1) One gene interferes with or prevents the expression of another gene located at a different locus.

2) 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.