Can you help me understand this please? Quote is from “Fundamentals of Psychobiology” published by Sanz and Torres (Fundamentos de Psicobiología) Quotes in italics: Epistasis consists of the interaction between genes which determin characteristics in such a way as that one gene masks the efect of the other. (the example that wikipedia gives of this is very nice, a fly who has a gene “for long wings” and another gene for “wrinkly wings” but neither of these are expressed in the phenotype because he also has a gene for “no wings”) I can't get my head round the more detailed example in my textbook. The part in bold is the problem. Two genes are involved in congenital deafness in humans (a and b) each of which presents 2 alleles. If one of these appears in recessive homozygosis, deafness is manifested independly of which allele the other presents. However, if there is at least one dominant allele in each gene, deafness does not manifest. That is, any of the following genotypes: Aabb; Aabb; aabb; aaBB or aaBb, means the person will be deaf. Any of the following: AABB; AABb; AaBB or AaBb; will prevent deafness. I think an explanation of why might help me understand, or maybe a more detailed example. For example applying the example to the genotypes. I don't understand what genes A and B are actually doing in the example. Unlike the example with the flies where I can imagine A=long wings, a = short wings, B = wings, b = no wings, so Aabb = fly with long wings while AABB = fly with no wings. Also it says that if there is one dominant allele then deafness will not manifest, but surely all but one of the list of genotypes resulting in deafness had a dominant allele?