Final answer:
In mice, the agouti phenotype is due to the interaction between the A and C genes. The A gene determines coat color, with agouti being dominant. The C gene, which is responsible for pigment production, is epistatic to the A gene.
Step-by-step explanation:
In mice, the agouti phenotype, which refers to the different size and coat color of offspring, is due to the interaction between two genes: the A gene and the C gene. The A gene determines coat color, with the mottled agouti coat color (A) being dominant to solid colorations such as black or gray. The C gene is responsible for pigment production, and when it is present as the recessive homozygote (cc), it negates any expression of pigment from the A gene, resulting in an albino mouse.
The interaction between the A and C genes is an example of epistasis, where one gene masks the effects of another gene. In this case, the C gene is epistatic to the A gene. When a cross is made between heterozygotes for both genes (AaCc x AaCc), the offspring exhibit a phenotypic ratio of 9 agouti:3 solid color:4 albino.