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What is the interaction of genes. Interaction of allelic and non-allelic genes. The phenomenon of pleiotropy. Non-allelic genes and their interaction

With the accumulation of scientific experience, contradictions appeared with Mendel's third law of independent inheritance. The offspring were divided by phenotype in a ratio of 15:1 or 9:7, and not 9:16 according to Mendel. This indicates a certain relationship of non-allelic genes.

Mechanism

Non-allelic genes are located in different parts of the chromosomes and encode different types of proteins. Genes do not directly affect each other, so the interaction occurs in the cytoplasm at the level of proteins that are encoded by certain genes.

Rice. 1. Non-allelic genes.

The interaction mechanism can proceed according to one of three scenarios:

simultaneous action of two enzymes that encode two non-allelic genes;
one non-allelic gene forms a protein that affects the work of another non-allelic gene (suppresses or activates);
two proteins encoded by two non-allelic genes act on the same process, enhancing or restoring the same trait.

One gene may be responsible for several phenotypic traits, or several genes may be responsible for one trait.

Kinds

There are several types of interaction of non-allelic genes, the main of which are described in detail in the table.

Rice. 2. Complementarity.

*View*	*Description*	*Example*
	A trait caused by two different genes appears only when two dominant alleles are combined. Such genes are called complementary. A trait is not formed in the absence of one gene. Segregation of phenotypic traits in F2 occurs in the ratio 9:7, 9:6:1, 9:3:4	A cross between sweet peas and white flowers. In F1, all offspring have purple flowers, because. a combination of dominant genes A and B encode an anthocyanin, giving a purple color. Individual genes do not form purple. Splitting occurs in F2 - 9 magenta (AB), 7 white (3 - Abb, 3 - aaB, 1 - aabb)
	One pair of genes suppresses the other, preventing the phenotypic trait from manifesting itself. The suppressing gene is called epistatic (suppressor or inhibitor gene), the suppressed is called hypostatic. The inhibitor is designated by the letter I, i. Epistasis can be dominant - suppression by the dominant gene (I>B, b) and recessive - suppression by the recessive gene (i>B, b). With dominance, gene splitting occurs in the ratio 7:6:3, 12:3:1, 13:3, with recessive manifestation - 9:3:4, 9:7, 13:3	Coloring of oat grain: A - black, B - gray. In F1, all grains will be black if gene A is epistatic (AaBB or IiBB). In F2, there will be a splitting according to the color of the grain - 12 black, 3 gray and 1 white. In 12 plants, the I-gene is necessarily present, in 3 it will be in a recessive state - i. One plant will get the iibb genes (no black and gray), so it will be white
Polymerism	Quantitative or dimensional traits that cannot be clearly separated by phenotype (height, amount of milk, fat content of livestock) are determined by a combination of genes. There are cumulative and non-cumulative types. In the first case, the manifestation of a trait depends on the sum of the actions of the genes (the more dominant genes, the brighter the trait). In the second case, the trait manifests itself with a dominant gene, the number of genes does not affect the manifestation of the phenotype. With a cumulative form in F2, splitting is observed in a ratio of 1:4:6:4:1, with a non-cumulative - 15:1. Polymer genes are designated with a single letter (A, a, B, b, etc.), and alleles with a number. For example, A1a1A2a2	Human skin color depends on the action of four genes: A1A1A2A2 - black, a1a1a2a2 - white, A1A1A2a2, A1a1A2A2, A1a1A2a2, A1A1a2a2, a1a1A2A2, A1a1a2a2, a1a1A2a2 - intermediate values from dark (almost black) to light (almost white) shade

Rice. 3. Epistasis.

The multiple action of genes is called pleiotropy. The action of one gene, as a rule, is due to interaction with other genes. Most genes have this effect, so the genotype is a system of interacting genes.

What have we learned?

Learned briefly about the types of interaction of non-allelic genes. There are three types of interaction - complementarity, epistasis, polymerization. For a trait to be complementary, two dominant genes must be present. Epistasis is characterized by suppression of the action of the second gene by one gene. Polymeria - the interaction of a set of genes. The interaction of many genes is called pleiotropy.

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Now let us turn to the problem of the interaction of non-allelic genes. If the development of a trait is controlled by more than one pair of genes, then this means that it is under polygenic control. Several main types of gene interaction have been established: complementarity, epistasis, polymerization and pleiotropy.

The first case of non-allelic interaction was described as an example of a deviation from Mendel's laws by the English scientists W. Betson and R. Pennet in 1904 when studying the inheritance of the comb shape in chickens. Different breeds of chickens are characterized by different comb shapes. Wyandottes have a low, regular, papilla-covered crest, known as the "pink". Brahms and some fighting chickens have a narrow and high crest with three longitudinal elevations - “pea-shaped”. Leghorns have a simple or leaf-shaped crest, consisting of a single vertical plate. Hybridological analysis showed that the simple comb behaves as a completely recessive trait in relation to the rose and pea. The splitting in F 2 corresponds to the formula 3: 1. When crossing races with a rose-shaped and pea-shaped comb, hybrids of the first generation develop a completely new shape of the comb, resembling a half of a walnut kernel, in connection with which the comb was called "nut-shaped". When analyzing the second generation, it was found that the ratio of different comb shapes in F 2 corresponds to the formula 9: 3: 3: 1, which indicated the dihybrid nature of the crossing. A crossover scheme was developed to explain the mechanism of inheritance of this trait.

Two non-allelic genes are involved in determining the shape of the crest in chickens. The dominant R gene controls the development of the pink crest, and the dominant P gene controls the development of the pisiform. The combination of recessive alleles of these rrpp genes causes the development of a simple crest. The walnut crest develops when both dominant genes are present in the genotype.

The inheritance of the crest shape in chickens can be attributed to the complementary interaction of non-allelic genes. Complementary, or additional, are genes that, when combined in the genotype in a homo- or heterozygous state, determine the development of a new trait. The action of each of the genes individually reproduces the trait of one of the parents.

Scheme illustrating the interaction of non-allelic genes,
determining the shape of the comb in chickens

The inheritance of the genes that determine the shape of the crest in chickens fits perfectly into the dihybrid cross scheme, since they behave independently during distribution. The difference from the usual dihybrid crossing is manifested only at the level of the phenotype and boils down to the following:

F 1 hybrids are not similar to either parent and have a new trait;
In F 2, two new phenotypic classes appear, which are the result of the interaction of either dominant (nut-shaped comb) or recessive (simple comb) alleles of two independent genes.

Mechanism complementary interaction studied in detail on the example of the inheritance of eye color in Drosophila. The red color of the eyes in wild-type flies is determined by the simultaneous synthesis of two pigments, brown and bright red, each of which is controlled by a dominant gene. Mutations affecting the structure of these genes block the synthesis of either one or the other pigment. Yes, a recessive mutation. brown(the gene is located on the 2nd chromosome) blocks the synthesis of a bright red pigment, and therefore the homozygotes for this mutation have brown eyes. recessive mutation scarlet(the gene is located on the 3rd chromosome) disrupts the synthesis of brown pigment, and therefore homozygotes stst have bright red eyes. With the simultaneous presence in the genotype of both mutant genes in the homozygous state, both pigments are not produced and the eyes of the flies are white.

In the described examples of complementary interaction of non-allelic genes, the phenotype splitting formula in F 2 corresponds to 9: 3: 3: 1. Such splitting is observed if the interacting genes individually have an unequal phenotypic manifestation and it does not coincide with the phenotype of the homozygous recessive. If this condition is not met, other ratios of phenotypes take place in F 2 .

For example, when two varieties of figured pumpkin with a spherical fruit are crossed, the hybrids of the first generation have a new feature - flat or disc-shaped fruits. When hybrids are crossed with each other in F 2, splitting is observed in the ratio of 9 disc-shaped: 6 spherical: 1 elongated.

Analysis of the scheme shows that two non-allelic genes with the same phenotypic manifestation (spherical shape) are involved in determining the shape of the fetus. The interaction of the dominant alleles of these genes gives a disc-shaped form, the interaction of recessive alleles - an elongated one.

Another example of complementary interaction is the inheritance of coat color in mice. The wild gray coloration is determined by the interaction of two dominant genes. Gene BUT responsible for the presence of the pigment, and the gene IN for its uneven distribution. If only the gene is present in the genotype BUT (A-bb), then the mice are uniformly colored black. If only the gene is present IN (aaB-), then the pigment is not produced and the mice are unstained, as is the homozygous recessive aabb. This action of the genes leads to the fact that in F 2 the splitting according to the phenotype corresponds to the formula 9: 3: 4.

	AB	Ab	aB	ab
AB	AABB ser.	AABb ser.	AaBB ser.	AaBb ser.
Ab	AABb ser.	AAbb black	AaBb ser.	Aabb black
aB	AaBB ser.	AaBb ser.	aaBB white	aaBb white
ab	AaBb ser.	Aabb black	aaBb white	aabb white

F 2: 9 Ser. : 3 black : 4 Bel.

A complementary interaction has also been described in the inheritance of flower color in sweet peas. Most of the varieties of this plant have purple flowers with purple wings, which are characteristic of the wild Sicilian race, but there are also varieties with a white color. By crossing plants with purple flowers with plants with white flowers, Betsson and Pennet found that the purple color of the flowers completely dominates the white, and in F 2 there is a ratio of 3: 1. But in one case, from crossing two white plants, offspring were obtained, consisting only of plants with colored flowers. During self-pollination of F 1 plants, offspring were obtained, consisting of two phenotypic classes: with colored and uncolored flowers in the ratio 9/16: 7/16.

The results obtained are explained by the complementary interaction of two pairs of non-allelic genes, the dominant alleles of which ( FROM And R) individually are not able to provide the development of purple color, as well as their recessive alleles ( ssrr). Coloring appears only if both dominant genes are present in the genotype, the interaction of which ensures the synthesis of the pigment.

purple
F2

	CP	cp	cP	cp
CP	CCPP purple	CCPp purple	CCPP purple	CcPp purple
cp	CCPp purple	CCpp white	CcPp purple	ccpp white
cP	CCPP purple	CcPp purple	ccPP white	ccPp white
cp	CcPp purple	ccpp white	ccPp white

F 2: 9 magenta : 7 Bel.

In the given example, the splitting formula in F 2 - 9: 7 is due to the absence of their own phenotypic manifestation in the dominant alleles of both genes. However, the same result is also obtained if the interacting dominant genes have the same phenotypic expression. For example, when crossing two varieties of corn with purple grains in F 1, all hybrids have yellow grains, and in F 2 there is a splitting of 9/16 yellow. : 7/16 fiol.

epistasis- another type of non-allelic interaction, in which the suppression of the action of one gene by another non-allelic gene occurs. A gene that prevents the expression of another gene is called epistatic, or a suppressor, and one whose action is suppressed is called hypostatic. Both a dominant and a recessive gene can act as an epistatic gene (respectively, dominant and recessive epistasis).

An example of dominant epistasis is the inheritance of coat color in horses and fruit color in pumpkins. The inheritance pattern of these two traits is exactly the same.

	CB	Cb	cB	cb
CB	CCBB ser.	CCBB ser.	CCBB ser.	CcBb ser.
Cb	CCBb ser.	CCbb ser.	CcBb ser.	ccbb ser.
cB	CCBB ser.	CcBb ser.	ccBB black	ccBb black
cb	CcBb ser.	ccbb ser.	ccBb black	ccbb red

F 2: 12 Ser. : 3 black : 1 red

The scheme shows that the dominant gene for gray color FROM is epistatic with respect to the dominant gene IN, which causes the black color. In the presence of a gene FROM gene IN does not show its effect, and therefore F 1 hybrids carry a trait determined by the epistatic gene. In F 2, the class with both dominant genes merges in phenotype (gray color) with the class in which only the epistatic gene is present (12/16). Black color appears in 3/16 hybrid offspring, in the genotype of which there is no epistatic gene. In the case of a homozygous recessive, the absence of a suppressor gene allows the recessive c gene to appear, which causes the development of a red color.

Dominant epistasis has also been described in the inheritance of feather color in chickens. The white color of the plumage in Leghorn chickens dominates over the colored black, pockmarked and other colored breeds. However, the white coloration of other breeds (such as Plymouth Rocks) is recessive in relation to colored plumage. Crosses between individuals with a dominant white color and individuals with a recessive white color in F 1 produce white offspring. In F 2, splitting is observed in a ratio of 13: 3.

An analysis of the scheme shows that two pairs of non-allelic genes are involved in determining feather color in chickens. Dominant gene of one pair ( I) is epistatic with respect to the dominant gene of the other pair, causing color development ( C). In this regard, only those individuals whose genotype contains the gene FROM, but no epistatic gene I. In recessive homozygotes ccii they lack an epistatic gene, but they do not have a gene that provides pigment production ( C), so they are white in color.

As an example recessive epistasis you can consider the situation with the albinism gene in animals (see above for the inheritance pattern of coat color in mice). The presence in the genotype of two alleles of the albinism gene ( aa) does not allow the dominant color gene to appear ( B) — genotypes aaB-.

Polymer type of interaction was first established by G. Nielsen-Ehle while studying the inheritance of grain color in wheat. When crossing a red-grain wheat variety with a white-grain one in the first generation, the hybrids were colored, but the color was pink. In the second generation, only 1/16 of the offspring had a red grain color and 1/16 - white, the rest had an intermediate color with varying degrees of expression of the trait (from pale pink to dark pink). The analysis of splitting in F 2 showed that two pairs of non-allelic genes are involved in determining the color of the grain, the action of which is summed up. The severity of the red color depends on the number of dominant genes in the genotype.

Polymeric genes are usually denoted by the same letters with the addition of indices, in accordance with the number of non-allelic genes.

The action of dominant genes in this crossing is additive, since the addition of any of them enhances the development of the trait.

	A 1 A 2	A 1 a 2	a 1 A 2	a 1 a 2
A 1 A 2	A 1 A 1 A 2 A 2 red	A 1 A 1 A 2 Aa 2 bright pink.	A 1 a 1 A 2 A 2 bright pink.	A 1 a 1 A 2 a 2 pink
A 1 a 2	A 1 A 1 A 2 a 2 bright pink.	A 1 A 1 a 2 a 2 pink	A 1 a 1 A 2 a 2 pink	A 1 a 1 a 2 a 2 pale pink.
a 1 A 2	A 1 a 1 A 2 A 2 bright pink.	A 1 a 1 A 2 a 2 pink	a 1 a 1 A 2 A 2 pink	a 1 a 1 A 2 a 2 pale pink.
a 1 a 2	A 1 a 1 A 2 a 2 pink	A 1 a 1 a 2 a 2 pale pink.	a 1 a 1 A 2 a 2 pale pink.	a 1 a 1 a 2 a 2 white

F 2: 15 color : 1 Bel.

The described type of polymerization, in which the degree of development of a trait depends on the dose of the dominant gene, is called cumulative. This nature of inheritance is common for quantitative traits, which should include color, as well. its intensity is determined by the amount of pigment produced. If we do not take into account the degree of coloring, then the ratio of colored and uncolored plants in F 2 corresponds to the formula 15: 1.

However, in some cases, polymerization is not accompanied by a cumulative effect. An example is the inheritance of the form of seeds in a shepherd's purse. Crossing of two races, one of which has triangular fruits, and the other ovoid, gives in the first generation hybrids with a triangular fruit shape, and in the second generation splitting according to these two characters is observed in the ratio of 15 triangles. : 1 eggs.

This case of inheritance differs from the previous one only at the phenotypic level: the absence of a cumulative effect with an increase in the dose of dominant genes determines the same severity of the trait (triangular shape of the fetus), regardless of their number in the genotype.

The interaction of non-allelic genes also includes the phenomenon pleiotropy- multiple action of the gene, its influence on the development of several traits. The pleiotropic effect of genes is the result of a serious metabolic disorder due to the mutant structure of this gene.

For example, Irish cows of the Dexter breed differ from the closely related Kerry breed by shortened legs and head, but at the same time by better meat qualities and fattening ability. When crossing cows and bulls of the Dexter breed, 25% of the calves have signs of the Kerry breed, 50% are similar to the Dexter breed, and in the remaining 25% of cases, miscarriages of ugly bulldog calves are observed. Genetic analysis made it possible to establish that the cause of death of part of the offspring is the transition to the homozygous state of a dominant mutation that causes underdevelopment of the pituitary gland. In the heterozygote, this gene leads to the appearance of dominant traits of short legs, short head and increased ability to deposit fat. In the homozygote, this gene has a lethal effect, i.e. in relation to the death of offspring, it behaves like a recessive gene.

The lethal effect upon transition to the homozygous state is characteristic of many pleiotropic mutations. Thus, in foxes, dominant genes that control the platinum and white-faced fur colors, which do not have a lethal effect in the heterozygote, cause the death of homozygous embryos at an early stage of development. A similar situation occurs with the inheritance of gray wool color in Shirazi sheep and underdevelopment of scales in mirror carp. The lethal effect of mutations leads to the fact that animals of these breeds can only be heterozygous and, with intrabreeding, they give splitting in the ratio of 2 mutants: 1 norm.

F1
F 1: 2 boards : 1 black

However, most lethal genes are recessive, and individuals heterozygous for them have a normal phenotype. The presence of such genes in the parents can be judged by the appearance in the offspring of homozygous freaks, abortions and stillborns. Most often, this is observed in closely related crosses, where parents have similar genotypes, and the chances of passing harmful mutations into a homozygous state are quite high.

Pleiotropic genes with a lethal effect are found in Drosophila. Yes, dominant genes Curly- upturned wings star- starry eyes Notch— the jagged edge of the wing and a number of others in the homozygous state cause the death of flies in the early stages of development.

Known recessive mutation white, first discovered and studied by T. Morgan, also has a pleiotropic effect. In the homozygous state, this gene blocks the synthesis of eye pigments (white eyes), reduces the viability and fertility of flies, and alters the shape of the testes in males.

In humans, an example of pleiotropy is Marfan's disease (spider finger syndrome, or arachnodactyly), which is caused by a dominant gene that causes increased finger growth. At the same time, it determines the anomalies of the lens of the eye and heart disease. The disease occurs against the background of an increase in intelligence, in connection with which it is called the disease of great people. A. Lincoln, N. Paganini suffered from it.

The pleiotropic effect of the gene, apparently, underlies the correlative variability, in which a change in one trait entails a change in others.

The interaction of non-allelic genes should also include the influence of modifier genes, which weaken or enhance the function of the main structural gene that controls the development of the trait. In Drosophila, modifier genes are known that modify the process of wing venation. At least three modifier genes are known that affect the amount of red pigment in the hair of cattle, as a result of which the coat color in different breeds ranges from cherry to fawn. In humans, modifier genes change the color of the eyes, increasing or decreasing its intensity. Their action explains the different color of the eyes in one person.

The existence of the phenomenon of gene interaction has led to the emergence of such concepts as “genotypic environment” and “gene balance”. Under the genotypic environment is meant the environment in which the newly emerging mutation falls, i.e. the whole complex of genes present in a given genotype. The concept of “gene balance” refers to the ratio and interaction between genes that affect the development of a trait. Usually, genes are designated by the name of the trait that occurs when a mutation occurs. In fact, the manifestation of this feature is often the result of a violation of the function of the gene under the influence of other genes (suppressors, modifiers, etc.). The more complex the genetic control of a trait, the more genes involved in its development, the higher the hereditary variability, since the mutation of any gene disrupts the gene balance and leads to a change in the trait. Consequently, for the normal development of an individual, not only the presence of genes in the genotype is necessary, but also the implementation of the entire complex of inter-allelic and non-allelic interactions.

The basic patterns of inheritance were first developed by Gregor Mendel. Any organism has many hereditary traits. G. Mendel proposed to study the inheritance of each of them regardless of what is inherited by others. Having proved the possibility of inheritance of 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 affect several different traits and, secondly, genes interact with each other. This discovery became the basis for the development of modern theory, which 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 impact of the entire genotype. Modern ideas about the interaction of genes are presented in Fig. one.

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 differ greatly from each other in phenotype.

Codominance- a phenomenon in which both parental traits are manifested in heterozygotes, 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 is white, and the heterozygote has a roan color - some of the hairs are red and some of the hairs are white (Fig. 2).

Rice. 2. Wool 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 loci of the chromosomes. The 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 with respect to both. Knowing that there can be only two of the three alleles in the genotype, we can assume that combinations can correspond to four blood types (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 the chromosomes and encoding unequal proteins. Non-allelic genes can interact with each other. In all cases of gene interaction, Mendelian patterns are strictly observed, with either one gene causing the development of several traits, or, conversely, one trait is manifested under the action of a combination of several genes. The interaction of non-allelic genes manifests itself in four main forms: epistasis, complementarity, polymerism 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. So, two enzymes take part in the formation of chlorophyll in barley, if they are in the genotype together, the green color of chlorophyll develops, if only one gene is found, the plant will have a yellow color. In the absence of both genes, the plant will have a white color and will not be viable.

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

dominant gene - A, is responsible for the white color;

recessive gene - a, for color;

dominant gene - B, is responsible for the black color;

recessive gene - in, for brown color.

At the same time, white color suppresses the manifestation of black (Fig. 4).

Rice. 4. An example of epistasis of white leghorns ()

When crossing the spirit of heterozygotes, white chicken and white rooster, we see the results of crossing in the Punnett lattice: 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 a polymer is the inheritance of skin color in humans. Two pairs of genes are responsible for coloring the skin color in humans:

if all four alleles of these genes are dominant, then the 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 Caucasoid skin type (Fig. 5).

Rice. 5. Polymeria, inheritance of human skin color ()

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 of such a marriage 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 the secondary reactions of biosynthesis.

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

Rice. 6. Marfan syndrome ()

Such a violation leads to the fact that a person develops a dislocation of the lens of the eye, heart valve defects, long and thin fingers, vascular malformations and frequent dislocations of the joints.

Today we learned that the genotype is not a simple collection 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

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

Volna.org().
Bannikov.narod.ru ().
Studopedia.ru ().

Homework

Define allelic genes, name their forms of interaction.
Define non-allelic genes, name their forms of interaction.
Solve the problems given to the topic.

The transmission of traits from generation to generation is due to the interaction between different genes. What is a gene, and what are the types of interaction between them?

What is a gene?

Under the genome at the present time, they mean the unit of transmission of hereditary information. Genes are located in DNA and form its structural sections. Each gene is responsible for the synthesis of a specific protein molecule, which determines the manifestation of a particular trait in humans.

Each gene has several subspecies or alleles, which cause a variety of traits (for example, brown eyes are due to the dominant allele of the gene, while blue is a recessive trait). Alleles are located in the same areas and the transmission of one or another chromosome causes the manifestation of one or another trait.

All genes interact with each other. There are several types of their interaction - allelic and non-allelic. Accordingly, the interaction of allelic and non-allelic genes is distinguished. How do they differ from each other and how do they manifest themselves?

Discovery history

Before the types of interaction of non-allelic genes were discovered, it was generally accepted that it was only possible (if there is a dominant gene, then the trait will appear; if it is not there, then there will be no trait). The doctrine of allelic interaction, which for a long time was the main tenet of genetics, prevailed. Dominance has been carefully studied, and such types of dominance as complete and incomplete dominance, co-dominance and overdominance have been discovered.

All these principles obeyed the first one, which stated the uniformity of hybrids of the first generation.

Upon further observation and study, it was noticed that not all signs were adjusted to the theory of dominance. With a deeper study, it was proved that not only the same genes affect the manifestation of a trait or group of properties. Thus, forms of interaction of non-allelic genes were discovered.

Reactions between genes

As has been said, for a long time the doctrine of dominant inheritance prevailed. In this case, an allelic interaction took place, in which the trait manifested itself only in the heterozygous state. After various forms of interaction of non-allelic genes were discovered, scientists were able to explain hitherto inexplicable types of inheritance and get answers to many questions.

It was found that gene regulation directly depended on enzymes. These enzymes allowed genes to react differently. At the same time, the interaction of allelic and non-allelic genes proceeded according to the same principles and patterns. This led to the conclusion that inheritance does not depend on the conditions in which genes interact, and the reason for the atypical transmission of traits lies in the genes themselves.

Non-allelic interaction is unique, which makes it possible to obtain new combinations of traits that determine a new degree of survival and development of organisms.

Non-allelic genes

Non-allelic genes are those genes that are localized in different parts of non-homologous chromosomes. They have one synthesis function, but they encode the formation of various proteins that cause different signs. Such genes, reacting with each other, can cause the development of traits in several combinations:

One trait will be due to the interaction of several genes that are completely different in structure.
Several traits will depend on one gene.

Reactions between these genes are somewhat more complicated than with allelic interaction. However, each of these types of reactions has its own features and characteristics.

What are the types of interaction of non-allelic genes?

Epistasis.
Polymerism.
Complementarity.
The action of modifier genes.
Pleiotropic interaction.

Each of these types of interaction has its own unique properties and manifests itself in its own way.

It is necessary to dwell in more detail on each of them.

epistasis

This interaction of non-allelic genes - epistasis - is observed when one gene suppresses the activity of another (the suppressing gene is called an epistatic, and the suppressed gene is called a hypostatic gene).

The reaction between these genes can be dominant or recessive. Dominant epistasis is observed when the epistatic gene (usually denoted by the letter I, if it does not have an external, phenotypic manifestation) suppresses the hypostatic gene (it is usually denoted B or b). Recessive epistasis occurs when the recessive allele of the epistatic gene inhibits the expression of any of the alleles of the hypostatic gene.

Splitting according to the phenotypic trait, with each of these types of interactions, is also different. With dominant epistasis, the following picture is more often observed: in the second generation, according to phenotypes, the division will be as follows - 13:3, 7:6:3 or 12:3:1. It all depends on which genes converge.

With recessive epistasis, the division is: 9:3:4, 9:7, 13:3.

complementarity

The interaction of non-allelic genes, in which, when the dominant alleles of several traits are combined, a new, hitherto unseen phenotype is formed, and is called complementarity.

For example, this type of reaction between genes is most common in plants (especially pumpkins).

If the plant genotype has a dominant allele A or B, then the vegetable gets a spherical shape. If the genotype is recessive, then the shape of the fetus is usually elongated.

If there are two dominant alleles (A and B) in the genotype at the same time, the pumpkin becomes disc-shaped. If, however, further crossbreeding is carried out (i.e., this interaction of non-allelic genes with pumpkins of a pure line is continued), then in the second generation, 9 individuals with a disc-shaped shape, 6 with a spherical shape and one elongated pumpkin can be obtained.

Such crossbreeding makes it possible to obtain new, hybrid forms of plants with unique properties.

In humans, this type of interaction determines the normal development of hearing (one gene - the development of the cochlea, the other - the auditory nerve), and in the presence of only one dominant trait, deafness appears.

Polymerism

Often, the manifestation of a trait is based not on the presence of a dominant or recessive allele of a gene, but on their number. The interaction of non-allelic genes - polymerism - is an example of such a manifestation.

The polymeric action of genes can occur with or without cumulative action. During cumulation, the degree of manifestation of a trait depends on the overall gene interaction (the more genes, the more pronounced the trait). The offspring with a similar effect is divided as follows - 1: 4: 6: 4: 1 (the degree of expression of the trait decreases, that is, in one individual the trait is maximally pronounced, in others its extinction is observed up to complete disappearance).

If no cumulative action is observed, then the manifestation of the trait depends on the dominant alleles. If there is at least one such allele, the trait will take place. With a similar effect, splitting in the offspring proceeds in a ratio of 15:1.

Action of modifier genes

The interaction of non-allelic genes, controlled by the action of modifiers, is relatively rare. An example of such an interaction is as follows:

Such an interaction of non-allelic genes in humans is quite rare.

Pleiotropy

With this type of interaction, one gene regulates the expression or affects the degree of expression of another gene.

In animals, pleiotropy manifested itself as follows:

In mice, dwarfism is an example of pleiotropy. It was noticed that when crossing phenotypically normal mice in the first generation, all mice were dwarfed. It was concluded that dwarfism is caused by a recessive gene. Recessive homozygotes stopped growing, their internal organs and glands were underdeveloped. This dwarfism gene influenced the development of the pituitary gland in mice, which led to a decrease in hormone synthesis and caused all the consequences.
Platinum coloration in foxes. Pleiotropy in this case was manifested by a lethal gene, which, when a dominant homozygote was formed, caused the death of embryos.
In humans, the pleiotropic interaction is illustrated by the example of phenylketonuria, as well as

The role of non-allelic interaction

In evolutionary terms, all the above types of interaction of non-allelic genes play an important role. New gene combinations cause the appearance of new features and properties of living organisms. In some cases, these signs contribute to the survival of the organism, in others, on the contrary, they cause the death of those individuals that will stand out significantly among their species.

Non-allelic interaction of genes is widely used in breeding genetics. Some species of living organisms are preserved due to such gene recombination. Other species acquire properties that are highly valued in the modern world (for example, breeding a new breed of animal with greater endurance and physical strength than its parent individuals).

Work is underway on the use of these types of inheritance in humans in order to exclude negative traits from and create a new, defect-free genotype.

Deviation from Mendel's laws is caused by various types of gene interaction (with the exception of complete dominance), due to the genomic level of organization of hereditary material.

There are interactions of allelic and non-allelic genes.

The interaction of genes of one allele is called intraallelic. The following types of it are distinguished: complete dominance, incomplete dominance, overdominance, codominance and allelic exclusion.

The interaction of genes of different alleles is called interallelic. There are the following types of it: complementarity, epistasis, polymerization and "position effect".

At complementarity the presence in one genotype of two dominant (recessive) genes from different allelic pairs leads to the emergence of a new variant of the trait. There are three types of complementary gene interaction.

I. Two dominant non-allelic genes separately do not have a phenotypic manifestation, but complementing each other, they determine a new variant of the trait.

The 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 normal development of the auditory nerve (DdEe). In recessive homozygotes dd the snail will be underdeveloped, and with the genotype her- auditory nerve. People with genotypes D-ee, ddE- and ddee will be deaf.

In mammals and humans, a specific protein is produced to protect against viruses. interferon. Its synthesis in the human body is due to the complementary interaction of two non-allelic genes located in different ( second and fifth) chromosomes.

Human hemoglobin contains 4 polypeptide chains, each of which is encoded by a separate independent gene. Therefore, 4 complementary genes are involved in the synthesis of hemoglobin.

II. One of the dominant complementary genes has a phenotypic expression, and the second does not; their simultaneous presence in the genotype determines a new variant of the trait. So in mice, the agouti coat color is inherited (at the base and at the end of the hair there is a black pigment, and in the middle part there is a yellow ring). Gene BUT determines the synthesis of black pigment, its allele a does not provide information for the synthesis of the pigment. Gene IN distributes the pigment along the hair unevenly, and its allele b- evenly:

Splitting - in the ratio 9:3:4.

III. Each of the complementary genes has its own phenotypic expression; their simultaneous presence in the genotype determines the development of a new variant of the trait. This is how the shape of the comb is inherited in chickens:

Splitting - in the ratio 9:3:3:1.

At epistasis a dominant (recessive) gene from one allelic pair suppresses the action of a dominant (recessive) gene from another allelic pair. This phenomenon is the opposite of complementarity. The suppressor gene is called suppressor (inhibitor) . Distinguish between dominant and recessive epistasis. An example dominant epistasis can serve polydactyly. Sometimes occurs in "perfectly healthy" parents. It is assumed that the effect of this allele in parents was suppressed by other genemi.

An example recessive epistasis is the "Bombay Phenomenon". In a woman who received the allele I B from her mother, 1 (0) blood group was determined phenotypically. In a detailed study, it was found that the action of gene I B (synthesis of antigen B in erythrocytes) was suppressed by a rare recessive gene, which in the homozygous state had an epistatic effect. In the manifestation of some hereditary metabolic diseases (fermentopathies), the main role is played by the epistatic interaction of genes, when the presence or absence of the products of one gene impedes the formation of active enzymes encoded by another gene.

At polymers genes from different allelic pairs affect the degree of manifestation of the same trait. Polymeric genes are usually denoted by a single letter of the Latin alphabet with numerical indices, for example, A 1 A 1 A 2 a 3 a 3, etc. Traits determined by polymeric genes are called polygenic(multifactorial). Thus, many quantitative and some qualitative traits are inherited in animals and humans: height, body weight, blood pressure, skin color, etc. The degree of manifestation of these traits depends on the number of dominant genes in the genotype (the more there are, the more pronounced the trait) and largely on the influence of environmental conditions. A person may have a predisposition to various diseases: hypertension, obesity, diabetes mellitus, schizophrenia, etc. These signs, under favorable environmental conditions, may not appear or be mild. This distinguishes polygenic inherited traits from monogenic ones. By changing environmental conditions and taking preventive measures, it is possible to significantly reduce the frequency and severity of some multifactorial diseases. The summation of "doses" of polymeric genes ( additive action) and the influence of the environment ensure the existence of continuous series of quantitative changes. Human skin pigmentation is determined by five or six polymeric genes. Dominant alleles predominate among the indigenous people of Africa, while recessive alleles predominate among representatives of the Caucasoid race. Mulattos are heterozygous and have intermediate pigmentation. Mulatto parents have both white and black children. The minimum number of polymeric genes at which a trait is expressed is called threshold effect.

Under "position effect" understand the mutual influence of genes of different alleles occupying nearby loci of the same chromosome. It manifests itself in a change in their functional activity. Rh-affiliation of a person is determined by three genes located in the short arm of the first chromosome at a close distance from each other (closely linked). Each of them has a dominant and a recessive allele ( FROM, D, E and c, d, e). Organisms with a set of genes CDE/cDe And CDe/cDE genetically identical (they have the same overall balance of genes). However, in individuals with the first combination of genes, a lot of antigen is formed E and little antigen FROM, and in individuals with the second combination of alleles, on the contrary, there is little antigen E and a lot of antigen FROM. Probably, the close proximity of the E allele to the C allele (the first case) reduces the functional activity of the latter.