This document discusses epistasis in mice related to hair color. It explains that in mice, black hair (B) is dominant over brown hair (b). However, a cross between two heterozygous black mice would produce offspring in a 56.25% black, 18.75% brown, 25% white ratio, rather than the simple Mendelian ratio, due to epistasis between genes B and C. Epistasis occurs when one gene alters the expression of another gene.