Final answer:
The dark-colored mice may be less common because a gradual phenotypic shift from light to dark would require intermediate forms that are less fit and more vulnerable to predators, limiting natural selection's ability to favor such a transition.
Step-by-step explanation:
The presence of dark-colored mice at location A can be explained by the concept of phenotype suitability in a given environment. Dark-colored mice may be less common because an intermediate phenotype between dark and light coloration is less advantageous for survival. Mice that are in-between cannot blend well with the environment, which makes them more vulnerable to predators. Since these phenotypic changes happen gradually, natural selection would not favor an immediate shift from a light to a dark phenotype due to the decreased fitness of intermediate forms. This serves as a constraint on the direction of evolutionary change.
A phenotypic ratio can offer insights into the genetic distribution of traits like coat color in a population. According to the details provided, the mottled agouti coat color (A) is dominant, and coloration is also influenced by another gene (C) which controls pigment production. When considering epistasis, where one gene can mask the effect of another gene at a different locus, we can understand how these genetic interactions affect coat color. The C gene's role is noted to be epistatic to the A gene, and this influences the phenotypic expression of color in the mice.