Which statement below defines epistasis?

a. when a single gene affects several phenotypes

b. a heterozygote displays a phenotype intermediate between its two alleles

c. a measurement of phenotype intensity

d. when one locus affects or covers the outcome of another locus

The correct assertive is letter D. When one locus affects or coverbs the outcome of another locus.

Explanation:

Epistasis is when alleles of one gene inhibit an action of alleles of another pair, which may or may not be on the same chromosome. The gene that exerts inhibitory action is called epistatic, and the gene that inhibits it is called hypostatic. This fact about epistasis is common among animals and plants.