Examples of interaction of non-allelic genes. Interaction of allelic and non-allelic genes

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.

Nonallelic genes can also interact with each other. Moreover, their principle of interaction is somewhat different than the dominant-recessive relationship as in the case of allelic genes.

It is more correct to talk not about the interaction of genes, but about the interaction of their products, i.e., the interaction of proteins that are synthesized on the basis of genes.

Complementary interaction of non-allelic genes- this is their interaction in which their products complement each other’s actions.

An example of complementary gene interaction is the eye color of the Drosophila fly. Flies with the S-B genotype have ordinary red eyes, ssbb - white, S-bb - brown, ssB- - bright scarlet. Thus, if both non-allelic genes are recessive, then no pigment is synthesized and the eyes become white. If only the dominant S gene is present, a brown pigment appears, and only the dominant B gene appears bright scarlet. If there are two dominant genes, then their products interact with each other, forming the red color.

With complementary interaction of genes when crossing heterozygotes (AaBb), different phenotypic splittings are possible (9:6:1, 9:3:3:1, 9:3:4, 9:7).

Epistasis- this is the interaction of non-allelic genes when the action of one gene suppresses the action of another. Both a dominant and a recessive allele of a given gene can have an epistatic (suppressive) effect on another gene. Phenotype cleavage in dominant epistasis differs from recessive epistasis. An epistatic gene is usually designated by the letter I.

An example of epistasis is the appearance of colored plumage in the second generation when crossing white chickens of different breeds. Some have genotype IIAA, others have iiaa. F 1 - IaAa. In F 2, the usual genotype splitting occurs: 9I-A-: 3I-aa: 3iiA-: 1iicc. In this case, birds with genotype iiA- are colored, which determines the dominant gene A, which in one parent was suppressed by the dominant inhibitor gene I, and in the other was present only in a recessive form.

At polymeric interaction of non-allelic genes the degree of expression of the trait (its quantity) depends on the number of dominant allelic and non-allelic genes. The more genes involved in polymer interaction, the more different degrees of expression of the trait there are. This occurs during cumulative polymerization, when all genes participate in the accumulation of a trait. With non-cumulative polymerization, the number of dominant genes does not affect the degree of expression of the trait; at least one is sufficient; and a phenotypically distinct form is observed only in individuals in which all polymer genes are recessive.

Polymerism, for example, determines the color of human skin. Four genes (or four pairs of alleles according to other sources) have an effect. Let's consider a situation with two pairs. Then A 1 A 1 A 2 A 2 will determine the darkest color, and a 1 a 1 a 2 a 2 will determine the lightest. Average skin color will appear if any two genes are dominant (A 1 a 1 A 2 a 2, A 1 A 1 a 2 a 2, a 1 a 1 A 2 A 2). The presence of one dominant gene will result in a skin color close to light, but darker, and three dominant genes - close to dark, but lighter.

It happens that one gene determines several traits. This gene action is called pleiotropy. It is clear that here we are not talking about the interaction of genes, but with the multiple actions of one gene.

♦ Properties of genes and features of their manifestation in traits:
■ the gene is discrete in its action, i.e. isolated in its activity from other genes;
■ one gene is responsible for the manifestation of one strictly defined trait or several traits (pleiotropy);
■ one trait can be the result of the action of several genes (allelic or non-allelic);
■ a gene can increase the degree of manifestation of a trait with an increase in the number of its dominant alleles;
■ a gene can interact with other genes; this leads to the appearance of new signs;
■ changes in the position of a gene in a chromosome or the influence of environmental factors can modify its manifestation in traits;
■ a gene has the ability to mutate.

Interaction of allelic genes

Gene interaction- a phenomenon when several genes (or alleles) are responsible for one trait.

Allelic interaction- this is the interaction of alleles of the same gene (several alleles of the same gene are responsible for a trait).

❖ Types of allelic interactions:
■ dominance,
■ incomplete dominance,
■ overdominance,
■ co-dominance.

Domination- type of interaction of two alleles of one gene, when one (dominant) of them completely eliminates action of another (recessive). Examples: dominance in a person of dark hair over light hair, brown eyes over blue.

Incomplete dominance- the degree of activity of the dominant allele is not sufficient to completely suppress the effect of the recessive allele and ensure the full expression of the dominant trait.

■ In this case, heterozygotes develop intermediate(in relation to parental characteristics) sign- occurs intermediate nature of inheritance. This trait will be observed in hybrids first generation And heterozygotes of the second generations. In the second generation, the splitting by phenotype and genotype turns out to be the same 1:2:1 (one part is a dominant homozygote AA with pronounced dominant characteristic, two parts are made up of a heterozygote Ahh with an intermediate trait and one part is homozygote ahh with recessive sign).

■ Examples incomplete dominance: inheritance of the shape (curliness) of hair in humans, the color of cattle, the color of flowers in the night beauty plant (see table).

Overdominance- stronger manifestation of the trait in a heterozygous individual (Ah) than any of the homozygotes (AA and aa).

Codominance- both alleles are equivalent, do not suppress each other and participate in determining the trait in a heterozygous individual. Example: inheritance of blood group IV in humans, which is determined by the simultaneous presence in the genotype of two codominant genes I A and I B. The first of these genes determines the synthesis of antigen protein in erythrocytes A, the second is the synthesis of antigen protein IN; the presence of both of these genes in the genotype leads to the fact that in people with blood group IV, red blood cells contain an antigen protein A, and protein-antigen IN.

Interaction of nonallelic genes

Nonallelic(or interallelic) interaction- this is the interaction of alleles of different genes, i.e. genes located on non-homologous chromosomes or different loci of homologous chromosomes.

■ Non-allelic interaction of genes leads to modification of Mendelian phenotypic segregation 9: 3: 1 , i.e. to the appearance in the offspring of a heterozygote of other splits, for example 9: 3: 4; 9: 6: 1; 12: 3: 1 and etc.

❖ Main types of interallelic interactions:
■ complementarity;
■ epistasis;
■ polymer.

Comment: complementary and epistatic interactions occur in cases where a trait is controlled by one pair of non-allelic genes.

Complementary, or additional, interaction is a type of inter-allelic interaction of genes in which the simultaneous presence in the genotype of a hybrid of dominant genes of different allelic pairs leads to the appearance of a new trait that is absent in both parents.

Example: inheritance of flower color of sweet peas (parental plants with genotypes A-bb, aaB- have white flowers, hybrids with the genotype A-B- purple; see table).

Comment: the “-” sign in the genotype formula means that this place can be occupied by both a dominant and a recessive allele.

Explanation: purple pigment is formed using a special enzyme, which is synthesized only in the presence both dominant genes: how A, so IN. The flowers of the parents are white because only one of these genes is present in the genotype of each of them.

In second generation during self-pollination, which ensures equally probable (random) formation of gametes and zygotes of different types, a splitting according to phenotype is observed in the ratio of purple and white color of flowers as 9: 7 (9 purple: A-B- and 7 white: ZA-Bb, ZaaV-. 1aabb).

Epistasis- this type of inter-allelic interaction of genes in which the alleles of one gene suppress the manifestation of an allelic pair of another gene, and the suppressed trait does not appear.

Suppressor(or inhibitor gene) is a gene that suppresses the action of other non-allelic genes. A suppressor can be either a dominant or a recessive gene.

Dominant epistasis- epistasis in which the suppressor is a dominant gene. With dominant epistasis in the second generation, a splitting phenotype of 12:3:1 or 13:3 is observed.

Recessive epistasis (cryptomeria)- epistasis in which the suppressor is a recessive gene. With recessive epistasis in the second generation, a 9:3:4 phenotype is observed.

Example of epistasis: inheritance of coat color in domestic rabbits. The synthesis of black pigment determines recessive gene With, dominant allele I another gene is a suppressor, suppressing the action of the gene With. Then rabbits with genotypes C-I-, ccl- will be white, rabbits with genotypes C-ii- gray, and with the genotype ccii- black.

Many traits are controlled by two or more pairs of non-allelic genes (called polymer genes in this case).

Polymerism— interaction of several non-allelic polymer genes. With polymerization, the degree of expression of a phenotypic trait often depends on the number of polymer genes responsible for its manifestation. At cumulative polymer gene action sums up ; examples: body weight, milk production of cattle, egg production of chickens, some parameters of human mental abilities, etc. When non-cumulative polymer the degree of manifestation of a trait does not depend on the number of dominant genes in the genotype ( example: feathered legs in chickens).

Pleiotropy- dependence of several traits on one gene. Each pleiotropic gene has some basic effect, but modifies the expression of other genes.

Linkage of genes. Morgan's experiments

Linked genes- any genes located on the same chromosome.

Clutch group- all genes located on the same chromosome.
■ The number of linkage groups is equal to the number of pairs of chromosomes (i.e., the haploid number of chromosomes). Humans have 46 chromosomes, i.e. 23 clutch groups.
■ Inheritance of traits for which genes from the same linkage group are responsible, does not obey Mendel's laws .

T. Morgan's experiments(1911-1912): inheritance analysis two pairs of alternative features in fruit flies - gray (IN) and black (b) body color and normal (V) or shortened (v) wing lengths.

First series of experiments: crossing homozygous dominant (BBVV) individuals (with a gray body color and normal wing length) with a homozygous recessive (bbvv) a black individual with short wings. All descendants of F 1, in accordance with Mendel's first law, are dominant heterozygous (BbVv) individuals of gray color with normal wings.

Second series of experiments: analyzing the crossing of first generation hybrids - homozygous recessive (black short-winged) female (bbvv) with a diheterozygous (gray with normal wings) male (BbVv). If we assume that two genes belonging to different allelic pairs are localized on different chromosomes, then in a diheterozygote we should expect the formation (in equal quantities) of four types of gametes: BV, bV, Bv and bv. Then, according to Mendel’s third law, four different phenotypes should be present in the offspring in equal numbers (25% each). Actually present only two phenotypes (in a 1:1 ratio).

■ This means that dominant genes IN And V, belonging to different allelic pairs, are localized on one chromosome (from a pair of homologous chromosomes) and fall into one gamete, and both recessive genes V And v localized on another chromosome and together enter another gamete. Therefore, a diheterozygous male Drosophila fly produces not four types of gametes (when the genes are located on different chromosomes), but only two: B.V.(50%) and bv(50%), and, therefore, the descendants of F 2 will have two combinations of traits.

Third series of experiments: testing the assumption of complete gene linkage by analysis cross diheterozygous (gray with normal wings) female (BbVv) from generation F 1 with a homozygous recessive (black short-winged) male (bbvv) from the parental generation. As a result, descendants of four phenotypes were obtained in the following ratio: 41.5% of individuals each with a gray body and normal wings (genotype BbVv) and individuals with a black body and short wings (genotype bbvv), and 8.5% of gray short-winged individuals (genotype Bbvv) and black individuals with normal wings (genotype bbVv).

It follows that linked genes, i.e. genes localized on the same chromosome are not always transmitted together, i.e. the clutch may be incomplete. This is due to the phenomenon crossing over . the probability of which in this case is 17%.

Crossing over

Crossing over- the phenomenon of exchange of sections of homologous chromatids during their conjugation in prophase of meiosis I.
■ In heterozygous organisms, crossing over leads to recombination of genetic material.
■ Crossing over does not always occur; its frequency depends on the distance between genes (see below for more information on the distance between genes).
■ Crossing over occurs in all plants and animals, with the exception of the male Drosophila fly and the female silkworm.
■ Crossover value: it allows you to create new combinations of genes and thereby increase the hereditary variability necessary to expand the ability of organisms to adapt to changed environmental conditions.

Recombination- the emergence of new gene combinations as a result of crossing over, the free combination of chromosomes during the formation of gametes or during their fusion during fertilization.

Crossover (or recombinant) individuals are individuals formed from gametes with a new combination of alleles obtained by crossing over.

Morgan's Law (Law of Linked Inheritance): genes located on the same chromosome (i.e. included in the same linkage group) are inherited preferentially, i.e. most likely, together (linked).

Chromosomal theory of heredity

The chromosomal theory of heredity was experimentally substantiated by T. Morgan and his colleagues in 1911-1926.

♦Basic provisions of the chromosomal theory of heredity:
■ genes responsible for the storage and transmission of hereditary information are localized in certain regions (loci) of chromosomes; different chromosomes have different numbers of genes;
■ allelic genes occupy identical loci on homologous chromosomes;
■ on chromosomes, genes are located in a linear sequence (one after another) and do not overlap;
■ genes of homologous chromosomes form a linkage group and are inherited predominantly together; the number of linkage groups is equal to the haploid set of chromosomes (i.e., the number of pairs of homologous chromosomes);
■ exchange of allelic genes (crossing over) is possible between homologous chromosomes;
■ the probability of crossing over is proportional to the distance between genes on chromosomes.

Genetic maps

Genetic map of chromosomes- a diagram that displays the relative position of genes located in the same linkage group, taking into account their sequence order and the relative distances between them.
■ The ability to map chromosomes is based on the constant percentage of crossing over between specific genes.

The distance between genes is expressed in morganids.

■ One Morganid- the distance between genes at which the probability of crossing over is 1%.

♦ The meaning of genetic maps:
■ in breeding, they make it possible to predict the possibility of obtaining organisms with certain combinations of traits;
■ are the basis for genetic engineering;
■ in medicine they are used to diagnose a number of severe hereditary human diseases;
■ comparison of genetic maps of different species of living organisms helps to establish the features of the evolutionary process.

Cytoplasmic inheritance- heredity associated with the action of genes located in cytoplasmic organelles containing DNA (mitochondria and plastids).
■ Such genes are capable of autonomous replication and uniform distribution between daughter cells.
■ In the transmission of characters, cytoplasmic inheritance is of secondary importance.
■ Cytoplasmic inheritance occurs only through the maternal organism (in male germ cells there is little cytoplasm and there are no mitochondria and plastids in it). Examples: mutations in genes localized in mitochondria are associated with the inheritance of disorders in the action of respiratory enzymes in yeast; In particular, the inheritance of variegation in a number of plants (night beauty, snapdragon, etc.) is associated with mutations of genes localized in plastids.

Interaction of allelic genes in the genotype (depending on the phenotypic effect): dominance, incomplete dominance, codominance, interallelic complementation, allelic exclusion.

Domination- this is an interaction of allelic genes in which the manifestation of one of the alleles (A) does not depend on the presence of another allele (A") in the genotype and heterozygotes AA" do not differ phenotypically from homozygotes for this allele (AA). At incomplete dominance first generation hybrids have a phenotype intermediate between the phenotypes of the parents; in second-generation hybrids, the splitting is 1:2:1 both by phenotype and genotype, since each genotype has its own phenotype; splitting on the basis of colored: uncolored is 3:1. A demonstration of incomplete dominance can be hereditary diseases in humans, which manifest themselves clinically in heterozygotes for mutant alleles, and in homozygotes ending in death (sickle cell anemia). Sometimes heterozygotes have an almost normal phenotype, and homozygotes are characterized by reduced viability.

Codominance- a type of interaction of allelic genes, when at the level of the final trait the products of both genes appear in the phenotype (for example, the formation of the blood group IV (AB) trait in a person).

Interallelic complementation– a type of interaction of allelic genes, when due to the formation of a hybrid protein in a heterozygote, the normal phenotype is restored. This phenomenon can occur if both allelic genes are mutant, but the mutation is in different parts of the genes.

Allelic exclusion– a type of interaction of allelic genes, when one of the allelic genes (subgenes or an entire chromosome) from a pair does not work - the gene product is not formed (for example, turning off a subgene during the synthesis of antibodies or heterochromatinization of one of the X chromosomes in women).

Types of interaction of non-allelic genes: modifying influence, complementarity, epistasis, gene position effect.

Modifying influence is a type of interaction of non-allelic genes when the product of one pair of genes modifies (changes) the phenotypic effect of another pair of genes. Modifier genes affect the penetrance or expressivity of another gene. Modifier gene in the ABO(H) blood group system: the presence of A, B or H antigens in saliva (and other secretions) depends on the secretory gene Se (located in chrome 19). Secretaries: SeSe, Sese. Nonsecretors: sese. For example: ABSeSe, ABSese - antigens A and B are detected in saliva. ABsese - antigens A and B are not detected in saliva. OOSese - antigen H is detected in saliva.

Complementarity- a type of interaction of non-allelic dominant genes, as a result of which a new final character is formed.

A and B are complementary genes that determine the development of normal hearing.

R AaBv x AaBv

normal.sl normal.sl

F AB Av aV av

normal.sl. deaf

Epistasis is a type of interaction of non-allelic genes, when an allele from one pair of genes suppresses (enhances) the phenotypic effect of another pair of genes. With dominant epistasis, when the dominant allele of one gene (A) prevents the expression of another gene (B or b), segregation in the offspring depends on their phenotypic significance and can be expressed in a ratio of 12:3:1 or 13:3. In recessive epistasis, the gene that determines a certain trait (B) does not appear in homozygotes for the recessive allele of another gene (aa). The splitting in the offspring of two diheterozygotes for such genes will correspond to a ratio of 9:3:4.

Gene position effect- the phenotypic effect of a gene depends on neighboring genes. If a gene ends up in the heterochromatin zone as a result of gene recombination, its activity will be reduced.

General characteristics of the interaction: a) allelic genes, b) non-allelic genes.

15. Patterns of linked inheritance of traits. Clutch groups. ( Cis- and trans-phases of gene linkage. Full and incomplete clutch. Crossing over, its genetic effect. Syntenic genes. Detection of linkage based on the results of analyzing crosses. Application of results on close linkage of genes for the purposes of medical genetic counseling. Genetic maps of human chromosomes.)

At linked inheritance non-allelic genes are located in one pair of homologous chromosomes. Each chromosome represents clutch group genes. The number of linkage groups in a diploid organism is equal to the haploid set of chromosomes (in women - 23 G.S., in men - 24).

Phases of gene linkage:

Cis phase A B Gametes: AB and av a in 50% 50%

If the genes are in the cis phase (both dominant genes are localized on one chromosome, and their recessive alleles on the other): gametes AB and ab (50% each), genotype of the offspring AaBb and aabb (50% each).

Trans phase A to Gametes: Av and aB

If the genes are in the trans phase (one dominant gene is localized in one chromosome, and the other in a homologous one): gamete types - Ab and aB (50% each), genotype of the offspring Aabb, aaBb (50% each).

Full grip– crossing over does not occur. Linked genes are always inherited together. Examples: rRNA genes, 40 to 50 copies on each nucleolus-forming chromosome.

Incomplete clutch– crossing over occurs, the frequency of crossing over depends on the distance between linked genes: close grip– crossing over occurs rarely, genes are more often inherited together, examples: Rh-complex genes (SDE) on chromosome 1, HLA-complex genes (AVSD) on chromosome 6; synthetic clutch– crossing over often occurs between genes located far from each other on a large chromosome ( syntenic genes), syntenic genes are inherited almost independently.

Cause of clutch failure - crossing over– exchange of homologous chromosomes by homologous regions occurs in prophase I of meiosis. The frequency of linkage failure is constant for each pair of linked genes. Crossing over occurs more often in women than in men. The biological significance of crossing over is that it increases combinative variability. In case of incomplete linkage, the diheterozygote produces 4 types of gametes and 4 phenotypic classes in the offspring in unequal quantitative ratios (and there are always fewer crossover recombinant individuals). Gametes: AB and AB are non-crossover, more of them are formed, AB and AB are crossover, fewer of them are formed. When crossover gametes merge, recombinants are formed (individuals whose genetic information is recombined). The percentage of individuals resulting from the fusion of crossover gametes depends on the distance between the genes. The strength of linkage between genes is inversely proportional to the distance between them. The unit of distance between genes is the conventional unit - morganida. 1 morganid corresponds to the distance in the chromosome at which crossing over occurs in 1% of gametes. When the distance between genes is 50 or more morganids, traits are inherited independently. Crossing over can be single, double (multiple). Crossing over frequency is used to map chromosomes (determine the order of genes on a chromosome and the relative distance between them).

Linked inheritance differs from independent inheritance in the quantitative ratio of gametes in descendants, which is revealed when analyzing dihybrid cross.

Gene position effect– change in the phenotypic effect of genes when they are closely linked. Rh complex (CDE, cde) - antigens are detected: C, D, E, c, d, e. Antigen-D is the strongest, it determines positive Rh. All others are negative.

Genotypes:

CDe - genes C and D are linked in cis-phase, while the activity of gene D is reduced by gene C

cde and blood gives a weakly positive reaction, because little D antigen.

Cde - genes C and D are linked in trans-phase. Gene C has no effect on activity

cDe gene D and blood gives a normal positive reaction

Genetic map of a chromosome– diagram of the relative arrangement of genes located in the same linkage group. The distance between genes on the genetic map of a chromosome is determined by the frequency of crossing over between them.

In some cases, the action of different genes is relatively independent, but, as a rule, the manifestation of traits is the result of the interaction of products of different genes. These interactions may be related to both allelic, so with non-allelic genes.

Interaction between alleles genes are carried out in three forms: complete dominance, incomplete dominance and independent manifestation (co-dominance).

Previously, Mendel's experiments were reviewed, which revealed the complete dominance of one allele and the recessivity of the other. Incomplete dominance is observed when one gene from a pair of alleles does not provide the formation of its protein product sufficient for the normal manifestation of the trait. With this form of gene interaction, all heterozygotes and homozygotes differ significantly in phenotype from each other. At co-dominance in heterozygous organisms, each of the allelic genes causes the formation of a trait controlled by it in the phenotype. An example of this form of interaction of alleles is the inheritance of human blood groups according to the ABO system, determined by the I gene. There are three alleles of this gene, Io, Ia, Ib, which determine blood group antigens. The inheritance of blood groups also illustrates the phenomenon plural allelism: in the gene pools of human populations, gene I exists in the form of three different alleles, which are combined in individual individuals only in pairs.

Interaction of nonallelic genes. In some cases, one trait of an organism can be influenced by two (or more) pairs of non-allelic genes. This leads to significant numerical deviations of phenotypic (but not genotypic) classes from those established by Mendel during dihybrid crossing. The interaction of non-allelic genes is divided into main forms: complementarity, epistasis, polymerization.

At complementary interaction, the trait manifests itself only in the case of the simultaneous presence of two dominant non-allelic genes in the genotype of the organism. An example of a complementary interaction is crossing two different varieties of sweet peas with white flower petals.

The next type of interaction of non-allelic genes is epistasis, in which the gene of one allelic pair suppresses the effect of the gene of the other pair. A gene that suppresses the action of another is called epistatic genome(or suppressor). The suppressed gene is called hypostatic. Epistasis can be dominant or recessive. An example of dominant epistasis is the inheritance of plumage color in chickens. Gene C in a dominant form determines normal pigment production, but the dominant allele of another gene I is its suppressor. As a result of this, chickens that have a dominant allele of the color gene in their genotype turn out to be white in the presence of a suppressor. The epistatic effect of a recessive gene illustrates the inheritance of coat color in house mice. The color of agouti (reddish-gray coat color) is determined by the dominant gene A. Its recessive allele a, in the homozygous state, causes black coloration. The dominant gene of another pair C determines the development of pigment; homozygotes for the recessive allele c are albinos with white fur and red eyes (lack of pigment in the fur and iris of the eyes).

Inheritance of a trait, the transmission and development of which is determined, as a rule, by two alleles of one gene, is called monogenic. In addition, genes from different allelic pairs are known (they are called polymeric or polygenes), having approximately the same effect on the trait.

The phenomenon of simultaneous action on a trait of several non-allelic genes of the same type is called polymerization. Although polymer genes are not allelic, since they determine the development of one trait, they are usually designated by one letter A (a), with numbers indicating the number of allelic pairs. The action of polygenes is most often additive.

Chained inheritance

An analysis of the simultaneous inheritance of several traits in Drosophila, carried out by T. Morgan, showed that the results of analytical crossing of F1 hybrids sometimes differ from those expected in the case of their independent inheritance. In the descendants of such a cross, instead of freely combining the characteristics of different pairs, a tendency was observed to inherit predominantly parental combinations of characteristics. This inheritance of traits was called linked. Linked inheritance is explained by the location of the corresponding genes on the same chromosome. As part of the latter, they are transmitted from generation to generation of cells and organisms, preserving the combination of alleles of the parents.

The dependence of linked inheritance of traits on the localization of genes on one chromosome gives grounds to consider chromosomes as separate clutch groups. An analysis of the inheritance of the eye color trait in Drosophila in T. Morgan's laboratory revealed some features that forced us to distinguish it as a separate type of inheritance of traits. sex-linked inheritance.

The dependence of the experimental results on which parent was the carrier of the dominant variant of the trait allowed us to suggest that the gene that determines eye color in Drosophila is located on the X chromosome and does not have a homologue on the Y chromosome. All features of sex-linked inheritance are explained by the unequal dose of the corresponding genes in representatives of different sexes - homo- and heterogametic. The X chromosome is present in the karyotype of each individual, therefore the characteristics determined by the genes of this chromosome are formed in both female and male representatives. Individuals of the homogametic sex receive these genes from both parents and pass them on to all offspring through their gametes. Representatives of the heterogametic sex receive a single X chromosome from the homogametic parent and pass it on to their homogametic offspring. In mammals (including humans), the male sex receives X-linked genes from the mother and passes them on to daughters. At the same time, the male sex never inherits the paternal X-linked trait and does not pass it on to his sons

Actively functioning genes on the Y chromosome, which do not have alleles on the X chromosome, are present in the genotype only of the heterogametic sex, and in a hemizygous state. Therefore, they manifest themselves phenotypically and are transmitted from generation to generation only in representatives of the heterogametic sex. Thus, in humans, the sign of hypertrichosis of the auricle (“hairy ears”) is observed exclusively in men and is inherited from father to son.

Cytoplasmic inheritance

The presence of a certain amount of hereditary material in the cytoplasm in the form of circular DNA molecules of mitochondria and plastids, as well as other extranuclear genetic elements, gives reason to specifically focus on their participation in the formation of the phenotype in the process of individual development. Cytoplasmic genes do not obey Mendelian patterns of inheritance, which are determined by the behavior of chromosomes during mitosis, meiosis and fertilization. Due to the fact that the organism formed as a result of fertilization receives cytoplasmic structures mainly with the egg, cytoplasmic inheritance of characteristics occurs through the maternal line. This type of inheritance was first described in 1908 by K. Correns in relation to the trait of variegated leaves in some plants.