During which genetic process does a second gene affect the expression of a first gene?

Epistasis occurs when the expression of one gene is influenced by one or more additional genes. This can manifest in various ways, such as when the alleles of one gene mask or modify the phenotypic expression of alleles of another gene. The classic example of epistasis is seen in coat color in certain dog breeds, where one gene determines the pigment type (black or brown), while another gene may determine whether the pigment is expressed at all (such as a gene that determines if the coat is yellow or not).

Transformation, transduction, and conjugation are all forms of genetic exchange or recombination processes that contribute to genetic diversity in organisms, particularly bacteria. However, they do not involve the regulatory interactions between genes that characterize epistasis. Transformation involves the uptake of naked DNA from the environment, transduction involves the transfer of DNA from one bacterium to another via a virus, and conjugation is characterized by direct transfer of DNA between bacteria through cell-to-cell contact. None of these processes directly address how the expression of one gene can be modified by another gene. Therefore, the best answer for the genetic interaction described in the question is epistasis.