The genetic phenomenon in which one gene influences multiple phenotypic traits is pleiotropy. A pleiotropic gene may affect several body systems because its protein is active in different tissues, participates in multiple pathways, or influences development at more than one stage.
Phenylketonuria, or PKU, illustrates the idea. Mutations in the PAH gene affect production of phenylalanine hydroxylase, causing phenylalanine to accumulate and potentially producing effects involving the nervous system, skin, and pigmentation. One genetic change therefore has several observable consequences.
Pleiotropy is not the same as polygeny, in which several genes contribute to one trait. It is also distinct from epistasis, where one gene modifies or masks the effect of another gene. The term was coined by German biologist Ludwig Plate in 1910. Modern genome-wide studies frequently find pleiotropic variants associated with multiple traits, although some apparent associations can result from linked genes or statistical artifacts.