Epistasis is when a gene is the on and off switch for a different gene; therefore, two genes contribute to the same phenotype or physical appearance. A specific gene masks or modifies how the second gene will express itself.
For instance, if two heterozygous mice (meaning it is Bb, one dominant and one recessive for the b trait, and Ee, one dominant and one recessive for the E trait) mate, this is what the punnet square would look like (down below). This is a dihybrid cross that studies how two genes affect the offspring's genotype (genetic constitution) and phenotype (physical appearance). The B represents a dominant trait representing black fur, while bb is recessive and will represent brown fur. The E represents the ability to develop pigmentation in the fur coat. So, you must have at least one E (capital "e") to show off the pigment seen through black and brown-furred mice. However, if the mouse is ee (homozygous recessive), the mouse would not be able to produce a pigment as seen in the tannish colored squares. The usual ratio for a dihybrid cross is 9:3:3:1; however, in this dihybrid cross, the ratio is 9:3:4 (represents the effects epistasis has on the offspring.