The transmission of characters from generation to generation is due to the interaction of 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, mean the unit of transmission of hereditary information. Genes are in DNA and form its structural sites. Each gene is responsible for the synthesis of a particular protein molecule, which determines the manifestation of a particular trait in humans.
Each gene has several subspecies or alleles, which determine a variety of characters (for example, brown eye color is due to the dominant gene allele, while blue is a recessive trait). Alleles are located in identical regions of homologous chromosomes, and the transmission of a chromosome causes the manifestation of a particular 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 isolated. How do they differ from each other and how are they manifested?
Discovery story
Before the types of interaction of non-allelic genes were discovered, it was believed that only complete dominance is possible (if there is a dominant gene, then the sign will appear; if it is not, then there will be no sign). The doctrine of allelic interaction prevailed, which for a long time was the main tenet of genetics. Domination was thoroughly investigated, and its types were discovered, such as complete and incomplete dominance, coding, and overdomination.
All these principles obeyed the first law of Mendel, which stated the uniformity of hybrids of the first generation.
Upon further observation and research, it was noticed that not all attributes were tuned to the theory of dominance. A deeper study showed 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.
Genetic Reactions
As was said, for a long time, the doctrine of dominant inheritance prevailed. In this case, there was an allelic interaction in which the trait appeared only in the heterozygous state. After various forms of interaction of non-allelic genes were discovered, scientists were given the opportunity to explain hitherto inexplicable 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 enter into reactions in different ways. In this case, 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 under which genes interact, and the reason for the atypical transmission of traits lies in the genes themselves.
Non-allelic interaction is unique, which allows one to obtain new combinations of traits that determine a new degree of survival and development of organisms.
Non-allelic genes
Non-allelic are those genes that are localized in different parts of non-homologous chromosomes. They have one synthesis function, however, they encode the formation of various proteins that cause different characters. Such genes, reacting with each other, can determine 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 one gene.
The 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 inhibits the activity of another (the suppressing gene is called the epistatic, and the suppressed is the hypostatic gene).
The reaction between these genes can be dominant and recessive. Dominant epistasis is observed when the epistatic gene (usually indicated 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 occurs when the recessive allele of the epistatic gene inhibits the manifestation of any of the alleles of the hypostatic gene.
Phenotypic splitting, for each of the types of these 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 recurrent 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 characters are combined, a new phenotype not yet met is formed and is called complementarity.
For example, most often this type of reaction between genes occurs in plants (especially pumpkins).
If the genotype of a plant has a dominant allele A or B, then the vegetable gets a spherical shape. If the genotype is recessive, then the fetal shape is usually elongated.
In the presence of two dominant alleles (A and B) in the genotype simultaneously, the pumpkin acquires a discoid shape. If we continue to cross (i.e., continue this interaction of non-allelic genes with clean line pumpkins), then in the second generation you can get 9 individuals with a disk-shaped shape, 6 - with a spherical and one elongated pumpkin.
Such 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 if there is only one dominant sign, deafness is manifested.
Polymerism
Often the basis for the manifestation of the trait is not the presence of a dominant or recessive allele of the gene, but their number. The interaction of non-allelic genes - polymerism - is an example of such a manifestation.
The polymer action of genes can occur with or without cumulative (cumulative) effect . During cumulation, the degree of manifestation of a trait depends on the general gene interaction (the more genes, the more pronounced the trait is). The offspring with a similar effect is divided as follows - 1: 4: 6: 4: 1 (the severity of the trait decreases, i.e., in one individual the trait is most pronounced, in others its extinction is observed until it disappears completely).
If no cumulative effect is observed, then the manifestation of the sign depends on the dominant alleles. If there is at least one such allele, the sign 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 observed relatively rarely. An example of such an interaction is as follows:
- For example, there is a D gene responsible for the color intensity. In the dominant state, this gene regulates the appearance of color, while the formation of a recessive genotype for this gene, even if there are other genes that directly control the color, will show the “color dilution effect”, 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 the color of the coat. With the formation of a recessive genotype, the coat will be colored unevenly, with the appearance, for example, of white spots in a particular area of the body.
This interaction of non-allelic genes in humans is quite rare.
Pleiotropy
In this type of interaction, one gene regulates the manifestation or affects the severity of another gene.
In animals, pleiotropy was manifested as follows:
- In mice, dwarfism is an example of pleiotropy. It was noted that when crossing phenotypically normal mice in the first generation, all mice were dwarf. It was concluded that dwarfism is due to the recessive gene. Recessive homozygotes ceased to grow, and underdevelopment of their internal organs and glands was observed. This dwarfism gene affected 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. In this case, pleiotropy was manifested by a lethal gene, which, when a dominant homozygote was formed, caused the death of the embryos.
- In humans, pleiotropic interaction is shown by the example of phenylketonuria, as well as Marfan syndrome.
The role of non-allelic interaction
In evolutionary terms, all of the above types of interaction of non-allelic genes play an important role. New gene combinations determine the appearance of new traits and properties of living organisms. In some cases, these signs contribute to the survival of the body, in others - on the contrary, cause the death of those individuals that will significantly stand out from their species.
Non-allelic gene interaction is widely used 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 that have greater endurance and physical strength than its parental 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.