Interaction of non-allelic genes: types and forms

The transmission of characteristics 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 gene at the present time, we mean the unit of transmission of hereditary information. Genes are found in DNA and form its structural regions. Each gene is responsible for the synthesis of a certain protein molecule, which determines the manifestation of a particular trait in humans.

Each gene has several subspecies or alleles, which both cause a variety of symptoms (for example, brown eye color is caused by the dominant allele of the gene, while blue color is a recessive sign). Alleles are located in the same areas of homologous chromosomes, and the transmission of a particular chromosome causes the manifestation of one or another characteristic.

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 singled out . What are they different from each other and how are they manifested?

History of the discovery

Before the types of interaction of non-allelic genes were discovered, it was considered that only complete dominance is possible (if there is a dominant gene, then the symptom will manifest itself, if not, then there will be no sign). The predominance of the doctrine of allelic interaction, which for a long time was the main dogma of genetics. Domination was thoroughly investigated, and its types were discovered, such as complete and incomplete domination, codomination and overdominance.

All these principles obeyed Mendel's first law, which referred to the uniformity of hybrids of the first generation.

With further observation and research, it was observed that not all the 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 the trait or group of properties. Thus, the forms of interaction of non-allelic genes were discovered.

Reactions between genes

As it was said, the dominant inheritance doctrine prevailed for a long time. In this case, there was an allelic interaction, in which the symptom was manifested only in the heterozygous state. After the different forms of interaction of non-allelic genes were discovered, scientists were able to explain previously unexplained types of inheritance and get answers to many questions.

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

Non-allelic interaction is unique, which allows us to obtain new combinations of characteristics that cause a new degree of survival and development of organisms.

Non-allelic genes

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

• One sign will be due to the interaction of several, completely different in structure of the genes.
• A few symptoms will depend on one gene.

The reactions between these genes proceed somewhat more complicatedly than in the case of allelic interaction. However, each of these types of reactions has its own features and peculiarities.

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

• Epistasis.
• Polymerism.
• Complementarity.
• The action of modifying 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 epistatic, and the suppressed gene is called the hypostatic gene).

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

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

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

Complementarity

The interaction of non-allelic genes, in which a new phenotype, which has not been met before, is formed when the dominant alleles of several characters combine, and is called complementarity.

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

If there is a dominant allele A or B in the plant genotype, the vegetable gets a spherical shape. If the genotype is reciprocal, 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 we continue to cross (ie continue this interaction of non-allelic genes with pumpkins of a clean line), then in the second generation it is possible to get 9 individuals with a discoid shape, 6 - with a spherical and one pumpkin of an elongated shape.

Such a crossing allows to obtain new, hybrid forms of plants with unique properties.

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

Polymerism

Often the basis for the manifestation of the sign is not the presence of a dominant or recessive allele of the gene, but their number. The interaction of non-allelic genes, the polymer, is an example of such a manifestation.

The polymeric action of genes can occur with or without a cumulative (cumulative) effect. With cumulation, the degree of manifestation of a sign depends on the overall gene interaction (the more genes, the stronger the sign is expressed). The offspring with this effect is divided as follows - 1: 4: 6: 4: 1 (the degree of expression of the trait decreases, ie in one specimen the sign is maximally expressed, in others its extinction is observed until complete disappearance).

If no cumulative effect is observed, then the manifestation of the trait depends on the dominant alleles. If there is at least one such allele, the symptom will take place. With this effect, splitting in the progeny 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:

• For example, there is a gene D responsible for the intensity of the color. In the dominant state, this gene regulates the appearance of color, while in the formation of a recurrent genotype for a given gene, even if there are other genes that control directly the color, a "color dilution effect" will appear, as is often observed in milky white mice.
• Another example of such a reaction is the appearance of spotting on the body of animals. For example, there is a gene F, the main function of which is uniformity of dyeing of wool. When a recessive genotype is formed, the coat will be colored unevenly, with the appearance, for example, of white spots in one or another area of the body.

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

Pleiotropy

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

In animals, pleiotropy was manifested as follows:

• In mice, example of pleiotropy is dwarfism. It was noted that when mating phenotypically normal mice in the first generation, all mice turned out to be dwarfish. It was concluded that dwarfism is caused by a recessive gene. Recessive homozygotes ceased to grow, and 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 the synthesis of hormones and caused all the consequences.
• Platinum color in foxes. Pleiotropia in this case was manifested by a lethal gene, which during the formation of a dominant homozygote caused the death of embryos.
• In humans pleiotropic interaction is shown on the example of phenylketonuria, as well as Marfan syndrome.

The role of non-allelic interaction

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

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

Work is underway to use these types of inheritance in humans to eliminate negative signs from the human genome and create a new, defect-free genotype.