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The genotype includes a large number of diverse genes, which in turn act as a whole. The founder of genetics, Mendel, described in his writings that he discovered only one possibility of the interaction of allelic genes - when there is absolute dominance (predominance) of one of the alleles, while the second remains completely recessive (passive, i.e. does not participate in interaction). But let’s say right away that the phenotypic manifestation of genes (external, visible to the eye) cannot depend on only one or a pair of genes, because it is a consequence of the interaction of the whole system.
In reality, proteins and enzymes interact, not genes.
In the basics of genetics , only 2 types of gene interaction are distinguished - the first is the interaction of allelic genes, the second, respectively, non-allelic. It is only necessary to understand the material side of this question, because not some concepts from the textbook interact, but proteins that are synthesized according to a specific program in the cytoplasm of cells, and the amount of these proteins is in the millions. The program itself, according to which proteins will be synthesized, and, as a result, their further interaction will develop, is embedded in the genes that give external commands while in the chromosomes of cells (ultrascopic organelles of cells).
What genes are called allelic?
Allelic genes are genes that occupy the same “places” (or loci) in the chromosomes. Every living organism has allelic genes in pairs. The interaction of allelic genes can occur in several ways, which are called: coding, overdomination, complete and incomplete dominance.
Allelic genes interact on the principle of complete dominance in the event that the action of the dominant gene completely overlaps the action of the recessive. Incomplete dominance can be called the relationship in which the recessive gene is not completely suppressed and takes, albeit minimal, participation in the formation of phenotypic characters.
Codomination occurs if allelic genes exhibit their properties independently of each other. Probably the most significant example of coding is the AB0 blood system, in which both A and B genes function independently.
Overdomination is an increase in the quality of phenotypic manifestations of a dominant gene if it is “in conjunction” with a recessive one. That is, if there are 2 dominant genes in one allele , then they appear worse than the dominant gene that is “in conjunction” with the recessive one.
Multiple allelism
As mentioned earlier, each living creature can have only 2 allelic genes, but alleles can be much more than two - this phenomenon is called multiple allelism. We must say right away that only one pair of alleles can manifest phenotypic characters, that is, while some work, others rest.
Almost always, homologous (identical) alleles are responsible for the development and manifestation of the same trait, but they differ in the quality of its manifestation. Multiple allelism is also characterized by various forms of gene interaction. That is, although they are responsible for one and the same sign, but, firstly, they manifest it in different ways, and secondly, using different methods (complete, incomplete domination, and so on).
It would seem, why such confusion? Everything is simple - only one pair of homologous alleles can enter the reproductive cell of a living creature, but which of all the available ones already solves the case. It is precisely because of this that species variability is achieved, which plays a major role in the evolution of living things.