Epistasis is a situation in genetics in which the expression of a particular gene depends on the presence of a modifier gene. It is different from dominance, which is when one allele of a gene is expressed over another allele of the same locus. Epistasis has causes at the genetic and molecular level.
The University of Utah defines dominance as an inheritance pattern of certain traits. Organisms that result from sexual reproduction have one copy of each gene from each parent. These copies are called alleles. The dominant allele is the copy of the gene that is expressed. For example, children receive genes for eye color from each of their parents. If one parent has hazel eyes and the other has blue eyes but the allele for hazel eyes is dominant, then the child has hazel eyes.
Epistasis is something separate and more complex. Nature.com indicates that epistasis is a major factor in determining genetic expression in organisms. There are several causes of epistasis. Often, it occurs when multiple genes in an organism act in parallel to achieve the same effect. Epistasis within the genome is the result of interactions between genes. If the genes inhibit each other's activity, modify one another or are separate components of a multi-component protein, there is a direct interaction of epistasis.