Epistasis is a circumstance where the expression of one gene is affected by the expression of one or more independently inherited genes. For example, if the expression of gene #2 depends on the expression of gene #1, but gene #1 becomes inactive, then the expression of gene #2 will not occur. In this example, gene #1 is said to be epistatic to gene #2.
Epistasis is actually a complicated term that's misused quite a bit in the literature. But it refers to a circumstance where the expression of one gene is affected by the expression of one or more independently inherited genes. Okay, that doesn't really make it much easier. Let me give you an example. If the expression of gene 2 depends upon the expression of gene 1, but gene 1 becomes inactive, it actually also effects the expression of gene 2. So you can't really tell what's going on with gene 2 if gene 1's altered. Gene 1 is then said to be epistatic to gene 2. Okay, that also sounds kind of complicated. Let me give you an example. If you look at the color of animals... One of the genes that's needed to make color is tyrosinase, and if you don't have tyrosinase, if it's mutated, it results in a condition call albinism. So the fur or the skin's completely white because you can't make the pigment. If there's another gene involved in making pigment--for example, making brown pigment--if you're wild-type for tyrosinase, you can see the brown mutation, and you have a brown animal. But if you have a tyrosinase mutation and a brown mutation, it still looks like tyrosinase mutation; it's all white. You don't see whether it has the brown mutation or not. So therefore, tyrosinase is epistatic to brown.