Pleiotropy (Greek: 'many turns') occurs when a single gene (allele) influences two or more apparently unrelated phenotypic traits. Classic examples: (1) The PKU gene affects pigmentation, brain development, and metabolism; (2) Sickle cell anaemia allele affects red blood cell shape, immunity to malaria, and multiple organ systems; (3) Marfan syndrome gene affects connective tissue in the skeleton, eyes, heart, and aorta. Pleiotropy reflects the multiple biochemical roles of a single gene product.