The types of interaction of non-allelic genes include. Interaction of allelic genes. II. Dihybrid cross

Allelic genes - genes located in the same regions of homologous chromosomes and controlling the development of variations of one trait.

Non-allelic genes - located in different parts of homologous chromosomes, control the development of different traits.

1. The concept of the action of genes.

A gene is a section of a DNA or RNA molecule that codes for a nucleotide sequence in tRNA and rRNA or an amino acid sequence in a polypeptide.

Characteristics of the action of genes:

The gene is discrete

The gene is specific - each gene is responsible for the synthesis of a strictly specific substance

The gene acts gradually

Pleiotropic action - 1 gene acts on the change or manifestation of several signs (1910 Plate) phenylketonuria, Marfan syndrome

Polymer action - several genes are needed for the expressiveness of a trait (1908 Nilson-Ele)

Genes interact with each other through protein products determined by them.

Environmental factors influence the expression of genes

1. List the types of interaction between allelic and non-allelic genes.

Between alleles:

Complete dominance

incomplete dominance

Codominance

overdominance

Between non-allelic: (a trait or properties are determined by two or more non-allelic 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 Mendeleian patterns of splitting).

Complimentary

• Polymerism

1. The essence of complete dominance. Examples.

Complete dominance - a type of interaction of allelic genes, in which the dominant gene (A) completely suppresses the action of the recessive gene (a) (freckles)

1. incomplete dominance. Examples.

Incomplete dominance - a type of interaction of allelic genes, in which the dominant allele does not completely suppress the action of the recessive allele, forming a trait with an intermediate degree of degeneracy (eye color, hair shape)

1. Overdominance as the basis of heterosis. Examples.

Overdominance is a type of interaction of allelic genes, in which a gene that is in a heterozygous state has a greater phenotypic manifestation of a trait than a homozygous one.

Sickle cell anemia. A - hemoglobin A, a - hemoglobin S. AA - 100% normal red blood cells, more susceptible to malaria; aa - 100% mutated (die), Aa - 50% mutated, practically not susceptible to malaria. already amazed

1. Codominance and its essence. Examples.

Codominance is a type of interaction of allelic genes, in which several alleles of a gene participate in the determination of a trait and a new trait is formed. One allelic gene complements the action of another allelic gene, the new trait differs from the parental ones (ABO blood groups).

The phenomenon of independent manifestation of both alleles in the heterozygous phenotype, in other words, the absence of dominant-recessive relationships between alleles. Most famous example- interaction of alleles that determine the fourth human blood group (AB). A multiple series is known, consisting of three alleles of gene I, which determines the trait of a person's blood group. Gene I is responsible for the synthesis of enzymes that attach certain polysaccharides to proteins located on the surface of red blood cells. (It is these polysaccharides on the surface of red blood cells that determine the specificity of blood groups.) Alleles 1 A and 1 b encode two different enzymes; allele 1° does not encode any. At the same time, the 1° allele is recessive both with respect to 1 A and with respect to I B, and there are no dominant-recessive relationships between the last two. People with the fourth blood group carry two alleles in their genotype: 1 A and 1 B. Since there are no dominant-recessive relationships between these two alleles, both enzymes are synthesized in the body of such people and the corresponding phenotype is formed - the fourth blood group.

Science has acquired an extensive base of new research on the substratum of evolution - genetic code. It is in it that information is laid down about all past and future changes for the development of the organism.

The ratio of heredity and variability allows you to save only best qualities, and instead of unsuccessful ones, acquire new ones, improving the structure and contributing to the 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 from a condensed DNA complex (chromatin), from which information is read in the process of protein synthesis.

Genetics is based on several concepts: a gene (a section of DNA encoding a specific single trait), (a set of genes and traits 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 the same parts of the 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 traits. According to 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 schemes of gene interaction

The American geneticist T.H. Morgan studied the forms among themselves. He outlined the results of his research in According to her, 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, recombination of genes also occurs by crossing over - the exchange of chromosomes by different sections among themselves. At the same time, it is absolutely logical and proven that the genes located directly one after another are not separated 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, then the interaction of allelic genes with each other occurs in three ways. simple circuits: with obtaining a pure dominant trait, incomplete dominance with obtaining an intermediate trait and codominance with inheritance of both traits. Non-allelic genes, on the other hand, are more difficult to inherit: according to the schemes of complementarity, polymerization, or epistasis. In this case, both traits will be inherited, but to a different extent.

The human genotype includes a huge number of genes that carry information about the properties and qualities of our body. Despite this a large number of, they interact as a single integral system.

From the school biology course, we know the laws of Mendel, who studied the patterns of inheritance of traits. In the course of his research, the scientist discovered dominant genes and recessive ones. Some are able to suppress the manifestation of others.

In fact, the interaction of genes goes far beyond the Mendelian laws, although all the rules of inheritance are respected. You can see the difference in the nature of splitting by phenotype, because the type of interaction may differ.

Gene characteristics

A gene is a unit of heredity, it has certain characteristics:

1. The gene is discrete. It determines the degree of development of a particular trait, including the features of biochemical reactions.
2. Has a gradual effect. Accumulating in the cells of the body, it can lead to an increase or decrease in the manifestation of a symptom.
3. All genes are strictly specific, that is, they are responsible for the synthesis of a particular protein.
4. One gene can have multiple effects, affecting the development of several traits at once.
5. Different genes can take part in the formation of one trait.
6. All genes can interact with each other.
7. The external environment influences the manifestation of the action of the gene.

Genes can act on two different levels. The first is the genetic system itself, which determines the state of the genes and their work, stability and variability. The second level can be considered already when working in the cells of the body.

Types of interaction of allelic genes

All cells in our body have a diploid set of chromosomes (it is also called double). The 23 chromosomes of the egg are fused with the same number of chromosomes of the sperm. That is, each trait is represented by two alleles, so they are called allelic genes.

Such allelic pairs are formed during fertilization. They can be either homozygous, that is, consisting of the same alleles, or heterozygous, if different alleles are included.

The forms of interaction of allelic genes are clearly presented in the table.

Complete and incomplete dominance

We can speak of complete dominance when one of the genes can provide the manifestation of a trait, and the second is unable to do so. A strong gene is called dominant, and its opponent is called recessive.

Inheritance in this case occurs completely according to the laws of Mendel. For example, the color of pea seeds: in the first generation we see all green peas, that is, this color is a dominant trait.

If, during fertilization, the gene gets together brown eyes and blue, then the child's eyes will be brown, because this allele completely suppresses the gene that is responsible for blue eyes.

With incomplete dominance, one can see the manifestation of an intermediate trait in heterozygotes. For example, when crossing a homozygous dominant nocturnal beauty with red flowers with the same individual, only with a white corolla, one can see hybrids in the first generation Pink colour. The dominant red trait does not completely suppress the manifestation of the recessive white, so in the end something in between is obtained.

Codominance and overdominance

Such an interaction of genes, in which each provides its own trait, is called codominance. All genes in one allelic pair are absolutely equivalent. Neither can suppress the action of the other. It is this interaction of genes that we observe in the inheritance of blood groups in humans.

Gene O provides the manifestation of the 1st blood group, gene A - the second, gene B - the third, and if genes A and B fall together, then none can suppress the manifestation of the other, therefore a new sign is formed - the 4th blood group.

Overdominance is another example of the interaction of allelic genes. In this case, heterozygous individuals for this trait have a more pronounced manifestation of it compared to homozygous individuals. This interaction of genes underlies such a phenomenon as heterosis (the phenomenon of hybrid strength).

When two tomato varieties are crossed, for example, a hybrid is obtained that inherits the traits of both original organisms, since the traits become heterozygous. In the next generation, splitting according to traits will already begin, so it will not be possible to obtain the same offspring.

In the animal world one can even observe the barrenness of such hybrid forms. Such examples of gene interaction can often be found. For example, when a donkey and a mare are crossed, a mule is born. He inherited all the best qualities of his parents, but he himself cannot have offspring.

In humans, sickle cell anemia is inherited by this type.

Non-allelic genes and their interaction

Genes that are located in different couples ah chromosomes are called non-allelic. If they are together, they may well influence each other.

The interaction of non-allelic genes can be carried out in different ways:

1. Complementarity.
2. Epistasis.
3. polymer action.
4. Pleiotropy.

All these types of gene interaction have their own distinctive features.

complementarity

In this interaction, one dominant gene complements another, which is also dominant, but not allelic. Getting together, they contribute to the manifestation of a completely new feature.

An example of the manifestation of color in sweet pea flowers can be given. The presence of pigment, which means that the color of the flower is provided by a combination of two genes - A and B. If at least one of them is absent, then the corolla will be white.

In humans, such an interaction of non-allelic genes is observed during the formation of the hearing organ. Normal hearing can be only if both genes - D and E - are present in the dominant state. In the presence of only one dominant or both in a recessive state, hearing is absent.

epistasis

This interaction of non-allelic genes is completely opposite to the previous interaction. In this case, one non-allelic gene is able to suppress the manifestation of another.

The forms of gene interaction in this variant can be different:

• dominant epistasis.
• Recessive.

In the first type of interaction, one dominant gene suppresses the manifestation of another dominant one. Recessive genes are involved in recessive epistasis.

According to this type of interaction, the color of the fruit in the pumpkin is inherited, the color of the coat in horses.

Polymer action of genes

This phenomenon can be observed when several dominant genes are responsible for the manifestation of the same trait. If at least one dominant allele is present, then the trait will definitely appear.

The types of gene interaction in this case can be different. One of them is storage polymer, when the degree of manifestation of a trait depends on the number of dominant alleles. This is how the color of wheat grains or the color of the skin in humans is inherited.

Everyone knows that all people have different colour skin. For some, it is completely light, some have dark skin, and representatives of the Negroid race are completely black. Scientists are of the opinion that skin color is determined by the presence of three different genes. For example, if the genotype contains all three in a dominant state, then the skin is the darkest, like that of blacks.

In the Caucasian race, judging by the color of our skin, there are no dominant alleles.

It has long been found that the interaction of non-allelic genes by the type of polymer affects most of the quantitative traits in humans. These include: height, body weight, intellectual abilities, body resistance to infectious diseases and some others.

It can only be noted that the development of such traits depends on environmental conditions. A person may be predisposed to overweight, but if you follow the diet, it is possible to avoid this problem.

Pleiotropic action of genes

Scientists have long been convinced that the types of gene interaction are rather ambiguous and very versatile. Sometimes it is impossible to predict the manifestation of certain phenotypic traits, because it is not known how genes interact with each other.

This statement is only emphasized by the fact that one gene can influence the formation of several traits, that is, have a pleiotropic effect.

It has long been noted that the presence of red pigment in beet fruits is necessarily accompanied by the presence of the same, but only in the leaves.

In humans, a disease such as Marfan's syndrome is known. It is associated with a defect in the gene that is responsible for the development of connective tissue. As a result, it turns out that wherever this tissue is in the body, problems can be observed.

Such patients have long "spider" fingers, a dislocation of the lens of the eye, a heart disease are diagnosed.

The influence of environmental factors on the action of genes

Influence external factors environment on the development of organisms cannot be denied. These include:

• Nutrition.
• temperature.
• Light.
• The chemical composition of the soil.
• Humidity, etc.

Factors external environment are fundamental in the processes of selection, heredity and variability.

When we consider the forms of interaction of allelic or non-allelic genes, we must always take into account the influence of the environment. An example can be given: if primrose plants are crossed at a temperature of 15-20 degrees, then all hybrids of the first generation will have a pink color. At a temperature of 35 degrees, all plants will turn out white. So much for the influence of the environmental factor on the manifestation of signs, it does not matter here which gene is dominant. In rabbits, it turns out that the color of the coat also depends on the temperature factor.

Scientists have long been working on the question of how to control the manifestations of signs by exerting various external influences. This may provide an opportunity to control the development of congenital traits, which is especially relevant for humans. Why not use your knowledge to prevent certain hereditary ailments from manifesting?

All types of interaction of allelic genes, and not only them, can be so different and multifaceted that it is impossible to attribute them to any particular type. Only one thing can be said, that all these interactions are equally complex both in humans and in representatives of all species of plants and animals.

The genotype is not just a mechanical set of genes, it is a historically established system of genes interacting with each other. More precisely, it is not the genes themselves (sections of DNA molecules) that interact, but the products formed on their basis (RNA and proteins).

Both allelic and non-allelic genes can interact.

Interaction of allelic genes

There are three types of interaction of allelic genes: complete dominance, incomplete dominance, codominance.

1. Complete dominance- a phenomenon when a dominant gene completely suppresses the work of a recessive gene, as a result of which a dominant trait develops.
2. incomplete dominance- a phenomenon when a dominant gene does not completely suppress the work of a recessive gene, as a result of which an intermediate trait develops.
3. Codominance (independent manifestation)- the phenomenon when both alleles participate in the formation of a trait in a heterozygous organism. A person with a series of multiple alleles has a gene that determines the blood group. In this case, the genes that determine blood types A and B are codominant with respect to each other and both are dominant with respect to the gene that determines blood group 0.

Interaction of non-allelic genes

There are four types of interaction of non-allelic genes: cooperation, complementarity, epistasis and polymerization.

Cooperation- a phenomenon when, with the mutual action of two dominant non-allelic genes, each of which has its own phenotypic manifestation, a new trait is formed.

complementarity- the phenomenon when a trait develops only with the mutual action of two dominant non-allelic genes, each of which individually does not cause the development of a trait.

epistasis- the phenomenon when one gene (both dominant and recessive) suppresses the action of another (non-allelic) gene (both dominant and recessive). Suppressor gene(suppressor) can be dominant (dominant epistasis) or recessive (recessive epistasis).

Polymerism- the phenomenon when several non-allelic dominant genes are responsible for a similar effect on the development of the same trait. The more such genes are present in the genotype, the more pronounced the trait. The phenomenon of polymerism is observed in the inheritance of quantitative traits (skin color, body weight, cow milk yield).

In contrast to polymers, there is such a phenomenon as pleiotropy- multiple gene action, when one gene is responsible for the development of several traits.

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

It is more correct to speak 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 action.

An example of a complementary interaction of genes is the eye color of the Drosophila fly. In flies with the genotype S-B- regular 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. In the presence of only the dominant S gene, a brown pigment appears, and only the dominant B gene produces a bright scarlet. If there are two dominant genes, then their products interact with each other, forming a red color.

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

epistasis- this is such an interaction of non-allelic genes, when the action of one gene suppresses the action of another. An epistatic (suppressive) effect on another gene can have both a dominant and a recessive allele of this gene. Cleavage by phenotype in dominant epistasis differs from recessive. An epistatic gene is usually referred to as I.

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

At polymer 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 the polymer interaction, the more different degrees of expression of the trait. This occurs with cumulative polymerism, when all genes are involved in the accumulation of a trait. With non-cumulative polymerism, the number of dominant genes does not affect the severity of the trait, at least one is enough; and a form that is excellent in phenotype is observed only in individuals in which all polymeric genes are recessive.

Polymeria, for example, determines the color of a person's skin. Four genes (or four pairs of alleles according to other sources) have an influence. Consider a situation with two pairs. Then A 1 A 1 A 2 A 2 will determine the darkest color, a 1 a 1 a 2 a 2 - the lightest. Medium color skin 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 skin color close to light but darker, and three dominant genes will result in skin color 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 talking not about the interaction of genes, but with the multiple action of one gene.