The transfer of traits from generation to generation due to the interaction between different genes. What is a gene, and what kind of interaction are there between them?
What is a gene?
Under the gene now, implyunit of transmission of hereditary information. Genes are located in the DNA and form its structural regions. Each gene is responsible for the synthesis of a specific protein molecule, which causes the manifestation of a particular trait in humans.
Each gene has several subspecies or alleles,which cause a variety of traits (for example, the brown color of the eyes is due to the dominant allele of the gene, while the blue color is a recessive trait). Alleles are located in the same areas of homologous chromosomes, and the transmission of a particular chromosome causes the manifestation of a particular trait.
Все гены взаимодействуют между собой.There are several types of their interaction - allelic and nonallelic. Accordingly, the interaction of allelic and nonallelic genes is distinguished. What is the difference between them and how do they appear?
History of discovery
Before interaction types were discoverednonallelic genes, it was assumed that only complete domination is possible (if there is a dominant gene, then the sign will manifest; if it is not, then there will be no sign). The theory of allelic interaction prevailed, which for a long time was the main dogma of genetics. Dominance was carefully investigated, and its types such as complete and incomplete dominance, codominance and overdominance were discovered.
All these principles obeyed the first law of Mendel, which spoke about the uniformity of hybrids of the first generation.
With further observation and research wasIt is noted that not all signs adapt to the theory of dominance. With a deeper study, it was proved that not only the same genes influence the manifestation of a trait or a group of properties. In this way, the forms of interaction of nonallelic genes were discovered.
Reactions between genes
As has been said, for a long time the doctrine prevailed.about dominant inheritance. In this case, there was an allelic interaction in which the trait manifested itself only in the heterozygous state. After various forms of interaction of nonallelic genes were discovered, scientists were able to explain the previously unexplained types of inheritance and get answers to many questions.
It was found that gene regulation is directlydepended on enzymes. These enzymes allowed genes to react in different ways. At the same time, the interaction of allelic and nonallelic 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 cause of the atypical transfer of characters lies in the genes themselves.
Nonallelic interaction is unique, which allows to obtain new combinations of signs, causing a new degree of survival and development of organisms.
Nonallelic genes
Non-allele are those genes that are localized indifferent sites of non-homologous chromosomes. They have the same synthesis function, but they encode the formation of various proteins causing different characters. Such genes, reacting with each other, can cause the development of traits in several combinations:
- One sign will be due to the interaction of several genes that are completely different in structure.
- Several traits will depend on a single 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 nonallelic genes?
- Epistasis
- Polymerism.
- Complementarity.
- Action 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 stop in more detail on each of them.
Epistasis
Данное взаимодействие неаллельных генов – эпистаз - it is observed in the case when one gene suppresses the activity of another (the suppressor gene is called epistatic, and the suppressed gene is called the hypostatic gene).
The reaction between these genes can bedominant and recessive. The 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 (usually denoted 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 hypostatic gene.
Phenotypic cleavage, withEach of the types of these interactions is also different. In the case of dominant epistasis, the following picture is more often observed: in the second generation, according to phenotypes, the separation will be as follows: 13: 3, 7: 6: 3 or 12: 3: 1. It all depends on which genes converge.
With recurrent epistasis, the separation is: 9: 3: 4, 9: 7, 13: 3.
Complementarity
The interaction of nonallelic genes, in which, when combining the dominant alleles of several traits, a new, hitherto unseen phenotype is formed, is called complementarity.
For example, most often this type of reaction between genes occurs 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.
In the presence of two in the genotype simultaneouslydominant alleles (A and B) pumpkin acquires a discoid shape. If, however, further crossing is carried out (i.e., to continue this interaction of nonallelic genes with pure line pumpkins), then in the second generation, 9 individuals can be obtained with a disc shape, 6 with a spherical and one elongated pumpkin.
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 - the development of the cochlea, the other - the auditory nerve), and if there is only one dominant trait, deafness is manifested.
Polymerism
Often the basis of the manifestation of a trait is not the presence of a dominant or recessive allele of a gene, but their number. The interaction of nonallelic genes — the polymer — is an example of such a manifestation.
The polymer action of genes can occur withcumulative (cumulative) effect or without it. When cumulative, the degree of manifestation of the trait depends on the general gene interaction (the more genes, the stronger the trait is expressed). With a similar effect, the offspring is divided as follows: 1: 4: 6: 4: 1 (the severity of the trait decreases, that is, in one specimen the trait is maximally pronounced, in others, its extinction is observed until it disappears completely).
If no cumulative action is observed, thenthe manifestation of the trait depends on the dominant alleles. If there is at least one such allele, the sign will take place. With this effect, splitting in the progeny proceeds in a ratio of 15: 1.
The effect of modifier genes
The interaction of nonallelic 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 that is responsible forcolor intensity. In the dominant state, this gene regulates the appearance of the color, while the formation of the recurrent genotype for this gene, even if there are other genes that directly control the color, will show the “dilution effect of the color”, which is often observed in milky-white mice.
- Another example of a similar reaction isthe appearance of spotting on the body of animals. For example, there is a gene F, the main function of which is the uniformity of dyeing wool. When a recessive genotype is formed, the wool will be colored unevenly, with the appearance, for example, of white spots in a particular area of the body.
Such interaction of nonallelic genes in humans is quite rare.
Pleiotropy
With 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, an example of pleiotropism isdwarfism. It was observed 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 ceased to grow, and underdevelopment of their internal organs and glands was observed. This gene of dwarfism 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 coloring in foxes. In this case, pleiotropy manifested itself as a lethal gene, which, during the formation of the dominant homozygote, caused the death of embryos.
- In humans, pleiotropic interaction is shown by the example of phenylketonuria, as well as Marfan syndrome.
The role of nonallelic interaction
In evolutionary terms, all of the above typesinteractions of nonallelic genes play an important role. New gene combinations cause the emergence 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, they cause the death of those individuals that will significantly stand out among their species.
Nonallelic gene interaction is wideused in breeding genetics. Some species of living organisms are preserved due to similar gene recombination. Other species acquire properties that are highly valued in the modern world (for example, breeding a new breed of animals 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 the human genome and create a new, defect-free genotype.
