Practice question

Epistasis describes interaction between alleles at different loci where genotype at one locus alters phenotypic expression of genotype at second locus. Molecularly genes may encode sequential enzymes in shared biosynthetic pathway; loss-of-function upstream prevents downstream step regardless of downstream genotype, or regulatory protein suppresses transcription of target gene. Phenotypic consequence is masking or modification, altering expected 9:3:3:1 into 12:3:1, 9:3:4, 9:7, 15:1. Concept differs from pleiotropy where single gene impacts multiple traits, polygenic inheritance where many genes additively affect one quantitative trait, and linkage concerning physical proximity.