Interaction of non-allelic genes: complementarity, epistasis, polymery, pleiotropy. Sex-cohesion tasks in birds Example of inheritance of the “feathered legs” trait in chickens

In chickens, a sex-linked lethal gene (a) is found that causes the death of embryos; heterozygotes for this gene are viable. A normal chicken was crossed with a rooster heterozygous for this gene (in birds, the heterogametic sex is female). Draw up a scheme for solving the problem, determine the genotypes of the parents, the sex and genotype of the possible offspring and the probability of hatching chickens from the total number of viable offspring.

Answer

X A X A - normal male
X A X a - normal male
X a X a - death before birth
X A Y - normal female
X a Y - death before birth

The proportion of females from the total number of viable offspring is 1/3.

In chickens, striped feather color dominates over the absence of stripes (the gene is sex-linked), and the presence of a comb dominates over its absence (an autosomal trait). What kind of offspring can be expected from crossing a heterozygous striped hen with a comb and a heterozygous rooster with striped plumage without a comb? In chickens, males are homogametic and females are heterogametic. Draw up a scheme for solving the problem, determine the genotypes of the parents, the genotypes and phenotypes of the offspring, and the ratio of phenotypes in the offspring. Explain what laws apply in this case.

Answer

X A – striped feather color, X a – no stripes.
B – presence of a ridge, b – absence of a ridge.

X a YBb – chicken without stripes, with a comb (heterozygous)
X A X a bb - heterozygous rooster with striped plumage, without a comb

The phenotypic ratio is 1:1:1:1 (25% stripes, comb, 25% stripes, no ridge, 25% no stripes, comb, 25% no stripes, no ridge).

In canaries, the presence of a crest is a dominant autosomal trait (A); The sex-linked gene X B determines the green color of the plumage, and X b - the brown color. In birds, the homogametic sex is male and the heterogametic sex is female. A tufted green female was crossed with a male without a tuft and green plumage (heterozygote). The offspring included green crested chicks, green crestless chicks, brown crested chicks and brown chicks without crested chicks. Make a diagram for solving the problem. Determine the genotypes of parents and offspring, their gender. What laws of heredity are manifested in this case?

Answer

A – the presence of a tuft, and – no tuft.
X B – green plumage, X b – brown plumage.

Crested green female A_X B Y
Male without crest with green plumage (heterozygote) aaX B X b .

Among the offspring there were chicks without a crest. They received one gene a from their mother, one from their father. Therefore, the mother must have the gene a, therefore the mother is Aa.

P АаX В Y x ааX B X b

In this case, the law of independent inheritance (Mendel's third law) manifested itself.

In chickens, feathered legs are a dominant autosomal trait. The dominant gene for pockmarked plumage color is located on the X chromosome. The heterogametic sex in birds is female. By crossing a hen with feathered legs, pockmarked plumage, and a dihomozygous rooster with feathered legs, black plumage, offspring were obtained. Make a diagram for solving the problem. Determine all possible variants of the chicken genotypes and the resulting offspring, their sex. What laws of heredity are manifested in this case?

Answer

A – feathered legs, and – bare legs.
X B – speckled plumage color, X b – black color.

Chicken with feathered legs, speckled plumage A_X B Y
Rooster with feathered legs, black plumage (digomozygote) AAX b X b.

Option 1: chicken AaX B Y
P AаX B Y x ААX b X b

Sex-linked inheritance used for early diagnosis of sex in animals, which is important for agricultural production. In poultry farming, it is important to determine the sex of “day-old” chicks in order to place cockerels and hens on different diets, feeding the cockerels for meat. To diagnose sex, criss-cross inheritance of the feather color trait is used. When crossing a motley hen (dominant trait) with a black rooster (recessive trait) in F 1, all the cockerels that received the dominant gene from the mother will be motley, and the hens will be black.

In humans, sex-linked inheritance hereditary anomalies such as hemophilia and color blindness. Since the male sex is heterogametic in humans, these anomalies manifest themselves mainly in men. Women are usually carriers of such genes, having them in a heterozygous state.

When breeding silkworms, criss-cross inheritance is used to select males based on grena color (a sex-linked trait), since the yield of silk from the cocoons of male silkworms is 20–30% higher.

The picture of sex-linked inheritance may be distorted if there are isolated cases of non-disjunction of sex chromosomes during meiosis. Thus, when a white-eyed female Drosophila is crossed with a red-eyed male (see the criss-cross inheritance scheme above), in F1, in addition to red-eyed females and white-eyed males, single white-eyed females and red-eyed males appear. The reason for this deviation is the non-disjunction of the X chromosomes in the original female. During the process of gametogenesis, not one X chromosome enters the egg, but both, or, conversely, neither one, but both enter the polar body. When such eggs are fertilized by normal sperm, red-eyed males and white-eyed females develop.

The offspring, which is formed as a result of the primary nondisjunction of chromosomes in a female, has different, non-standard combinations and numbers of sex chromosomes. However, the genetic inertia of the Y chromosome makes individuals with a karyotype XXY female and viable, and with a karyotype X0- male and also viable. Zygotes that have not received an X chromosome ( Y0), die, as do (with rare exceptions) zygotes with three X chromosomes.

Scheme of inheritance of white eye color in Drosophila (white gene)
with nondisjunction of X chromosomes in a female

In Drosophila, a line has been bred ( double yellow- double yellow), in which the inheritance of a sex-linked trait - yellow body color - is disrupted from generation to generation. In females of this line, the X chromosomes are connected to each other in the proximal part and have one centromere. In this regard, in meiosis they behave like one chromosome and in anaphase they move to one pole.

Heterogamety of one sex determines the correspondence of the sex ratio in each generation of organisms to the formula 1: 1. This ratio coincides with the splitting during analytical crossing. Let's consider it using the example of Drosophila, in which sex determination corresponds to the Lygaeus type. The set of chromosomes in Drosophila consists of three pairs of autosomes and two sex chromosomes. The female forms one type of gametes with a haploid set (3A+X), and the male produces two types of gametes (3A+X) and (3A+Y) in equal quantities. As a result, the same number of females and males develops in the next generation.

4 . 2. EPISTASE

Epistasis (from the Greek epístasis - stop, obstacle), the interaction of two non-allelic (i.e. belonging to different loci) genes, in which one gene, called epistatic or a suppressor gene, suppresses the action of another gene, called hypostatic. Suppressor genes are known in animals (mammals, birds, insects) and plants. They are usually designated I or Su in the case of the dominant state of genes and i or su for their recessive alleles (from the English words inhibitor or suppressor). During epistasis, an allele of one of the genes suppresses the effect of alleles of other genes, for example, A > B or B > A, a > B or b > A, etc.

Epistatic interaction of genes is opposite in nature to complementary. In the case of complementary interaction, one gene is complemented by another. The epistatic effect of genes is very similar in nature to the phenomenon of dominance; the only difference is that with dominance, the allele suppresses the manifestation of a recessive allele belonging to the same allelomorphic pair. During epistasis, the allele of one gene suppresses the manifestation of an allele from another allelomorphic pair, i.e., a non-allelic gene.

Phenotypically, epistasis is expressed in a deviation from the segregation expected with digenic inheritance, however, there is no violation of G. Mendel’s laws in this case, since the distribution of alleles of interacting genes fully complies with the law of independent combination of traits.

Currently, epistasis is divided into two types: dominant and recessive. The most well-known examples of gene interaction by type of epistasis are given in Table. 13.

Table 13

Dominant epistasis(A > B or B > A). Dominant epistasis is understood as the suppression by a dominant allele of one gene of the action of an allelic pair of another gene.

The epistatic system has been found in chickens. Some chicken breeds have white plumage (white Leghorn, white Plymouth rock, Wyandotte, etc.), while other breeds have colored plumage (Australorp, New Hampshire, striped Plymouth rock, etc.). The white plumage of different chicken breeds is determined by several different genes. For example, the dominant white color is determined by the CCII genes (white leghorns), and the recessive white color is determined by the ccii genes (white Wyandottes). GeneS determines the presence of a pigment precursor (chromogen), i.e., the color of the feather, its allele - the absence of the chromogen and, therefore, the uncolored feather of the bird. Gene I is a suppressor of the action of gene C, the allele i does not suppress its action. In the presence of even one dose of gene I in the bird’s genotype, the effect of color genes will not manifest itself. Therefore, when crossing white leghorns (CCII) with white windottes (ccii), the F1 hybrids turn out to be white (CCIi). When F 1 hybrids are crossed with each other in the second generation, a split in color occurs in the ratio of 13 white: 3 colored (Fig. 38).

Rice. 38. Inheritance of color in chickens through the interaction of two pairs of genes (epistasis): I – suppresses color, i – does not suppress color,

C – presence of pigment, c – absence of pigment

Thus, suppression of the action of the dominant allele of the gene that determines the development of color (C) by the dominant allele of another gene (I) causes cleavage in F2 according to the 13: 3 phenotype.

Dominant epistasis can also give another ratio of phenotypes in F 2, namely: 12: 3: 1. In this case, the recessive homozygote (aabb) is phenotypically distinguishable from one of the heterozygous classes A-bb or aaB-. This type of splitting of characters has been established for the color of fruits in pumpkins (Fig. 39). This plant has three known fruit colors: white, yellow and green. The dominant allele of gene A determines the yellow color of the pumpkin, the recessive allele determines the green color. The second gene B exhibits an epistatic effect - it suppresses the formation of both yellow and green pigments, making them white. Recessive allele - does not affect the color of pumpkin fruits. When crossing plants with white (AABB) and green (aabb) fruits, all offspring F 1 will be white. BF 2 splitting of features will correspond to the formula 12: 3: 1.

F1: A-B-

Rice. 39. Inheritance of fruit color traits in pumpkins

In the above example, the suppressor gene itself does not determine any qualitative reaction or synthetic process, but only suppresses the action of other genes.

A slightly different mechanism of dominant epistasis is known for grain color in oats. In this case, the suppressor gene performs two functions: it ensures the manifestation of the trait and at the same time promotes

has an epistatic effect on another gene. In this crop, dominant genes were established that determine black (gene A) and gray (gene B) grain color. In addition, gene A exhibits an epistatic effect on gene B. When crossing parental forms of black-seeded (AABB) and white-seeded (aabb) in F 1, all offspring will be black-seeded (AaBb). Since gene A suppresses the expression of gene B, in F 1 all offspring will be black-seeded. BF 2 splitting will be 12: 3: 1:

Another example of dominant epistasis is the interaction of genes that determine coat color in horses. Gene B in the dominant state determines black color, and in the recessive state

- redhead. Gene C in the dominant state determines the gray color (causes early graying of horses). In addition, gene C additionally exhibits an epistatic effect on gene B, regardless of whether the latter is in a dominant or recessive state. As a result of the action of the suppressor gene C, the color of horses, regardless of the allelic state of the gene B, becomes gray. Therefore, from crossing gray horses of the BBCC genotype with red horses (bbcc) in F 1, gray offspring (BbCC) are born. When crossing horses of gray color with each other, splitting is observed in F 2

Lection 12:3:1.

12 gray: 3 black: 1 red

Recessive epistasis. Recessive epistasis is understood as this type of interaction when the recessive allele of one gene, being in a homozygous state, does not allow it to manifest itself

dominant or recessive alleles of another gene: aa > B;

aa>bb or bb>A; bb> aa.

We have already had the opportunity to become acquainted with the 9:3:4 cleavage as a result of the complementary interaction of genes. But these same cases can also be considered as an example of recessive epistasis.

An example of recessive epistasis is the coat color of Labrador dogs. Coat pigmentation is provided by the B gene, which in the dominant state produces black color, and in the recessive state (b) produces brown color. There is also a gene E, which in a dominant state does not affect the manifestation of color, but being in a recessive state (cc) suppresses the synthesis of both black and brown pigments. Such dogs become white. Cleavage in

9 black: 3 brown: 4 white

When crossing black mice (AAcc) with albinos (aaCC), all individuals F 1 (AaCc) have agouti-type coloring (an example of the complementary action of genes), and in F 2 9 parts of all individuals turn out to be agouti (A - C -), 3 parts are black (A -ss) and 4 – albinos (aaS - iaass). These results can be explained by assuming that recessive epistasis of type aa >C - occurs. In this case, mice of genotype aaaC turn out to be white, because the gene in the homozygous state, causing the absence of pigment, thereby prevents the manifestation of the pigment distribution gene C.

An interesting example of recessive epistasis is the Bombay blood phenotype. In rare cases, residents of Bombay (India), whose genotype contains dominant alleles I A or I B, have blood type zero (O). It was found that this defect is caused by a recessive mutation in the H gene, which is not homologous to the A or B loci, which in the homozygous state leads to the formation of a defective H substance during the synthesis of A or B antigens, with which the glycosyltransferase enzyme cannot interact and, therefore, , normal antigens are not formed. The nature of the H-substance has not yet been studied

on the. Antigens A and B are found only in individuals with the HH or Hh genotype. When both parents have genotype I A I B Hh, different blood groups may occur among the offspring in the following ratio:

ni: 3A: 6AB: 3B: 4O.

In addition to the described cases of recessive epistasis, there are also those when the recessive allele of each gene in the homozygous state simultaneously reciprocally suppresses the action of another pair of genes, i.e. aa epistatizes over B -, abb over A -. This interaction of two recessive genes is called double recessive epistasis (cryptomerism). In this case, in a dihybrid cross, the phenotypic split will correspond to 9: 7, as in the case of complementary gene interaction. Consequently, the same relationship can be interpreted both as a complementary interaction and as epistating. In itself, a genetic analysis of inheritance during the interaction of genes without taking into account the biochemistry and physiology of the development of a trait in ontogenesis cannot reveal the nature of this interaction. But without genetic analysis it is impossible to understand the hereditary determination of the development of these characteristics.

4 . 3. POLYMERISM

In the types of gene interactions considered so far, we have dealt with alternative, i.e., qualitatively different, traits. However, such properties of organisms as height, weight, egg production of chickens, the amount of milk and its fat content in livestock, wool length in sheep, the amount of protein in the endosperm of corn and wheat grains, the content of vitamins in plants, the rate of biochemical reactions, the properties of the nervous activity of animals and etc., cannot be divided into clear phenotypic classes. Such characteristics must be assessed in quantitative terms, which is why they are most often called quantitative or dimensional.

The study of the inheritance of polymeric traits began in the first decade of our century. Thus, when crossing wheat plants with red and white (uncolored) grains, the Swedish geneticist G. Nilsson-Ehle in 1908 discovered the usual monohybrid split in F 2 in a ratio of 3: 1. However, when crossing some lines of wheat that differ in the same characteristics, In F 2, a split is observed in the ratio of 15/16 colored and 1/16 white (Fig. 40).

The color of grains from the first group varied from dark red to pale red. Genetic analysis of wheat plants in F 3 from seeds

F 2 that plants grown from white grains and from grains with the darkest (red) color do not further split. From grains with an intermediate type of color, plants developed that in subsequent generations gave rise to splitting according to the color of the grain.

F3:

A 1A 2♂

A1 a2

a1 A2

a1 a2

A1 a1 A2 a2

Rice. 40. Inheritance of grain color in Triticum through the interaction of two pairs of genes (polymerism).

Analysis of the nature of the splitting made it possible to establish that in this case the red color of the grains is determined by two dominant alleles of two different genes, and the combination of their recessive alleles in a homozygous state determines the absence of color. The intensity of grain color depends on the number of dominant genes present in the genotype.

Polymerism is inherent in genes that are represented by independent units, i.e. are non-allelic, but their products perform the same function (Fig. 41).

Gene A 1

Such genes were called polymeric, and since they clearly influence the same trait, it was customary to designate them with one Latin letter indicating the index for different members: A 1, A 2, A 3, etc. Consequently, the original parental forms that gave 15:1 split in F 2 had genotypes A 1 A 1 A 2 A 2 ia 1 a 1 a 2 a 2 .

Obviously, in a trihybrid cross, if the F 1 hybrid has no number of polymer genes in the heterozygous state

vaniyah. From this comparison it is clear that the greater the number of dominant genes that determine a given trait, the greater the amplitude of variability.

When inheriting a quantitative trait, the offspring of a hybrid forms a continuous variation series according to the phenotypic manifestation of this trait.

The study of polymer genes is not only of theoretical, but also of great practical interest. It has been established that many economically valuable traits in animals and plants, such as milk production of livestock, egg production of chickens, animal weight and height, ear length, corn cob length, sugar content in beet roots, fertility and precocity of animals, length of the growing season in plants and many others are inherited according to the type of polymer.

The variability of a quantitative trait, in contrast to an alternative one, is assessed by the amplitude of its variation. The amplitude of variation of a trait itself is hereditarily determined and has adaptive significance in individual development. As an example of what has been said, let us cite the experiment of E. East on crossing two forms of corn - long-ear and short-ear. As can be seen from the results presented in Fig. 43, cobs along their length in the original corn lines No. 60 (short-cob) and No. 54 (long-cob), as well as in the hybrids of the first and second generations, are distributed with a certain pattern. It is easy to see that these two lines differ greatly from each other, but within each of them the length of the cobs varies slightly. This indicates that they are hereditarily relatively homogeneous. There is no variation in the size of the cobs in the parent forms.

In hybrid plants, the length of the ears appears to be intermediate, with little variability in the row. When F1 plants self-pollinate in the next generation (F2), the range of variability in cob length increases significantly. If we draw a distribution curve of classes according to the length of the cobs, plotting the size of the cobs on the abscissa and their number on the ordinate, it turns out to be similar to the distribution curve of polymeric dominant genes (see Fig. 42). Consequently, a continuous series of changes in the length of a corn cob can be represented as a series of genotypes during trihybrid crossing with a different number of dominant genes that determine a given quantitative trait.

F1:

F2:

Rice. 43. Inheritance and variability of cob length (in centimeters)

in Zea mays in F 1 and F 2

The fact that with a small number of second-generation plants studied, some of them reproduce the ear length characteristic of the parental forms may indicate the participation of a small number of polymer genes in determining the ear length in crossed forms. This assumption follows from the well-known formula 4n, which determines the number of possible combinations of gametes forming zygotes in F 2, depending on the number of gene pairs by which the original parental forms differed. The appearance in F2 of plants similar to the parental forms, with a sample size of 221 plants, indicates that the number of independently inherited genes determining ear length should not exceed three (43 = 64) or four (44 = 256).

The given examples of analysis of the inheritance of quantitative traits illustrate only one of the possible ways to study complex and fluctuating traits. Greater variability of a trait, first of all, indicates its complex genetic conditioning and, on the contrary, less variability of a trait indicates a smaller number of factors determining it.

Enhancing bird pigments.
Unlike mammals, whose skin pigmentation patterns and
hair depends mainly on two different types of pigments belonging to the group of melanins (eumelanin and pheomelanin), synthesized, like other proteins, in the body itself; in birds, in addition to melanins, there is also another yellow crystalline pigment - xanthophyll, which is formed higher plants and enters the body of birds in finished form. In some breeds of chickens, xanthophyll is deposited only in the skin, beak, and skin of the legs, but not in the feathers. When crossing them with breeds of birds in which this pigment is deposited only in fat, the ability to form yellow pigment in the skin, beak and legs does not appear in the first generation, that is, this trait behaves as recessive. In breeds of chickens in which this pigment is usually formed, it may not appear due to a lack of xanthophyll in the feed or begins to disappear as egg laying increases.

Fully pigmented bird breeds usually have another pigment in their skin, melanin, which, in the presence of xanthophyll, gives the skin of the legs a green tint, and in its absence, blue.

As for white chickens, the hereditary nature of such plumage
different. White Leghorns and Russian Whites will have this trait
dominant in relation to almost all pigmented colors (when crossed with New Hampshire and Rhode Island reds, a small number of pigmented feathers appear in the first generation crosses). On the other hand, in chicken breeds such as the White Wyandotte and White Plymouth Rock, this type of plumage is inherited as a typical recessive trait.

The black plumage of chickens of the Australorp, Minorca, black Leghorn, and Black Wyandotte breeds dominates over the red plumage of chickens of the Rhode Island, New Hampshire, and other breeds. When crossing black and some white (with single black spots in the plumage) breeds of chickens, the heterozygous offspring of the first generation has blue plumage (blue Andalusian chickens), which, with subsequent breeding of crosses “in itself”, gives splitting into white, blue and black in a ratio close to 1: 2: 1.

In turkeys, black plumage color dominates over bronze; white
turkey plumage behaves like a recessive trait. Peculiar
plumage is inherited in gray tabby Plymouth rock chickens. Firstly, sexual dimorphism is quite clearly expressed here: already one-day-old cockerels differ from pullets in that they have a rather large light spot on the back of the head (in pullets it is quite insignificant), and in adult birds the striping of feathers is much more pronounced in roosters. This sign dominates over continuous pigmentation. Secondly, the striping of Plymouth Rocks is a typical example of the inheritance of sex-linked traits: when such striped roosters are crossed with black hens, for example the Minorca breed, all the offspring of the first generation have gray striped plumage, while in the reciprocal crossing of black Minorca roosters with striped Plymouth Rock hens
the offspring differ in that all males will have striped plumage, while females will have black plumage. The manifestation of this trait in the second generation is also peculiar; from the first type of crossing in the second generation, all roosters will be gray-striped, and among the hens, half will be black and the other
half are gray striped; from the second type of reciprocal crossing in the second generation, both among the cockerels and among the hens, there will be an equal number of black and gray striped ones.

· 12: 3:1

Such splitting is possible if the recessive allele of an epistatic gene has its own phenotypic manifestation. A similar interaction of genes is observed during the inheritance of horse color.

Epistasis in horses

The black color is determined by the dominant gene B, the red color is determined by the recessive gene b, the dominant gene C, due to early graying of the hair, gives a gray color and suppresses the manifestation of the B gene (C>B). In the F 2 offspring from crossing gray (CCBB) and red (ccbb) horses, 12/16 are gray, 3/16 are black and 1/16 are red.

With recessive epistasis - cryptomeria a recessive homozygote of one gene suppresses the action of another dominant gene: aa>B.

With cryptomeria, a 9:3:4 split is observed in the offspring.

For example, in mice, the gray coat color is called “agouti” and is caused by the interaction of two dominant genes A and B.

Recessive epistasis in mice

Gene A determines the synthesis of black pigment, gene B promotes the distribution of pigment along the length of the hair, the recessive gene b does not affect coat color. The recessive gene a disrupts pigment synthesis and, in the homozygous state, suppresses the action of gene B (aaB - albinos).

When crossing black and white mice in F 1, only mice of the agouti type (AaBb) are obtained. In F 2, 9/16 mice are agouti, 3/16 are black, and 4/16 are white. The same splitting is characteristic of complementary gene interaction.

Characteristic signs of epistatic gene interaction:

1. the effect of two pairs of genes on one trait;

2. suppression of the manifestation of the hypostatic gene in F 1;

3. changing the dihybrid cleavage formula in F 2 by expanding the proportion of individuals with the suppressor gene phenotype, while the characteristic cleavage formulas for dominant epistasis are 13:3 and 12:3:1, for recessive epistasis – 9:3:4.

3. Polymeria – This is a type of non-allelic gene interaction in which several pairs of non-allelic genes influence the formation of one trait, causing similar changes.

The phenomenon of polymerization was discovered in 1909 by the Swedish geneticist Nilsson-Ehle, who described a series of uniquely acting genes that determine the color of the endosperm of wheat grain.

This is the case of the so-called cumulative polymer ( complex ) when the degree of manifestation of a trait depends on the number of dominant alleles in the genotype. This is how, for example, the length of the cob in corn is inherited.

Inheritance and variability of cob length (in centimeters) in Zea mays in F 1 and F 2

One of the original lines (No. 60) has cob lengths ranging from 5 to 8 cm, line No. 54 – from 13 to 21 cm. F 1 hybrids have average cob lengths. F 2 plants are phenotypically heterogeneous, the length of the ears varies from 7 to 21 cm. In this case, the length of the ear is proportional to the number (dose) of dominant genes in the genotype.

Human skin pigmentation is inherited by the type of cumulative polymer. For example, in the offspring of a black man and a white woman (or vice versa), children with intermediate skin color are born - mulattoes. A mulatto couple gives birth to children with skin color ranging from black to white, which is determined by the number of dominant alleles in the genotype:

A 1 A 1 A 2 A 2 × a 1 a 1 a 2 a 2

black white

A 1 a 1 A 2 a 2 × A 1 a 1 A 2 a 2

A 1 A 1 A 2 A 2 A 1 A 1 A 2 a 2 A 1 a 1 A 2 A 2 A 1 a 1 A 2 a 2 A 1 A 1 a 2 a 2 a 1 a 1 A 2 A 2 A 1 a 1 a 2 a 2 a 1 a 1 A 2 a 2 a 1 a 1 a 2 a 2

1/16 2/16 2/16 4/16 1/16 1/16 2/16 2/16 1/16

black “dark” mulatto “light” white

At non-cumulative polymer(simple), the presence of at least one dominant allele of polymer genes in the genotype determines the triangular shape of the fruit. For example, when crossing shepherd's purse plants with triangular fruits (pods) with a plant with oval fruits in F 1, plants with triangular-shaped fruits are formed.

Pod shape inheritance Capsella bursa pastoris when two pairs of genes interact

When they self-pollinate in F2, splitting into plants with triangular and oval fruits in a ratio of 15:1 is observed. If the splitting in F2 is 63:1, then 3 pairs of unambiguous genes are involved in the formation of the trait.

With the polymeric type of inheritance, manifestations are possible transgressions. Transgression is a form in which the degree of manifestation of a trait is greater than that of the parent forms.

Transgressions can be positive and negative:

P: A 1 A 1 a 2 a 2 A 3 A 3 a 4 a 4 ×a 1 a 1 A 2 A 2 a 3 a 3 A 4 A 4

F1: A 1 aA 2 a 2 A 3 a 3 A 4 a 4

F2: A 1 A 1 A 2 A 2 A 3 A 3 A 4 A 4 aa 1 a 2 a 2 a 3 a 3 a 4 a 4

positive negative transgression

Thus, transgressions appear in F 2 when the parent forms do not have extreme manifestations of traits and do not carry all dominant (with positive transgression) or all recessive (with negative transgression) alleles.

Modifier genes – genes that enhance or weaken the effect of the main gene.

The study of the coloration of mammals has shown that, along with extreme forms that have full development of pigment (black coloration) or its absence (albinos), there are a number of intermediate forms - grayish, brown, yellow. Coat color depends on the presence of modifier genes that do not have their own manifestation, but change the action of the main gene.

Modifier genes control the taste, color and aroma of fruits, so it is recommended to accumulate them to improve the traits of fruit crop varieties.

2. Penetrance and expressiveness. Norm of reaction. Pleiotropic effect of the gene.

Gene penetrance is the proportion of individuals that exhibit the expected phenotype.

Expressiveness is the degree of severity of the phenotype.

Many genes are fully penetrant and expressive. In Mendel's experiments, all peas carrying the dominant allele determining yellow color were yellow (both in homozygous and heterozygous states), and all peas homozygous for the allele determining green color were green. All people of genotype I A I A or I A i have blood group A, people of genotype I B I B or I B i have blood group B, and genotype I A I B determines blood group AB.

An example of incomplete penetrance and expressivity is the manifestation of a dominant gene that causes Huntington's chorea in humans. People carrying this dominant gene become ill at different ages, some of them remain healthy throughout almost their entire lives. The disease begins with involuntary twitching of the head, limbs and torso and, as it progresses, leads to degenerative changes in the nervous system, loss of physical and mental strength and death. The age at which Huntington's chorea first appears is from infancy to old age. This gene has incomplete penetrance because it is known that in some carriers it never manifests itself and does not develop the disease. This gene has variable expressivity: its carriers become ill at different ages, i.e. it affects their lives in different ways. The reasons that affect the expression of a gene in some individuals and not in others may be the influence of the external environment and genotype. The role of the environment in gene expression is also obvious in auxotrophic mutations and other conditional lethal mutations. For example, Drosophila, which carry some temperature-sensitive lethal mutations, have normal viability at 20 C, but at 29 C they lose mobility or die. The external environment also affects the expressiveness of morphological characters. For example, in humans, polydactyly occurs in the presence of the dominant D gene, which controls the number of radiates. The expressiveness of the Dd genotype varies within the same person, who may have five fingers on one hand and six on the other. In this case, varying expressiveness is determined by the internal environment of the developing organism. The penetrance and expressivity of a gene can also be influenced by other genes of a given individual, which is expressed in the case of modifier genes and epistatic genes.

Pleiotropy - a phenomenon in which one gene affects several traits.

For example, the influence of the pleiotropic gene for fur color in foxes on the viability of the offspring.

The platinum color gene is dominant to the silver-black color. However, in the homozygous state it leads to embryonic death (AA) in the early stages:

Platinum Platinum

2Aa : 1aa

Platinum Silver-black

Only platinum foxes that are heterozygous for this gene survive. According to the same scheme, the presence (aa) and absence (Aa) of scales in mirror carp, gray (Aa) and black (aa) coloration of the wool of astrakhan sheep are inherited.

In humans, a dominant gene is known that determines the trait “spider fingers” (Marfan syndrome). At the same time, the same gene determines an abnormality of the lens of the eye and heart disease.

Literature

1. Ayala, F. Modern genetics / F. Ayala, J. Caiger. – M.: Mir, 1987. – T.1. – 295 s; T.2. – 368 s; T.3.

2. Alikhanyan, S. I. General genetics / S. I. Alikhanyan, A. P. Akifev,
L. S. Chernin. – M.: Higher. school, 1985.

3. Bokut, S. B. Molecular biology: molecular mechanisms of storage, reproduction and implementation of genetic information / S. B. Bokut, N. V. Gerasimovich, A. A. Milyutin. – Mn.: Higher. school, 2005.

4. Dubinin, N. P. General genetics / N. P. Dubinin. – M.: Nauka, 1986.

5. Zhimulev, I. F. General and molecular genetics / I. F. Zhimulev. – Novosibirsk: Novosibirsk University Publishing House, 2002.

6. Zhuchenko, A. A. Genetics / A. A Zhuchenko, Yu. L. Guzhov,
V. A. Pukhalsky. – M.: Kolos, 2004.