What is the definition of epistasis

Question

A. When the allele of one gene interferes with the expression of another gene
B. When one gene affects multiple phenotypes
C. When one trait is determined by multiple genes
D. When a dominant allele for a specific trait is not completely expressed over the recessive allele

Answer 1

The correct definition of epistasis is:

A. When the allele of one gene interferes with the expression of another gene.

Epistasis occurs when the effect of one gene is dependent on the presence of one or more 'modifier genes'.