Practice question

Complementary gene action requires simultaneous presence of dominant alleles at both loci to produce distinct phenotype; absence at either locus yields same alternate phenotype. In dihybrid F2 selfing AaBb x AaBb, only genotype A-B- 9 carries both dominants and displays novel trait, while 3 A-bb plus 3 aaB- plus 1 aabb totaling 7 lack one or both and show recessive alternative, resulting in 9:7 ratio. This distinguishes complementation from 9:3:4 recessive epistasis or 15:1 duplicate dominant. Sweet pea purple flower inheritance with C and P genes completing anthocyanin pathway together exemplifies complementary requirement for trait expression.