Practice question

Recessive epistasis occurs when homozygous recessive genotype at epistatic locus aa masks expression of second locus, regardless of genotype at hypostatic locus. Phenotypically genotypes aaB- and aabb become indistinguishable because upstream block prevents downstream phenotype. Remaining segregating classes are 9 A-B- both dominant and 3 A-bb single dominant, plus combined 4 for masking class 3+1, producing ratio 9:3:4. Upstream gene may encode enzyme producing substrate required by downstream enzyme. Mouse albino cc preventing agouti and brown expression, and grain color maize are textbook illustrations.