Epistasis is a circumstance where the expression of one gene is modified (e.g., masked, inhibited or suppressed) by the expression of one or more other genes.
Epistasis. Interestingly, the word “epistasis” is composed of Greek roots that mean “standing upon.” A good example of epistasis is coat color in the popular dog breed the Labrador retriever. Labrador retriever coat color genes only come in black or chocolate. But you see yellow Labrador retrievers running around the dog park. This occurs when recessive epistatic genes called “extension genes” don't actually allow color pigment to reach the fur.
Chief & NIH Distinguished Investigator
Cancer Genetics and Comparative Genomics Branch