1 interaction of allelic and non-allelic difference genes. Interaction of allelic and non-allelic genes. The phenomenon of pleiotropy. Complete and incomplete dominance

Interaction of allelic genes

The genotype includes a large number of genes that function and interact as an integral system. G. Mendel in his experiments discovered only one form of interaction between allelic genes - complete dominance of one allele and complete recessiveness of the other. The genotype of an organism cannot be considered as a simple sum of independent genes, each of which functions independently of the others. The phenotypic manifestations of a particular trait are the result of the interaction of many genes.
There are two main groups of gene interactions: interaction between allelic genes and interaction between non-allelic genes. However, it should be understood that this is not the physical interaction of the genes themselves, but the interaction of primary and secondary products that will determine this or that trait. In the cytoplasm, interaction occurs between proteins - enzymes, the synthesis of which is determined by genes, or between substances that are formed under the influence of these enzymes.
The following are possible types of interaction:
1) for the formation of a certain trait, the interaction of two enzymes is necessary, the synthesis of which is determined by two non-allelic genes;
2) an enzyme that was synthesized with the participation of one gene completely suppresses or inactivates the action of an enzyme that was formed by another non-allelic gene;
3) two enzymes, the formation of which is controlled by two non-allelic genes that influence one trait or one process so that their joint action leads to the emergence and intensification of the manifestation of the trait.
Interaction of allelic genes. Genes that occupy identical (homologous) loci on homologous chromosomes are called allelic. Each organism has two allelic genes.
The following forms of interaction between allelic genes are known: complete dominance, incomplete dominance, codominance and overdominance.
The main form of interaction is complete dominance, which was first described by G. Mendel. Its essence lies in the fact that in a heterozygous organism, the manifestation of one of the alleles dominates the manifestation of the other. With complete dominance, the 1:2:1 genotype split does not coincide with the 3:1 phenotypic split. In medical practice, almost half of the two thousand monogenic hereditary diseases have a dominant manifestation of pathological genes over normal ones. In heterozygotes, the pathological allele manifests itself in most cases as signs of the disease (dominant phenotype).
Incomplete dominance- a form of interaction in which in a heterozygous organism (Aa), the dominant gene (A) does not completely suppress the recessive gene (a), as a result of which an intermediate trait between the parents appears. Here the splitting by genotype and phenotype coincides and is 1:2:1
At co-dominance in heterozygous organisms, each of the allelic genes causes the formation of a product dependent on it, that is, products of both alleles appear. A classic example of such a manifestation is the blood group system, in particular the ABO system, when human red blood cells carry antigens on their surface that are controlled by both alleles. This form of manifestation is called codominance.
Overdominance- when the dominant gene in the heterozygous state is more pronounced than in the homozygous state. Thus, Drosophila with the AA genotype has a normal life expectancy; Aa - extended trivatism of life; aa - lethal outcome.

Multiple allelism

Each organism has only two allelic genes. At the same time, often in nature the number of alleles can be more than two, if some locus can be in different states. In such cases we talk about multiple alleles or multiple allelomorphism.
Multiple alleles are designated by one letter with different indices, for example: A, A1, A3... Allelic genes are localized in identical regions of homologous chromosomes. Since a karyotype always contains two homologous chromosomes, even with multiple alleles, each organism can simultaneously have only two identical or different alleles. Only one of them enters the germ cell (along with the difference in homologous chromosomes). For multiple alleles, the characteristic effect of all alleles on the same trait. The difference between them lies only in the degree of development of the trait.
The second feature is that somatic cells or cells of diploid organisms contain a maximum of two alleles out of several, since they are located at the same chromosomal locus.
Another feature is inherent in multiple alleles. According to the nature of dominance, allelomorphic traits are placed in a sequential series: more often a normal, unchanged trait dominates over others, the second gene of the series is recessive relative to the first, but dominates the next, etc. One example of the manifestation of multiple alleles in humans is the ABO blood group.
Multiple allelism has important biological and practical significance, since it enhances combinative variability, especially genotypic variability.

Interaction of non-alelic genes

There are many cases where a trait or properties are determined by two or more non-alelic genes that interact with each other. Although here the interaction is conditional, because it is not the genes that interact, but the products controlled by them. In this case, there is a deviation from the Mendelian patterns of splitting.
There are four main types of gene interaction: complementarity, epistasis, polymerization and modifying effect (pleiotropy).
Complementarity This is a type of interaction of non-allelic genes when one dominant gene complements the action of another non-allelic dominant gene, and together they determine a new trait that is absent in the parents. Moreover, the corresponding trait develops only in the presence of both non-allelic genes. For example, gray coat color in mice is controlled by two genes (A and B). Gene A determines the synthesis of pigment, however, both homozygotes (AA) and heterozygotes (Aa) are albinos. Another gene, B, provides pigment accumulations mainly at the base and ends of the hair. Crossing of diheterozygotes (AaBb x AaBb) leads to the splitting of hybrids in a ratio of 9:3:4. Numerical ratios during complementary interaction can be as high as 9:7; 9:6:1 (modification of the Mendelian split).
An example of complementary gene interaction in humans can be the synthesis of a protective protein - interferon. Its formation in the body is associated with the complementary interaction of two non-allelic genes located on different chromosomes.
Epistasis is an interaction of non-allelic genes in which one gene suppresses the action of another non-allelic gene. Oppression can be caused by both dominant and recessive genes (A>B, a>B, B>A, B>A), and depending on this they are distinguished epistasis is dominant and recessive. The suppressive gene was named inhibitor or suppressor. Inhibitor genes generally do not determine the development of a particular trait, but only suppress the action of another gene.
The gene whose effect is suppressed is called hypostatic. With epistatic interaction of genes, the phenotypic split in F2 is 13:3; 12:3:1 or 9:3:4, etc. The color of pumpkin fruits and the color of horses are determined by this type of interaction.
If the suppressor gene is recessive, then cryptomeria(Greek hristad - secret, hidden). For a person, such an example could be the “Bombay Phenomenon”. In this case, the rare recessive allele “x” in the homozygous state (mm) suppresses the activity of the jB gene (determining the B (III) blood group of the ABO system). Therefore, a woman with the jb_xx genotype phenotypically has blood group I - 0 (I).

Polygenic inheritance of quantitative traits

Pleiotropy
- gene expressivity and penetrance
Most quantitative traits of organisms are determined by several non-allelic genes (polygenes). The interaction of such genes in the process of trait formation is called polymeric. In this case, two or more dominant alleles equally influence the development of the same trait. Therefore, polymer genes are usually designated by one letter of the Latin alphabet with a digital index, for example: A1A1 and a1a1. For the first time, unambiguous factors were identified by the Swedish geneticist Nilsson-Ehle (1908) while studying the inheritance of color in wheat. It was found that this trait depends on two polymer genes, therefore, when crossing dominant and recessive dihomozygotes - colored (A1A1, A2 A2) with colorless (a1a1, a2a2) - in F, all plants produce colored seeds, although they are lighter than the parent ones specimens that have red seeds. In F, when crossing individuals of the first generation, phenotypic cleavage appears in a ratio of 15:1, therefore only recessive digomozygotes (a1a1 a2a2) are colorless. In pigmented specimens, the color intensity is very different depending on the number of dominant alleles they received: maximum in dominant digomozygotes (A1A1, A2 A2) and minimum in carriers of one of the dominant alleles.
An important feature of polymers is the summation of the effect of non-allelic genes on the development of quantitative traits. If with monogenic inheritance of a trait there are three possible variants of gene “doses” in the genotype: AA, Aa, aa, then with polygenic inheritance their number increases to four or more. The summation of the “doses” of polymer genes ensures the existence of continuous series of quantitative changes.
The biological significance of polymers also lies in the fact that the traits encoded by these genes are more stable than those encoded by a single gene. An organism without polymer genes would be very unstable: any mutation or recombination would lead to sharp variability, and this in most cases is unfavorable.
Animals and plants have many polygenic traits, among them those that are valuable for the economy: growth rate, early maturity, egg production, amount of milk, content of sugary substances and vitamins, etc.
Skin pigmentation in humans is determined by five or six polymer genes. In indigenous Africans (the Negroid race), dominant alleles predominate, while in representatives of the Caucasian race, recessive alleles predominate. Therefore, mulattoes have intermediate pigmentation, but when mulattoes marry, they may have both more and less intensely pigmented children.
Many morphological, physiological and pathological characteristics of a person are determined by polymer genes: height, body weight, blood pressure, etc. The development of such characteristics in humans obeys the general laws of polygenic inheritance and depends on environmental conditions. In these cases, there is, for example, a tendency to hypertension, obesity, etc. These signs may not appear or appear slightly under favorable environmental conditions. These polygenic traits differ from monogenic ones. By changing environmental conditions, it is possible to prevent a number of polygenic diseases.

Pleiotropy

Pleiotropic action of genes- this is the dependence of several traits on one gene, that is, the multiple effects of one gene. In Drosophila, the gene for white eye color simultaneously affects the color of the body, length, wings, structure of the reproductive apparatus, reduces fertility, and reduces life expectancy. A hereditary disease is known in humans - arachnodactyly ("spider fingers" - very thin and long fingers), or Marfan's disease. The gene responsible for this disease causes a disorder in the development of connective tissue and simultaneously affects the development of several signs: disruption of the structure of the eye lens, abnormalities in the cardiovascular system.
The pleiotropic effect of a gene can be primary or secondary. With primary pleiotropy the gene exhibits its multiple effects. For example, in Hartnup disease, a gene mutation leads to impaired absorption of the amino acid tryptophan in the intestine and its reabsorption in the renal tubules. In this case, the membranes of intestinal epithelial cells and renal tubules are simultaneously affected, with disorders of the digestive and excretory systems.
With secondary pleiotropy there is one primary phenotypic manifestation of a gene, followed by a stepwise process of secondary changes leading to multiple effects. Thus, with sickle cell anemia, homozygotes exhibit several pathological signs: anemia, an enlarged spleen, damage to the skin, heart, kidneys and brain. Therefore, homozygotes with the sickle cell anemia gene usually die in childhood. All these phenotypic manifestations of the gene constitute a hierarchy of secondary manifestations. The root cause, the direct phenotypic manifestation of the defective gene, is abnormal hemoglobin and sickle-shaped red blood cells. As a result, other pathological processes occur successively: adhesion and destruction of red blood cells, anemia, defects in the kidneys, heart, brain - these pathological signs are secondary.
With pleiotropy, a gene, acting on one basic trait, can also change and modify the expression of other genes, and therefore the concept of modifier genes has been introduced. The latter enhance or weaken the development of traits encoded by the “main” gene.
Indicators of the dependence of the functioning of hereditary inclinations on the characteristics of the genotype are penetrance and expressivity.
When considering the effect of genes and their alleles, it is necessary to take into account the modifying influence of the environment in which the organism develops. If primrose plants are crossed at a temperature of 15-20 ° C, then in F1, according to the Mendelian scheme, all generations will have pink flowers. But when such crossing is carried out at a temperature of 35 ° C, then all hybrids will have white flowers. If crosses are carried out at a temperature of about 30 ° C, then a different ratio (from 3:1 to 100%) of plants with white flowers arises.
This fluctuation of classes during splitting depending on environmental conditions is called penetrance - the strength of phenotypic manifestation. So, penetrance- this is the frequency of manifestation of a gene, the phenomenon of the appearance or absence of a trait in organisms of the same genotype.
Penetrance varies significantly among both dominant and recessive genes. Along with genes whose phenotype appears only under a combination of certain conditions and fairly rare external conditions (high penetrance), humans have genes whose phenotypic manifestation occurs under any combination of external conditions (low penetrance). Penetrance is measured by the percentage of organisms with a phenotypic trait from the total number of examined carriers of the corresponding alleles.
If a gene completely determines phenotypic expression, regardless of the environment, then it has 100 percent penetrance. However, some dominant genes are expressed less regularly. Thus, polydactyly has a clear vertical inheritance, but there are generation gaps. Dominant anomaly- premature puberty is characteristic only of men, but sometimes the disease can be transmitted from a person who has not suffered from this pathology. Penetrance indicates what percentage of gene carriers exhibit the corresponding phenotype. So penetrance depends on genes, on environment, on both. Thus, this is not a constant property of a gene, but a function of genes under specific environmental conditions.
Expressiveness(Latin exprhessio - expression) is a change in the quantitative manifestation of a trait in different individuals carrying the corresponding alleles.
With dominant hereditary diseases, expressivity may fluctuate. In the same family, hereditary diseases can manifest themselves from mild, barely noticeable to severe: various forms of hypertension, schizophrenia, diabetes, etc. Recessive hereditary diseases within a family manifest themselves in the same way and have minor fluctuations in expressivity.

Science has acquired an extensive base of new research into the substrate of evolution - the genetic code. It contains information about all past and upcoming changes for the development of the body.

The ratio of heredity and variability allows us to preserve only the best qualities, and instead of unsuccessful ones, acquire new ones, improving the structure and contributing to victory in natural selection.

Basic concepts of genetics

In modern genetics, the chromosomal theory of inheritance is taken as the basis, according to which the main morphological substrate is the chromosome - a structure of a condensed DNA complex (chromatin), from which information is read in the process of protein synthesis.

Genetics is based on several concepts: gene (a section of DNA encoding a specific single trait), (a set of genes and characteristics of an organism), gametes (sex cells with a single set of chromosomes) and zygotes (cells with a diploid set).

Genes, in turn, are classified into dominant (A) and recessive (a) depending on the predominance of one trait over another, allelic (A and a) and non-allelic genes (A and B). Alleles are located on identical sections of chromosomes and encode one trait. Non-allelic genes are absolutely opposite to them: they are located in different areas and encode different traits. However, despite this, non-allelic genes have the ability to interact with each other, giving rise to the development of completely new characteristics. Based on the qualitative composition of allelic genes, organisms can be divided into homo- and heterozygous: in the first case, the genes are the same (AA, aa), in the other they are different (Aa).

Mechanism and patterns of gene interaction

The forms among themselves were studied by the American geneticist T.H. Morgan. He presented the results of his research in According to it, genes included in the same chromosome are inherited together. Such genes are called linked and form the so-called. clutch groups. In turn, within these groups, gene recombination also occurs through crossing over - the exchange of chromosomes by different sections with each other. At the same time, it is absolutely logical and proven that genes located directly next to each other are not subject to separation during the process of crossing over and are inherited together.

If there is a distance between the genes, then the probability of separation exists - this phenomenon is called “incomplete linkage of genes.” If we talk about this in more detail, the interaction of allelic genes with each other occurs according to three simple schemes: with the production of a pure dominant trait, incomplete dominance with the production of an intermediate trait, and codominance with the inheritance of both traits. Non-allelic genes are inherited in a more complex manner: according to patterns of complementarity, polymerization or epistasis. In this case, both characteristics will be inherited, but to varying degrees.

Genes that control the development of the same trait (for example, flower color), whether allelic or non-allelic, cannot act completely independently. A genotype is not a simple sum of its constituent genes; it is a complex system based on inter-allelic and non-allelic interactions. The interaction occurs at the level of protein products that are produced under the control of genes.

Different types of dominance are determined interaction of allelic genes. Complete dominance does not always mean that the function of a recessive gene is completely suppressed and it does not function. For example, in snapdragons, red flower color is dominant over light red. However, both allelic genes, dominant and recessive, are expressed, that is, they ensure the production of an enzyme that catalyzes pigment synthesis. But under the control of a recessive gene, an inactive form of the enzyme is produced, which cannot provide the final stage of the production of red pigment (cyanidin). As a result, recessive homozygotes produce only its precursor, a light red pigment (pelargonidin). In a heterozygote, the work of the dominant gene completely ensures the transformation of the light red pigment into red.

The nature of dominance can change under the influence of external conditions. So, for example, in wheat, under normal conditions, a normal ear dominates, and under short daylight hours, a branched one dominates. But a change in the nature of dominance does not lead to a change in the genotype and does not change the segregation in the hybrid offspring.

The interaction of allelic genes can be seen especially clearly in the example of the phenomenon multiple allelism. This term refers to the existence of several (sometimes many) alleles of the same gene, which form a series of multiple alleles. Such series are known in many animals and plants; in Drosophila their number reaches several dozen.

A classic example of multiple allelism is the series of genes that control eye color in Drosophila. It includes 12 mutant genes that determine different types of coloration: from white to dark red, characteristic of wild-type flies.

All members of a series of multiple alleles are designated by the same letter (the initial letter in the English name of the first member of the series). An index is added to it in the form of one or two letters - the first in the name of this member of the series. For example: the initial member of the above series of eye color in Drosophila - the recessive mutation white (white eyes) is designated as w, one of the subsequent mutant members of the series as w a(apricot - apricot eyes), and the dominant wild type gene is like W.

All members of the series are mutant forms of the same wild-type gene and therefore occupy the same locus on the chromosome. With a normal (diploid) number of chromosomes, only two members of this series can be represented in the genotype.

Each of the mutant genes in the series forms an allelic pair with any other member of the series, and they are all allelic to a single wild-type gene that causes the normal (red) coloration of Drosophila eyes. It is dominant in relation to any other member of the series. If the genotype contains two mutant alleles, then such individuals are called compounds. They are characterized by an intermediate state of the trait. So, for example, in heterozygotes for genes white And apricot Eye color is yellow. The difference between the interaction of allelic genes and non-allelic ones is that in heterozygotes for two mutant alleles, their action is not complementary and does not provide a return to the wild-type trait.

A series of allelic genes for eye color in Drosophila

In some series of multiple alleles, the wild-type gene may be recessive to the mutant gene. This indicates that the gene can mutate in different directions: both towards dominance and towards recessivity. An example of this situation is a series of three genes in Drosophila: Truncate(T dp - clipped wings) - Normal— dumpy(dp - shortened wings).

A series of multiple alleles were found in mice (coat color), rabbit, sable and fox (fur color), in buckwheat, tobacco (self-incompatibility), in humans (blood group genes), etc. The combination of allelic mutations is widely used by breeders to obtain new valuable traits .

The different fur colors of rabbits are determined by the genes of a series of multiple alleles.

In humans, a series of multiple alleles that control blood groups of the ABO system are well known. It includes three non-allelic genes: I A, I B and i. The dominant genes I A and I B interact with each other according to the type of codominance and both completely dominate the recessive gene i. Depending on the combination of these genes, one or another blood group is formed in a person.

Continue reading other topics in the book "Genetics and selection. Theory. Assignments. Answers".

The basic laws of inheritance were first developed by Gregor Mendel. Any organism has many hereditary characteristics. G. Mendel proposed to study the inheritance of each of them regardless of what is inherited by others. Having proved the possibility of inheriting one trait independently of others, he thereby showed that heredity is divisible and the genotype consists of separate units that determine individual traits and are relatively independent of each other. It turned out that, firstly, the same gene can influence several different traits and, secondly, the genes interact with each other. This discovery became the basis for the development of a modern theory that considers the genotype as an integral system of interacting genes. According to this theory, the influence of each individual gene on a trait always depends on the rest of the gene constitution (genotype) and the development of each organism is the result of the influence of the entire genotype. Modern ideas about gene interaction are presented in Fig. 1.

Rice. 1. Scheme of gene interaction ()

Allelic genes- genes that determine the development of the same trait and are located in identical regions of homologous chromosomes.

At complete dominance the dominant gene completely suppresses the manifestation of the recessive gene.

Incomplete dominance is of an intermediate nature. With this form of gene interaction, all homozygotes and heterozygotes are very different from each other in phenotype.

Codominance- a phenomenon in which heterozygotes exhibit both parental traits, that is, the dominant gene does not fully suppress the effect of the recessive trait. An example is the coat color of Shorthorn cows, the dominant color is red, the recessive color is white, and the heterozygote has a roan color - part of the hairs are red and part of the hairs are white (Fig. 2).

Rice. 2. Coat color of Shorthorn cows ()

This is an example of the interaction of two genes.

Other forms of interaction are also known, when three or more genes interact - this type of interaction is called multiple allelism. Several genes are responsible for the manifestation of such traits, two of which may be located in the corresponding chromosomal loci. Inheritance of blood groups in humans is an example of multiple allelism. A person's blood type is controlled by an autosomal gene, its locus is designated I, its three alleles are designated A, B, 0. A and B are codominant, O is recessive to both. Knowing that out of three alleles in a genotype there can be only two, we can assume that the combinations may be corresponding to the four blood groups (Fig. 3).

Rice. 3. Human blood groups ()

To consolidate the material, solve the following problem.

Determine what blood groups a child born from a marriage between a man with the first blood group - I (0) and a woman with the fourth blood group - IV (AB) can have.

Non-allelic genes- these are genes located in different parts of chromosomes and encoding different proteins. Nonallelic genes can interact with each other. In all cases of gene interaction, Mendelian patterns are strictly observed, with either one gene determining the development of several traits, or, conversely, one trait manifests itself under the influence of a combination of several genes. The interaction of non-allelic genes manifests itself in four main forms: epistasis, complementarity, polymerization and pleiotropy.

Complementarity- a type of gene interaction in which a trait can manifest itself if two or more genes are found in the genotype. Thus, two enzymes take part in the formation of chlorophyll in barley; if they are present in the genotype together, the chlorophyll will develop a green color; if only one gene is present, the plant will have a yellow color. If both genes are missing, the plant will have a white color and will not be viable.

Epistasis- interaction of genes, in which one non-allelic gene suppresses the manifestations of another non-allelic gene. An example is the plumage color of white leghorn chickens, which is controlled by two groups of genes:

dominant gene - A, responsible for white color;

recessive gene - a, for color;

dominant gene - B, responsible for black color;

recessive gene - in, for brown color.

In this case, white color suppresses the appearance of black (Fig. 4).

Rice. 4. Example of epistasis of white leghorns ()

When crossing the spirit of heterozygotes, a white hen and a white rooster, we see in the Punnett lattice the results of the crossing: splitting by phenotype in the ratio

12 white chickens: 3 black chickens: 1 brown chicken.

Polymerism- a phenomenon in which the development of traits is controlled by several non-allelic genes located on different chromosomes.

The more dominant alleles of a given gene, the greater the severity of this trait. An example of polymerization is the inheritance of skin color in humans. Two pairs of genes are responsible for the color of human skin:

if all four alleles of these genes are dominant, then a negroid type of skin color will appear;

if one of their genes is recessive, the skin color will be dark mulatto;

if two alleles are recessive, the color will correspond to the average mulatto; if only one dominant allele remains, the color will be light mulatto; if all four alleles are recessive, the color will correspond to the Caucasian skin type (Fig. 5).

Rice. 5. Polymeria, inheritance of skin color by humans ()

To consolidate the material, solve the problem.

The son of a white woman and a black man married a white woman. Can a son born from such a marriage turn out to be darker than his father?

Pleiotropy- an interaction in which one gene controls the development of several traits, that is, one gene is responsible for the formation of an enzyme that affects not only its own reaction, but also affects secondary biosynthesis reactions.

An example is Marfan syndrome (Fig. 6), which is caused by a mutant gene that leads to impaired development of connective tissue.

Rice. 6. Marfan syndrome ()

This disorder leads to a person developing a dislocated lens of the eye, heart valve defects, long and thin fingers, vascular malformations and frequent dislocations of the joints.

Today we learned that a genotype is not a simple set of genes, but a system of complex interactions between them. The formation of a trait is the result of the combined action of several genes.

Bibliography

1. Mamontov S.G., Zakharov V.B., Agafonova I.B., Sonin N.I. Biology. General patterns. - Bustard, 2009.
2. Ponomareva I.N., Kornilova O.A., Chernova N.M. Fundamentals of general biology. 9th grade: Textbook for 9th grade students of general education institutions / Ed. prof. I.N. Ponomareva. - 2nd ed., revised. - M.: Ventana-Graf, 2005.
3. Pasechnik V.V., Kamensky A.A., Kriksunov E.A. Biology. Introduction to general biology and ecology: Textbook for grade 9, 3rd ed., stereotype. - M.: Bustard, 2002.

1. Volna.org ().
2. Bannikov.narod.ru ().
3. Studopedia.ru ().

Homework

1. Define allelic genes and name their forms of interaction.
2. Define non-allelic genes and name their forms of interaction.
3. Solve the problems proposed for the topic.

These genes can be located in different loci of homologous chromosomes or in non-homologous chromosomes, and are usually responsible for the development of different traits.

Complementarity (lat. complementum - addition) - the presence in one genotype of two dominant (recessive) genes that complement each other’s action, and the trait is formed only with the simultaneous action of both genes.

Example: development of hearing in humans. For normal hearing, the human genotype must contain dominant genes from different allelic pairs -D and E. The D gene is responsible for the normal development of the cochlea, and the E gene is responsible for the development of the auditory nerve. In recessive homozygotes (dd), the cochlea will be underdeveloped, and with the genotype (ee), the auditory nerve will be underdeveloped. People with genotypes D..ee, ddE.. and ddee will be deaf.

Epistasis - this type of interaction in which a dominant (recessive) gene from one allelic pair suppresses the action of a dominant (recessive) gene from another allelic pair. Accordingly, epistasis can be either dominant or recessive. This phenomenon is the opposite of complementarity. The suppressive gene is called suppressor, inhibitor, epistatic. The suppressed gene is hypostatic. In humans, the “Bombay phenomenon” has been described in the inheritance of blood groups according to the ABO system. In a woman who received the J B allele from her mother, blood type I (O) was phenotypically determined. A detailed study revealed that the effect of the J B gene was suppressed by a rare recessive gene, which in the homozygous state had an epistatic effect.

Polymeria - Dominant genes from different allelic pairs affect the degree of manifestation of the same trait. Polymer genes are usually designated by one letter of the Latin alphabet with digital indices. So in humans, the amount of melanin pigment in the skin (and, therefore, skin color) is determined by four non-allelic genes: P 1 - P 4. Accordingly, people with the genotype P 1 P 1 P 2 P 2 P 3 P 3 P 4 P 4 will have dark brown skin color. The lightest skin color corresponds to the genotype p 1 p 1 - p 4 p 4. Intermediate variants will determine different intensities of pigmentation: For example, a person with a large number of dominant genes in the genotype will have darker skin. Traits determined by polymer genes are called polygenic; they are characterized by a large range of variability, i.e. wide reaction rate. Thus, many quantitative and some qualitative characteristics are inherited - height, body weight, blood pressure.

The basic patterns of inheritance of characteristics according to Mendel are realized thanks to the existence law (hypothesis) of gamete purity, put forward by G. Mendel in 1865.

The essence of the latter is that a pair of allelic genes that determine a particular trait: a) never mixes; b) during the process of gametogenesis, it diverges into different gametes, that is, one gene from an allelic pair ends up in each of them. Cytologically, this is ensured by meiosis: allelic genes lie in homologous chromosomes, which in anaphase of meiosis diverge to different poles and end up in different gametes.

II. Dihybrid cross

Previously, we studied the patterns of inheritance of 1 trait (monohybrid crossing)

In general and medical genetics, there is often a need to study the simultaneous inheritance of two or more traits (di- and polyhybrid crossing). If each of these traits is controlled by a pair of allelic genes, then we can assume the existence of two forms of inheritance: independent and linked. Fundamental differences will be determined by the location of genes on chromosomes. With linked inheritance, both pairs of allelic genes are located in one pair of homologous chromosomes (i.e., in the same linkage group). With independent inheritance, pairs of allelic genes are located in different pairs of homologous chromosomes.

The patterns and mechanisms of independent inheritance were identified and formulated by G. Mendel in the 3rd law "The Law of Independent Combination" signs": when crossing homozygous organisms that differ in two (or more) pairs of alternative characters, uniformity in geno- and phenotype is observed in the first generation, and when crossing hybrids of the first generation, in the second generation a splitting in phenotype 9:3:3:1 is observed, and when This produces organisms with combinations of characteristics not characteristic of the parental forms.”

For this purpose, Mendel used homozygous pea plants, differing in two pairs of alternative characters: yellow, smooth seeds and green, wrinkled seeds. In the first crossing he received AaBb plants with yellow, smooth seeds, i.e., the law of uniformity of first-generation hybrids manifests itself not only in monohybrid, but also in polyhybrid crossings if the parental forms are homozygous.

P: AABB x aabb

G: AB, AB ab, ab

F 1: AaBb

P (F 1): AaBb x AaBb

F 2:9:3:3:1

9 parts of plants with yellow, smooth peas, three parts with yellow wrinkled, 3 with green smooth, and 1 part with green, wrinkled, (3+1) n - splitting by phenotype, where n is the number of analyzed characters.

Organisms arise with new combinations of characteristics that are not characteristic of the parent forms.

Conditions for compliance with the law:

Traits are inherited monogenically (inheritance in each pair occurs independently)

The form of interaction of allelic genes is complete dominance

Pairs of allelic genes are located in different pairs of homologous chromosomes

In humans, eye color and hair color are independently inherited.

Reasons for the diversity of hybrids:

Independent divergence of pairs of chromosomes in anaphase I of meiosis (leads to the formation

nium of gametes with different combinations of non-allelic genes)

Random fusion of gametes during fertilization (various combinations arise

genes in the genotypes of offspring that determine the combination of traits)

New combinations of genes in the genotypes of descendants lead to the emergence of new combinations of traits in them - the main conclusion of the 3rd law.

In 1908 Sutton and Punnett discovered deviations from the free combination of characters according to Mendel's III law. In 1911-12 T. Morgan et al. Described the phenomenon of gene linkage - the joint transmission of a group of genes from generation to generation.

In Drosophila, the genes for body color (b+ - gray body, b - black body) and wing length (vg+ - normal wings, vg - short wings) are located on the same chromosome; these are linked genes located in the same linkage group. If you cross two homozygous individuals with alternative traits, then in the first generation, all hybrids will have the same phenotype with manifestations of dominant traits (gray body, normal wings).

This does not contradict G. Mendel’s law of uniformity of first generation hybrids. However, with further crossing of the first generation hybrids with each other, instead of the expected splitting according to the 9:3:3:1 phenotype, with linked inheritance, splitting occurred in a ratio of 3:1, individuals appeared only with the characteristics of the parents, and there were no individuals with recombination of characters.

This is due to the fact that in meiosis of gametogenesis, entire chromosomes diverge to the poles of the cell. One chromosome from a given homologous pair and all the genes that are in it go to one pole and subsequently end up in one gamete. The other chromosome from this pair moves to the opposite pole and ends up in another gamete. Joint inheritance of genes located on the same chromosome is called linked inheritance.

An example of complete linkage of genes in humans is the inheritance of the Rh factor. The presence of the Rh factor is due to three linked genes, so its inheritance occurs as a monohybrid cross.

However, genes located on the same chromosome can sometimes be inherited separately, in which case they speak of incomplete linkage of genes

Continuing his work on dihybrid crossing, Morgan conducted two experiments on analytical crossing and revealed that gene linkage can be complete and incomplete.

The reason for incomplete linkage of genes is crossing over. In meiosis, during conjugation, homologous chromosomes can cross over and exchange homologous regions. In this case, the genes of one chromosome are transferred to another, homologous to it.

During the growth period of gametogenesis, DNA reduplication occurs, the genetic characteristics of oocytes and spermatocytes are of the first order 2n4c, each chromosome consists of two chromatids that contain an identical set of DNA. During the prophase of the reduction division of meiosis, conjugation of homologous chromosomes occurs and an exchange of similar sections of homologous chromosomes can occur - crossing over. During the anaphase of reduction division, whole homologous chromosomes diverge to the poles; after completion of division, n2c cells are formed - oocytes and spermatocytes of the second order. During the anaphase of equational division, the chromatids diverge - nc, but at the same time they differ in the combination of non-allelic genes. New combinations of non-allelic genes - the genetic effect of crossing over.→ new combinations of traits in descendants → combinative variability.

The closer the genes are located to each other on a chromosome, the stronger the linkage between them and the less often their divergence occurs during crossing over, and, conversely, the farther the genes are from each other, the weaker the linkage between them and the more often its disruption is possible.

The number of different types of gametes will depend on the frequency of crossing over or the distance between the genes analyzed. The distance between genes is calculated in morganids: one unit of distance between genes located on the same chromosome corresponds to 1% crossing over. This relationship between distances and crossing-over frequency can be traced only up to 50 morganids.

Theoretical basis Patterns of linked inheritance are the provisions Chromosome theory heredity , which was formulated and experimentally proven by T. Morgan and his colleagues in 1911. Its essence is as follows:

The main material carrier of heredity are chromosomes with genes localized in them;

Genes are located on chromosomes in linear order at certain loci; allelic genes occupy identical loci on homologous chromosomes.

Genes localized on the same chromosome form a linkage group and are inherited predominantly together (or linked); the number of linkage groups is equal to the haploid set of chromosomes.

During gametogenesis (prophase I of meiosis), allelic exchange may occur.

genes - crossing over, which disrupts the linkage of genes.

The frequency of crossing over is proportional to the distance between genes. 1morganid is a distance unit equal to 1% crossing over.

This theory provided an explanation for Mendel's laws and revealed the cytological basis of the inheritance of traits.

The phenomenon of gene linkage underlies the compilation genetic maps of chromosomes- diagrams of the relative position of genes located in the same linkage group. Chromosome mapping methods are aimed at finding out which chromosome and in which locus (location) a gene is located, as well as determining the distance between neighboring genes

This is a straight line segment on which the order of genes is indicated and the distance between them in morganids is indicated; it is constructed based on the results of analyzing crossing. The more often traits are inherited together, the closer the genes responsible for these traits are located on the chromosome. In other words, the location of genes on a chromosome can be judged by the characteristics of the manifestation of traits in the phenotype.

When analyzing the linkage of genes in animals and plants, the hybridological method is used, in humans - the genealogical method, the cytogenetic method, as well as the method of hybridization of somatic cells.

A cytological map of a chromosome is a photograph or precise drawing of a chromosome that shows the sequence of genes. It is built on the basis of a comparison of the results of analyzing crossings and chromosomal rearrangements.