There are several forms of interaction of allelic genes. Firstly, complete dominance is a phenomenon that consists in the fact that one allelic gene completely suppresses the other and manifests itself in the form of a trait. For example, in peas, the gene that causes the yellow color of the seeds (A) suppresses the gene that determines the green color of the seeds (A). Therefore, in heterozygotes (Aa), the seed is colored yellow.
Secondly, incomplete dominance, expressed in the fact that none of the allelic genes completely suppresses the other allele. In a nocturnal beauty, gene A is responsible for the development of the red color of the corolla of the flower (AA), gene a – for the white color (aa). Heterozygous plants (Aa) have pink flowers.
The third form of interaction of allelic genes – coding – a joint manifestation of both alleles that do not affect each other. For example, when determining blood groups in humans (ABO system), gene I determines the development of group II (A), and gene Ib forms B. antigen (agglutinogen) B. located on red blood cells in individuals with blood group III (B).
Finally, overdomination is the phenomenon underlying heterosis (the effect of hybrid strength). Heterozygotes, the genotype of which contains two different alleles (Aa), exhibit increased vitality and fertility, incomparable with homozygous organisms (AA and aa).
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