上位refers to an antagonistic interaction betweengenessuch that one gene masks or interferes with the expression of another.

An example of epistasis is pigmentation in mice. The wild-type coat color, agouti (AA) isdominantto solid-colored fur (aa). However, a separate geneC, when present as therecessivehomozygote(cc), negates the expression of pigment from theAgene and results in an albino mouse. Therefore, thegenotypesAAcc, Aacc, aaccwill produce the same albinophenotype. A cross between杂合的for both genes (AaCcxAaCc) will generate offspring with a phenotypic ratio of 9 agouti : 3 black : 4 albino. In this case, we say that theCgene is epistatic to theAgene.