Some traits develop only as a result of the interaction of several non-allelic genes. For example, when two clean lines of sweet pea with white flowers are crossed, in hybrids of the first generation, all flowers will have a purple color. It turns out that the dominant genes A and B, individually, cannot provide the synthesis of the red pigment anthocyanin for coloring the flower. And only if there are two of these genes in one cell, anthocyanin begins to be synthesized there, and the flowers turn purple.
Another possible mechanism of additional interaction may be that gene A encodes the structure of one part (subunit) of the protein necessary for the manifestation of any trait, and gene B encodes the structure of another subunit of the same protein. And only in the presence of genes A and B taken together, is a full-fledged protein with a quaternary structure capable of providing the manifestation of this trait synthesized.
Consequently, this type of gene interaction is called complementary when the presence of non-allelic genes A and B is necessary for the manifestation of a trait. These genes are called complementary or complementary.
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