Practice question

Recessive epistasis often termed supplementary gene interaction because recessive homozygous epistatic genotype must act supplementarily with other gene to allow phenotype development. In this model, one gene supplements action of other, providing necessary substrate or cofactor; without supplement trait absent. Terminology supplementary distinguishes from complementary interaction where both dominants required simultaneously for novel trait, and duplicate interaction where either dominant suffices. Supplementary emphasizes recessive masking where homozygous recessive reveals epistasis. Historical literature uses supplementary to describe 9:3:4 pattern reflecting biochemical dependency between loci in pathway.