In several sets of alleles control the phenotype and each dominant allele present codes for a product which has a quantitative and additive effect.

Polygenic inheritance

Explanation:

Polygenic inheritance occurs when a genetic trait is regulated by more than one genes. All the alleles of these genes together determine the phenotype of the organism for the polygenic trait. Here, the phenotype is regulated by the total number of dominant alleles for all the genes that regulate a polygene trait.

For example, human skin color is a polygenic trait. The final phenotype depends on the total number of dominant alleles of all the genes that regulate the skin color in human.  

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