Pleiotropy (Greek: πλείων, pleion meaning "plurality", "plus", "more" and Greek: τρόπος, tropos, meaning "change", "turn") is the phenomenon whereby a single gene influences multiple phenotypic traits or different and unrelated characters. Genes that control multiple phenotypes are called pleiotropic.
Tuberous sclerosis is a rare multi-system genetic disease that causes non-malignant tumors to grow in the brain and on other vital organs such as the liver and the skin, for example. This disease demonstrates pleiotropy, resulting in a variety of apparently unrelated phenotypic features, such as skin hypopigmentation, multiple hamartomas and learning disability.
Albinism is also a pleiotropic trait affecting eyesight as well as resulting in a lack of pigmentation in skin, hair, and eyes.
Phenylketonuria is a disease that can cause mental retardation and can be caused by any of a large number of mutations in a single gene encoding an enzyme called phenylalanine hydroxylase present in the liver and necessary for the metabolism of phenylalanine. Phenylketonuria is a classic example of pleiotropy.
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