Practice question

Pleiotropy occurs when single gene influences multiple apparently unrelated phenotypic traits because gene product functions in several tissues or participates in branched metabolic pathway. Mutation in such gene therefore produces syndromic effects: e.g., fibrillin-1 mutation affecting skeleton, eyes, and aorta in Marfan; phenylalanine hydroxylase deficiency affecting hair color, skin pigmentation, and cognitive development. Mechanistic basis includes protein expression in diverse cell types or substrate used in multiple reactions. This contrasts with polygenic inheritance where many genes affect one trait, codominance where both alleles express, and epistasis involving interaction.