Question

ACTN3 is a gene that encodes for alpha-actinin-3, a protein expressed only in type-Il muscle fibers. The R allele has been associated with high endurance and speed in athletes. However, not all individuals that have the R allele show the "fast" phenotype. Lets pretend that the Rallele is dominant, and there is another gene involved in mitochondria efficiency, the A allele codes for highly efficient mitochondria and the a allele codes for low efficient mitochondria. When mitochondria efficiency is low, or the Rallele is not present, the fast phenotype is not expressed.

Answer 1

Gene 1 (ACTN3) would be the epistatic gene and Gene 2 (Mitochondria efficiency) would be the epistatic gene.

A gene that masks the phenotypic effect of another gene is called an epistatic gene. Here, gene 1 and 2 mask or modify the effect of "fast" phenotype.