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 gene at the present time, they mean a 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 regions of homologous chromosomes, and the transfer 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 interactionallelic and non-allelic genes. 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 only complete dominance is possible (if there is a dominant gene, then the trait will appear; if it is absent, then there will be no trait). The doctrine of allelic interaction, which for a long time was the main dogma of genetics, prevailed. Dominance has been extensively researched and types such as complete and incomplete dominance, co-dominance and over-dominance have been discovered.
All these principles were subject to Mendel's first law, which stated the uniformity of first-generation hybrids.
Following further observation and research, it was noticed that not all signs adjusted to the dominance theory. 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 unexplained types of inheritance and get answers to many questions.
It was found that gene regulation was directly dependent 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 completely different genes.
- Multiple 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.
- Polymeria.
- Complementarity.
- The action of modifier genes.
- Pleiotropic interaction.
Everyoneof these types of interaction has its own unique properties and manifests itself in its own way.
We should dwell on each of them in more detail.
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 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 genotype of the plant 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 we continue to cross (i.e. continue this interaction of non-allelic genes with pumpkins of a pure line), then in the second generation you can get 9 individuals with a disc-shaped shape, 6 with a spherical shape and one elongated pumpkin.
Such crossbreeding allows you to get new, hybrid forms of plants with unique properties.
In humans, this type of interaction causes the normal development of hearing (one gene for the development of the cochlea, the other for the auditory nerve), and in the presence of only one dominant trait, deafness appears.
Polymeria
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 - polymeria - is an example of such a manifestation.
The polymeric action of genes can proceed with a cumulative (cumulative) effect or without it. 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, i.e. in one individual the trait is maximally pronounced, in others its extinction is observed up to complete disappearance).
If no cumulative action is observed, thenthe manifestation of a trait depends on 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.
The 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 D gene responsible for color intensity. In the dominant state, this gene regulates the appearance of color, while in the formation of a recessive genotype for this gene, even if there are other genes that directly control color, the “color dilution effect” will appear, which 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 the F gene, the main function of which is the uniformity of wool coloring. With the formation of a recessive genotype, the coat will be colored unevenly, with the appearance, for example, of white spots in one or another area of the body.
Such an interaction of non-allelic genes in humans is quite rare.
Pleiotropy
In 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 has been observed that when crossing phenotypically normal mice inIn the first generation, all mice turned out to be dwarf. 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 affected 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, a pleiotropic interaction has been shown in phenylketonuria as well as Marfan's syndrome.
The role of non-allelic interactions
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 from 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 stamina and physical strength than its parents).
Work is underway on the use of these types of inheritance in humans toeliminating negative traits from the human genome and creating a new, defect-free genotype.
