Epistasis is best defined as:

Epistasis refers to the interaction between genes, specifically where the expression of one gene is influenced or suppressed by another gene. In this context, the correct definition highlights that one gene can mask or alter the phenotypic expression of another gene's alleles. This can have important implications in genetics, especially in predicting the outcomes of genetic crosses and understanding complex traits.

For example, if Gene A produces a pigment in flowers but Gene B controls whether that pigment is produced at all, the presence or absence of the pigment in the flowers depends on the interaction between these two genes. If Gene B inhibits the expression of Gene A, then regardless of the alleles present in Gene A, the trait will not be expressed if Gene B is expressed in a specific way.

This concept is crucial in genetics as it illustrates how various genes work together to determine physical traits, which is essential knowledge for anyone studying biology or preparing for assessments in the field. Understanding epistasis helps clarify the complexity of inheritance beyond simple Mendelian genetics.