Many quantitative and some qualitative characteristics in living organisms are determined not by one, but by several pairs of interacting genes. In this case, the more dominant genes in the genotype, the more the trait is manifested. This form of interaction between non-allelic genes is called polymerization.
Polymerism is the interaction of multiple pairs of non-allelic genes with the same effect.
Example: Human skin color is determined by four pairs of non-allelic genes responsible for the production of the pigment melanin. The more dominant genes there are, the more melanin in the skin, and the darker its color.
A person with dominant genes A1A1A2A2A3A3A4A4 has the darkest skin. A person whose genotype contains only recessive genes a1a1a2a2a3a3a4a5 has the lightest skin. Depending on the ratio of the number of dominant and recessive genes, skin color changes from light to dark.
This type of interaction is called cumulative polymer. With cumulation, the degree of manifestation of a trait depends on the number of dominant genes (the more genes, the more pronounced the trait). The offspring with cumulative polymerization is divided as follows – 1: 4: 6: 4: 1 (in one individual, the trait is expressed as much as possible, in the rest, its extinction is observed until it disappears completely).
According to this type, many important quantitative traits of agricultural animals and plants are inherited: milk yield of cows, egg production of chickens, sugar content in root crops, length of corn cob, length of an ear, coloration of an ear in cereals, etc.
In noncumulative polymerization, the manifestation of a trait does not depend on the number of dominant alleles. The trait will manifest itself if there is at least one dominant gene. With a similar effect, splitting is observed in the offspring in a ratio of 15: 1.
Example: The presence of feathering on the legs of chickens is determined by two non-allelic dominant genes – A1 and A2. Feathered legs will have chickens with genotypes: A1-A2-, A1a2a2 and a1a1A2-. Only birds with the genotype a1a1a2a2 will have unfeathered legs.