Practice question

Mouse coat color genetics demonstrates recessive epistasis where upstream locus C required for melanin synthesis masks downstream loci. Genotype cc lacks tyrosinase enzyme essential for pigment production, resulting in albino phenotype irrespective of agouti A or brown B alleles at other loci. Therefore cc acts epistatically over B locus hypostatic genes, preventing their expression. Dihybrid cross involving Cc x Bb yields modified 9:3:4 ratio where four represents albino class combining cc genotypes. This illustrates sequential biochemical pathway blockade and hierarchical gene interaction, not simple dominance, linkage, or incomplete dominance.