The ability of one gene to influence the expression of another gene, or even a group of genes. The existence of epistasis should not be surprising if one considers biochemical pathways. Several different genes are involved in such pathways, and if an early step in the pathway is blocked, then the precursor for a later gene is not made, and thereby the action of the later gene is disrupted. Expressions of alleles at that later point in the pathway can be blocked by epistasis.
