Pleiotropy is a condition in which a single gene influences more than one phenotypic trait in an organism. A phenotypic trait is any observable trait, in contrast to a genotypic trait, which is involved with the genetic composition of an organism. The word pleiotropy has a Greek origin; it comes from the words pleion, meaning "more," and trepein, meaning "to turn" or "to convert." If an animal had a gene that affected both the length and color of its fur, that gene would be considered pleiotropic. People commonly say pleiotrophic instead of pleiotropic, but this is incorrect; pleiotropic is the correct term to describe a gene that affects multiple traits.
Genes contain the genetic information needed to produce amino acids, which are the building blocks of proteins. Proteins have myriad purposes in almost all living things; they serve to regulate, facilitate, or directly cause countless different processes and reactions in most organisms. Some amino acids or proteins have several different effects in the body, so the gene that codes for such amino acids and proteins is considered to be pleiotropic. In most cases, the underlying mechanism of pleiotropy is said to be one gene that affects the production of a substance which influences several different parts of an organism. This is an important concept in evolutionary biology, in which the history and origin of different traits is considered to be very important.
Pleiotropy can, at times, cause harm to an organism. This happens when a defect in a single gene causes negative effects relating to several different traits of an organism. One example of this is the disease PKU, or phenylketonuria, which occurs in humans. It causes both mental retardation and a reduction in hair and skin pigmentation. The disease is caused by a genetic mutation and affects multiple traits, so it is an example of pleiotropy.
Some theories of human aging have come to rely heavily on antagonistic pleiotropy, a condition by which one gene codes for multiple traits that have different and competing effects. Some traits, such as testosterone production, help to increase general fitness early in life but can, later in life, lead to susceptibility to cancers and other diseases. The p53 gene presents a good example of antagonistic pleiotropy. While it suppresses cancer, it also suppresses stem cells that replenish old tissues. Studies focused on various bacteria have shown that antagonistic pleiotropy can be heavily based on the environment and on the various resources available to the organism.