Final answer:
When a homozygous dominant brown mouse is crossed with a heterozygous brown mouse (tan being the recessive color), all offspring will have the albino phenotype.
Step-by-step explanation:
In this scenario, we are crossing a homozygous dominant brown mouse (AA) with a heterozygous brown mouse (Aa). The tan color is the recessive color (aa). The gene responsible for pigment production (C) is epistatic to the coat color gene (A). A mouse with the homozygous recessive genotype (cc) for the pigment production gene will be albino, regardless of the allele present at the coat color gene locus (A).
The genotype of the homozygous dominant brown mouse is AACC, while the genotype of the heterozygous brown mouse is AACc. When these two mice are crossed, the possible genotypes of the offspring are AACC, AACC, AAcc, AAcc. All of these genotypes will produce the same albino phenotype due to the presence of the recessive c allele at the pigment production gene locus.
Therefore, the phenotypic ratio of the offspring in this cross will be 4 albino:0 brown, as all of the offspring will have the albino phenotype.