Pleiotropy

Pleiotropy occurs when a single gene influences multiple phenotypic traits. Consequently, a mutation in a pleiotropic gene may have an effect on some or all traits simultaneously. This can become a problem when selection on one trait favors one specific version of the gene (allele), while the selection on the other trait favors another allele.

Etymology
The term pleiotropy comes from the Greek πλείων pleion, meaning "more", and τροπή tropí, meaning "turn, convert". A common mistake is to use "pleiotrophic" instead of "pleiotropic".

Mechanism
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 signaling 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 the enzyme (phenylalanine hydroxylase), which converts the amino acid phenylalanine to tyrosine, another amino acid. 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. PKU is totally benign if a diet free from phenylalanine is maintained.

Antagonistic pleiotropy
Antagonistic pleiotropy refers to the expression of a gene resulting in multiple competing effects, some beneficial but others detrimental to the organism.

This is central to a theory of aging first developed by G. C. Williams in 1957. Williams suggested that some genes responsible for increased fitness in the younger, fertile organism contribute to decreased fitness later in life. An example is the p53 gene, which suppresses cancer, but also suppresses stem cells, which replenish worn-out tissue.

Whether or not pleiotropy is antagonistic may depend upon the environment; for instance, a bacterial gene that enhances glucose utilization efficiency at the expense of the ability to use other energy sources (such as lactose) has positive effects when there is plenty of glucose, but can be lethal if lactose is the only available food source.

Evolution
Pleiotropy of genes impacts the evolutionary rate of genes. Traditionally it has been expected that evolutionary rate of genes is related negatively with pleiotropy, however this has not been clearly found in empirical data. Contrarily to this traditional expectation, it has been theoretically demonstrated that evolutionary rate should be positively related with pleiotropy by itself as the square root of gene pleiotropy, however other effects of pleiotropy may explain why a clear relationship between evolutionary rate and gene pleiotropy has not been found at genomic scale.