Pleiotropy occurs when a single gene influences multiple phenotypic traits. Consequently, a new mutation in the gene will have an effect on all traits simultaneously. This can become a problem when selection on one trait favours one specific mutant, while the selection at the other trait favours another mutant.
The term pleiotropy comes from the Greek pleio, meaning "many", and trepein, meaning "influencing".
Pleiotropy describes the genetic effect of a single gene on multiple phenotypic traits. The underlying mechanism is that the gene codes for a product that is for example used by various cells, or has a signalling function on various targets.
A classic example of pleiotropy is the human disease PKU (phenylketonuria). This disease can cause mental retardation and reduced hair and skin pigmentation, and can be caused by any of a large number of mutations in a single gene that codes for an enzyme (phenylalanine hydroxylase) that converts the amino acid phenylalanine to tyrosine, another amino acid. PKU is totally benign if a diet free from phenylalanine is maintained. Depending on the mutation involved, this results in reduced or zero conversion of phenylalanine to tyrosine, and phenylalanine concentrations increase to toxic levels, causing damage at several locations in the body.
Antagonistic pleiotropy refers to a situation in which a single gene creates multiple competing effects, such that beneficial effects of a trait created by the gene are offset by 'losses' in other traits. One example is a theory of aging first developed by G. C. Williams in 1957. Williams suggested that one gene is responsible for increased fitness when young at the expense of decreased fitness later in life (i.e. aging). One such example in male humans is the level of the hormone testosterone, where earlier in life increases reproductive fitness but is also associated with decreased fitness later in life, such as susceptibility to prostate cancer. Another example might be a gene in a bacterium which confers increased glucose utilization efficiency at the expense of other carbon sources (such as lactose).