What describes epistasis in genetics?

Epistasis refers to a genetic interaction where the expression of one gene can suppress or alter the expression of another gene. This means that when two genes are involved in determining a particular trait, one gene (the epistatic gene) can mask or modify the effect of the second gene (the hypostatic gene). This interaction can significantly influence the phenotype, or observable traits, of an organism.

For instance, in certain coat color patterns in animals, the presence of one gene can determine whether another gene's associated color will be expressed. In such cases, it's not just a simple dominance or recessiveness that determines the observable trait; rather, the interaction between the genes introduces complexity in the final phenotype.

Other options describe different genetic concepts: one involves incomplete dominance, another relates to polygenic traits, and the last one addresses co-dominance. Each of these concepts illustrates various ways that genes can interact or express traits, but they do not accurately define epistasis.