Practice question

Duplicate recessive epistasis termed complementary gene interaction because phenotypic expression requires complementary presence of dominant alleles at two loci simultaneously to produce trait. Neither dominant alone can complete biochemical pathway; both must be present to generate end product like purple anthocyanin pigment in sweet pea or cyanogenic glucoside. Term complementary emphasizes cooperative interdependence where genes complement each other function. Contrasts with duplicate dominant where either locus sufficient, and recessive epistasis where one recessive masks. Complementary action yields diagnostic 9:7 ratio, hallmark of interdependent steps where loss at any step aborts pathway to same blocked phenotype.