Final answer:
Epistasis is when a gene at one locus affects the expression of a gene at a different locus, modifying or masking the effects of the second gene's alleles.
Step-by-step explanation:
A gene at one locus affecting the expression of a gene at a different locus is termed epistasis. This is a form of gene interaction where the expression of an allele for one gene masks or modifies the expression of an allele for another gene. Unlike dominance, where one allele of the same gene masks the effect of another, epistasis involves genes at different loci. It differs from independent assortment as well, which is the random distribution of alleles during gamete formation, ensuring that the inheritance of one trait does not affect the inheritance of another.
Epistasis can significantly alter the expected ratios of phenotypes in offspring, as it involves the complex interactions between multiple genes contributing to a single trait. An example of epistasis is in coat color in various animals, where one gene can prevent the expression of another gene that typically controls pigment production, resulting in an unexpected phenotype.