Practice question

Epistasis represents inter-locus interaction where genotype at one locus masks or modifies phenotypic expression of genotype at second non-allelic locus. Underlying mechanism often involves genes encoding sequential enzymes in same biosynthetic pathway where upstream mutation blocks substrate production, preventing downstream gene phenotype regardless of its genotype, or regulatory factor suppresses transcription of target. Outcome changes dihybrid phenotypic expectation from 9:3:3:1 to modified ratios like 12:3:1 or 9:3:4. Concept distinct from dominance which is intra-locus allele interaction and from linkage concerning physical chromosome proximity.