What is the role of epistasis in genetics?

The concept of epistasis refers to the interaction between genes, where the expression of one gene is affected by the presence or absence of one or more other genes. This means that an epistatic gene can mask or modify the expression of a gene located at a different locus. For instance, in coat color in certain dog breeds, one gene may determine whether pigment is produced (the color), while another gene can determine whether that pigment is deposited. If the gene controlling pigment deposition is non-functional, it doesn't matter what the first gene says about color; the resulting phenotype will be affected based on the interaction.

This role is significant in genetics as it helps explain the complexity of inheritance patterns beyond simple Mendelian genetics, where traits are assumed to be controlled by a single gene pair. Thus, the essence of epistasis is specifically about the alteration of gene expression due to the interactions between different genes, which makes the answer particularly relevant in discussions about genetic interactions and phenotypic outcomes.