Practice question

Duplicate recessive epistasis, also called complementary gene action, occurs when homozygous recessive at either locus produces same alternate phenotype, requiring dominant alleles at both loci together for distinct phenotype. Cross AaBb x AaBb gives 9 A-B- showing novel phenotype needing both dominants, while 3 A-bb plus 3 aaB- plus 1 aabb all display same recessive alternative, totaling 7. Resulting ratio 9:7 signals two genes must function cooperatively to complete biosynthetic pathway, such as two enzymes acting sequentially. Neither single dominant alone sufficient, creating complementary requirement for pathway completion.