Final answer:
Epistasis is a form of non-Mendelian inheritance where the phenotypic expression of one gene is masked by another gene, impacting the final appearance of traits such as eye color in humans or coat color in mice.
Step-by-step explanation:
Epistasis is a non-Mendelian trait where the expression of one gene depends on the presence of one or more 'modifier genes'. Essentially, the effect of one gene is overshadowed or masked by another gene. It is derived from the Greek 'epi-', meaning 'over', and '-stasis', meaning 'standing', which together suggest one gene standing over another to affect the phenotypic outcome. Unlike Mendelian inheritance, where traits are influenced by individual alleles of a single gene, epistasis involves multiple genes that can influence a single trait. For instance, in humans, eye color is a feature that results from the interplay of multiple genes, not just one.
An example that illustrates epistasis is the coat color in mice. In this case, a gene responsible for pigment production can suppress the effects of another gene that dictates the actual color variation in the coat. When considering a cross between heterozygotes for both pigmentation genes (AaCc), the expected offspring display a phenotypic ratio showing the effects of epistasis - in this scenario, typically a 9:3:4 ratio.