Epistasis is the interaction between two or more genes to control a single phenotype. We model epistasis with a two-locus two-allele problem. The resulting model allows us to examine both population sizes as well as genotypic and phenotypic frequencies. In the context of an example, we show that if epistasis results in an undesirable phenotype, such as a genetically transmitted disease or condition, elimination of undesirable phenotypes from the population through selective breeding may be possible, although such selective breeding could lead to an increase in less desirable phenotypes and possibly the elimination of desirable phenotypes.