Epistatic and non-epistatic gene interactions are fundamental concepts in genetics and evolutionary biology that explain how multiple genes influence phenotypic expression. Epistasis occurs when the effect of one gene is masked or modified by another gene at a different locus. In such cases, the phenotypic outcome is not simply the sum of individual gene effects but depends on the hierarchical interaction among genes. For example, in coat color inheritance in mice, the gene responsible for pigment deposition can mask the action of pigment color genes, producing a uniform phenotype regardless of the underlying alleles. Epistasis can be recessive, dominant, or even involve