The masking of the phenotypic effect of alleles at one gene by alleles of another gene. A gene is said to be epistatic when its presence suppresses the effect of a gene at another locus. Epistatic genes are sometimes called inhibiting genes because of their effect on other genes which are described as hypostatic.

Example:

Hh --| Ptc --| Smo --> Ci

Ci is epistatic to Hh, Ptc, and Smo.