Epistasis

Epistasis occurs when one gene affects the phenotypic expression of a second gene. This frequently occurs in the expression of pigmentation. One gene turns on (or off) the production of pigment, while a second gene controls either the amount of pigment produced or the color of the pigment. If the first gene codes for no pigment, then the expression of the second gene has no effect, regardless of the kind of pigmentation it encodes.

Epistasis occurs in the pigmentation of fur in mice. One gene codes for the presence or absence of pigmentation. A second gene codes for the color of pigmentation, black or brown. Thus, C and c represent the alleles for the presence and absence of color, and B and b represent the alleles for black and brown pigments. As the allele notation indicates, both genes are expressed by the complete dominance inheritance pattern. The phenotypic expressions of CCBB, CCBb, CcBB, and CcBb are all black, and the expressions of CCbb and Ccbb are both brown. However, whenever cc is inherited, no pigment is produced and the fur is white regardless of the color encoded by the B allele.