The process by which one gene can mask the phenotypic expression of another is known as:

The correct answer is epistasis, which refers to a genetic interaction where the expression of one gene can completely mask or alter the effect of another gene. This concept is essential in understanding how different genes interact to influence an organism’s phenotype. In epistasis, the action of one gene can overshadow the effects associated with another gene, resulting in a variety of phenotypic outcomes.

For example, in some cases, a gene that determines pigmentation in an organism can inhibit the expression of another gene that influences the pattern or color, leading to a particular trait being more visibly expressed or even completely silenced.

Other options like dominance, codominance, and incomplete dominance describe different relationships between alleles of a single gene, rather than interactions between two distinct genes. Dominance refers to scenarios where one allele predominates over the other in determining a trait. Codominance involves both alleles contributing to the organism's phenotype equally, while incomplete dominance refers to a blending of traits where neither allele is fully dominant. These concepts are distinct from epistasis, which specifically deals with the masking effect of one gene on another's expression.