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Epistasis occurs when the expression of one gene is affected by the expression of a different gene.
Example. Consider two proteins in a metabolic pathway. Enzyme AB takes molecule A and converts it into molecule B. Then Enzyme BC takes molecule B and converts it into molecule C.
Now say there are two different alleles for both enzyme AB and enzyme BC. We'll call them enzyme AB1, enzyme AB2, enzyme BC1 and enzyme BC2 to keep them straight. The two variants of both enzymes produce different products. Enzyme AB1 produces B1 and enzyme AB2 produces B2. Both enzymes BC1 and BC2 can ONLY bind product B1 (not B2) and converts it into molecules C1 and C2.
This is a case of epistasis because if the allele for enzyme AB only produces enzyme AB2, it will only produce molecule B2 which enzyme BC1 and BC2 can't bind. Therefore, it doesn't matter which alleles for enzyme BC are present - the reaction won't go because there is no bindable substrate.