Answer the following question given this paragraph: 

Epistasis is when a gene at one location (locus) alters the phenotypic expression of a gene at another locus. Epistasis takes place when the action of one gene is modified by one or several other genes. These genes are sometimes called modifier genes . The gene whose phenotype is expressed is said to be epistatic , while the phenotype that is altered is said to be hypostatic . Sometimes hypostatic phenotypes are completely suppressed. Epistatic genes are not dominant over the genes they alter or suppress. Dominance refers to an interaction between alleles of the same gene, not different genes.


Q: What is it called when a gene at one location (locus) alters the phenotypic expression of a gene at another locus?

 Choices:

- exogenous

- epigenesis

- spontaneous mutation

- epistasis

A:
epistasis
