Final answer:
Epistasis occurs when the expression of one gene pair masks or modifies the expression of another, impacting the phenotypic outcome of an organism, such as coat color in animals.
Step-by-step explanation:
When the expression of one gene pair masks or modifies the expression of another gene pair, this interaction is known as epistasis. This concept is distinct from other phenomena like incomplete dominance or codominance, where one allele doesn't completely dominate or mask the other, but results in a phenotype that is a blend or combination of the parent traits. In epistasis, however, one gene interferes with or suppresses the expression of another gene. An example of this can be seen in the way coat color in some animals is determined, where one gene can prevent the expression of another, resulting in a specific coat color or pattern.
Phenotypic ratios can be affected by epistasis, as seen in an example where one gene (C) masks the expression of another (A) for coat color. If the Callele is present, coat color is expressed; conversely, if it is absent (cc), no coat color is expressed, regardless of the alleles at the A gene.