Subtitles

Mendel's predecessors

At the beginning of the 19th century, J. Goss ( John Goss), experimenting with peas, showed that when crossing plants with greenish-blue peas and yellowish-white peas in the first generation, yellow-white ones were obtained. However, during the second generation, the traits that did not appear in the first generation hybrids and later called recessive by Mendel appeared again, and plants with them did not split during self-pollination.

Thus, by the middle of the 19th century, the phenomenon of dominance was discovered, the uniformity of hybrids in the first generation (all hybrids of the first generation are similar to each other), splitting and combinatorics of characters in the second generation. However, Mendel, highly appreciating the work of his predecessors, pointed out that they had not found a universal law for the formation and development of hybrids, and their experiments did not have sufficient reliability to determine numerical ratios. The discovery of such a reliable method and mathematical analysis of the results, which helped create the theory of heredity, is the main merit of Mendel.

Mendel's methods and progress of work

• Mendel studied how individual traits are inherited.
• Mendel chose from all the characteristics only alternative ones - those that had two clearly different options in his varieties (the seeds are either smooth or wrinkled; there are no intermediate options). Such a conscious narrowing of the research problem made it possible to clearly establish the general patterns of inheritance.
• Mendel planned and carried out a large-scale experiment. He received 34 varieties of peas from seed-growing companies, from which he selected 22 “pure” varieties (not producing segregation according to the studied characteristics during self-pollination) varieties. Then he carried out artificial hybridization of varieties, and crossed the resulting hybrids with each other. He studied the inheritance of seven traits, studying a total of about 20,000 second-generation hybrids. The experiment was facilitated by a successful choice of object: peas are normally self-pollinating, but it is easy to carry out artificial hybridization on them.
• Mendel was one of the first in biology to use precise quantitative methods to analyze data. Based on his knowledge of probability theory, he realized the need to analyze a large number of crosses to eliminate the role of random deviations.

Mendel called the manifestation of the trait of only one of the parents in hybrids as dominance.

Law of Uniformity of First Generation Hybrids(Mendel's first law) - when crossing two homozygous organisms belonging to different pure lines and differing from each other in one pair of alternative manifestations of the trait, the entire first generation of hybrids (F1) will be uniform and will carry the manifestation of the trait of one of the parents.

This law is also known as the "law of trait dominance." Its formulation is based on the concept clean line regarding the trait being studied - in modern language this means homozygosity of individuals for this trait. The concept of homozygosity was later introduced by W. Batson in 1902.

When crossing pure lines of purple-flowered peas and white-flowered peas, Mendel noticed that the descendants of the plants that emerged were all purple-flowered, with not a single white one among them. Mendel repeated the experiment more than once and used other signs. If he crossed peas with yellow and green seeds, all the offspring would have yellow seeds. If he crossed peas with smooth and wrinkled seeds, the offspring would have smooth seeds. The offspring from tall and short plants were tall.

Codominance and incomplete dominance

Some opposing characters are not in the relation of complete dominance (when one always suppresses the other in heterozygous individuals), but in the relation incomplete dominance. For example, when pure lines of snapdragons with purple and white flowers are crossed, the first generation individuals have pink flowers. When pure lines of black and white Andalusian chickens are crossed, gray chickens are born in the first generation. With incomplete dominance, heterozygotes have characteristics intermediate between those of recessive and dominant homozygotes.

The crossing of organisms of two pure lines, differing in the manifestations of one studied trait, for which the alleles of one gene are responsible, is called monohybrid crossing.

The phenomenon in which the crossing of heterozygous individuals leads to the formation of offspring, some of which carry a dominant trait, and some - a recessive one, is called segregation. Consequently, segregation is the distribution of dominant and recessive traits among the offspring in a certain numerical ratio. The recessive trait does not disappear in the first generation hybrids, but is only suppressed and appears in the second hybrid generation.

Explanation

Law of gamete purity- each gamete contains only one allele from a pair of alleles of a given gene of the parent individual.

Normally, the gamete is always pure from the second gene of the allelic pair. This fact, which could not be firmly established in Mendel's time, is also called the gamete purity hypothesis. This hypothesis was later confirmed by cytological observations. Of all the laws of inheritance established by Mendel, this “Law” is the most general in nature (it is fulfilled under the widest range of conditions).

Law of independent inheritance of characteristics

Definition

Law of independent inheritance(Mendel’s third law) - when crossing two 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).

When homozygous plants differing in several characters, such as white and purple flowers and yellow or green peas, were crossed, the inheritance of each character followed the first two laws, and in the offspring they were combined in such a way as if their inheritance had occurred independently of each other. The first generation after crossing had a dominant phenotype for all traits. In the second generation, a splitting of phenotypes was observed according to the formula 9:3:3:1, that is, 9:16 were with purple flowers and yellow peas, 3:16 were with white flowers and yellow peas, 3:16 were with purple flowers and green peas, 1 :16 with white flowers and green peas.

Explanation

Mendel came across traits whose genes were located in different pairs of homologous chromosomes (nucleoprotein structures in the nucleus of a eukaryotic cell, in which most of the hereditary information is concentrated and which are intended for its storage, implementation and transmission) of the pea. During meiosis, homologous chromosomes of different pairs are randomly combined in gametes. If the paternal chromosome of the first pair gets into the gamete, then with equal probability both the paternal and maternal chromosomes of the second pair can get into this gamete. Therefore, traits whose genes are located in different pairs of homologous chromosomes are combined independently of each other. (It later turned out that of the seven pairs of characters studied by Mendel in the pea, which has a diploid number of chromosomes 2n=14, the genes responsible for one of the pairs of characters were located on the same chromosome. However, Mendel did not discover a violation of the law of independent inheritance, since as linkage between these genes was not observed due to the large distance between them).

Basic provisions of Mendel's theory of heredity

In modern interpretation, these provisions are as follows:

• Discrete (separate, non-mixable) hereditary factors - genes (the term “gene” was proposed in 1909 by V. Johansen) are responsible for hereditary traits.
• Each diploid organism contains a pair of alleles of a given gene responsible for a given trait; one of them is received from the father, the other from the mother.
• Hereditary factors are transmitted to descendants through germ cells. When gametes are formed, each of them contains only one allele from each pair (the gametes are “pure” in the sense that they do not contain the second allele).

Conditions for the fulfillment of Mendel's laws

According to Mendel's laws, only monogenic traits are inherited. If more than one gene is responsible for a phenotypic trait (and the absolute majority of such traits), it has a more complex pattern of inheritance.

Conditions for fulfilling the law of segregation during monohybrid crossing

Splitting 3:1 by phenotype and 1:2:1 by genotype is performed approximately and only under the following conditions:

1. A large number of crosses (large number of offspring) are studied.
2. Gametes containing alleles A and a are formed in equal numbers (have equal viability).
3. There is no selective fertilization: gametes containing any allele fuse with each other with equal probability.
4. Zygotes (embryos) with different genotypes are equally viable.
5. The parent organisms belong to pure lines, that is, they are truly homozygous for the gene being studied (AA and aa).
6. The trait is truly monogenic

Conditions for the implementation of the law of independent inheritance

1. All conditions necessary for the fulfillment of the law of splitting.
2. The location of the genes responsible for the traits being studied is in different pairs of chromosomes (unlinked).

Conditions for fulfilling the law of gamete purity

1. The normal course of meiosis. As a result of chromosome nondisjunction, both homologous chromosomes from a pair can end up in one gamete. In this case, the gamete will carry a pair of alleles of all genes that are contained in a given pair of chromosomes.