What is the definition of epistasis in genetics?

The concept of epistasis refers to the interaction between genes where the presence of certain alleles at one locus can influence or mask the effects of alleles at another locus. In this case, the correct definition highlights that the interaction between two alleles leads to a differing outcome when they are combined, compared to the effects of each allele individually. This means that when two alleles are present together, their combined effect can be different, either enhancing or suppressing the expression of each other's traits, rather than acting independently.

This interaction illustrates the complexity of gene expression and how multiple genes can influence a trait in ways that may not be predicted by looking at each gene in isolation. While other options may touch on related concepts in genetics, they do not accurately encapsulate the essence of epistasis, which specifically involves the combinatory effect of multiple alleles that changes their individual contributions.