traits (see Figure 1). A mutation on a pleiotropic gene could influence all or only some of the traits it affects. Pleiotropy comes from Greek words pleio and tropic, which mean “many” and “affecting,” respectively. This is different from polygenic traits, which occur when a single phenotype arises from multiple genes.
There are direct and antagonistic pleiotropy. In direct pleiotropy, the effects of a single gene directly acts upon phenotypic traits whereas antagonistic pleiotropy occurs when one gene controls both beneficial and detrimental to fitness phenotypic traits. A ubiquitous example of direct pleiotropy is phenylketonuria (PKU), a human disease that results in mental retardation, hair loss, and skin pigmentation. This disease results from a mutation on a single gene that codes the enzyme that converts phenylalanine to tyrosine, two amino acids. Due to this, phenylalanine accumulates in the bloodstream, which becomes toxic to the nervous system and induces symptoms in infants. Tyrosine is responsible for producing melanin, a pigment found in skin and hair so lack of it leads to fairer skin and hair. Other common diseases caused by direct pleiotropy are albinism and sickle-cell anemia.
The antagonistic pleiotropy hypothesis was introduced in 1957 by George C. Williams. This idea is routed in evolution and was used by William to explain biological aging. An example of an antagonistic pleiotropy gene is p53, which has the beneficial effect of preventing cancer by suppressing tumors, but also the detrimental effect of thwarting stem cells that replace damaged or aged cells.
1. Wikipedia article: Pleiotropy. Date accessed: Dec 6, 2014.
2. Wikipedia article: Antagonistic Pleiotopy Hypothesis. Date accessed: Dec 6, 2014.
3. Lobo I. Pleiotropy: One Gene Can Affect Multiple Traits. Nature Education. 2008. Date accessed: Dec 6, 2014.