What is the most complete definition of epistasis?

The most complete definition of epistasis is centered on the concept of one gene influencing or masking the effects of another gene. This phenomenon occurs when the alleles of one gene can overshadow or hide the expression of alleles from another gene, leading to a specific phenotype that may not be the direct result of either gene alone.

For instance, in certain genetic interactions, the presence of a dominant allele from one gene can prevent the expression of the alleles at a second gene locus, thereby altering the expected phenotypic outcome. This interaction plays a significant role in the complexity of genetic traits and can explain variations that deviate from Mendelian ratios.

The other options, while related to genetic inheritance, do not accurately capture the essence of epistasis. The first option describes general gene interaction rather than focusing on the masking relationship. The second option refers to the cumulative effect of multiple genes contributing to a phenotype, which is more aligned with polygenic inheritance than specifically epistasis. The fourth option speaks about blending phenotypes, which is more typical of incomplete dominance rather than the specific interaction of epistasis.