Epistasis

- Some traits may be determined by two or more genes​- In epistasis, one gene affects the phenotype of another due to interaction of their gene products​ - [ IN FANCY WORDS: In epistasis, expression of a gene at one locus alters the phenotypic expression of a gene at a second locus​ ] - For example, in Labrador retrievers and many other mammals, coat color depends on two genes ​ - One gene determines the pigment color (with alleles B for black and b for brown) ​ - The other gene (with alleles E for color and e for no color) determines whether the pigment will be deposited in the hair - If heterozygous black labs (genotype BbEe) are mated, we might expect the dihybrid F2 ratio of 9:3:3:1​ - However, a Punnett square shows that the phenotypic ratio will be 9 black to 3 chocolate to 4 yellow labs ​- Epistatic interactions produce a variety of ratios, all of which are modified versions of 9:3:3:1​