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Pleiotropy occurs when one gene influences multiple phenotypic traits. Consequently, a mutation in a pleiotropic gene may have an effect on some or all traits simultaneously. Pleiotropic gene action can limit the rate of multivariate evolution when natural selection, sexual selection or artificial selection on one trait favours one specific version of the gene (allele), while selection on other traits favors a different allele. The underlying mechanism of pleiotropy in most cases is the effect of a gene on metabolic pathways that contribute to different phenotypes. Genetic correlations and hence correlated responses to selection are most often caused by pleiotropy.
The term "pleiotropie" was coined in a 1910 Festschrift written by the German geneticist Ludwig Plate, a former student of Ernst Haeckel. The term "polyphaeon" was also suggested in 1925 by Haeckel but did not persist. Even before the term was proposed there were examples of distinct traits that seemed to be inherited together. In his classic 1866 paper, Gregor Mendel listed his trait number three in peas as having brown seed coat, violet flowers, and axial spots.
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.
A gene recently discovered in laboratory house mice, termed "mini-muscle," causes a 50% reduction in hindlimb muscle mass as its primary effect (the phenotypic effect by which it was originally identified:), in addition to various effects on behavior, skeletal morphology, relative size of internal organs, and metabolism. The mini-muscle allele behaves as a Mendelian recessive.
Antagonistic pleiotropy 
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.
Pleiotropy of genes impacts the evolutionary rate of genes and allele frequencies. Traditionally, it has been expected that evolutionary rate of genes is related negatively with pleiotropy; however, this has not been clearly found in empirical studies. Contrary 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 the genomic scale.
See also 
- Stearns, F. W. (2010). One Hundred Years of Pleiotropy: A Retrospective. Genetics 186(3):767-773. http://www.genetics.org/content/186/3/767.abstract
- Garland T, Jr, Morgan MT, Swallow JG, Rhodes JS, Girard I, Belter JG, Carter PA (2002). "Evolution of a small-muscle polymorphism in lines of house mice selected for high activity levels". Evolution 56: 1267–1275.
- Hannon RM, Kelly SA, Middleton KM, Kolb EK, Pomp D, Garland T, Jr (2008). "Phenotypic effects of the "mini-muscle" allele in a large HR x C57BL/6J mouse backcross". Journal of Heredity 99: 349–354.
- Williams, G.C. (1957) Pleiotropy, natural selection, and the evolution of senescence. Evolution 11: 398–411
- Rodier F, Campisi J, Bhaumik D (2007). "Two faces of p53: aging and tumor suppression". Nucleic Acids Res 35 (22): 7475–84. doi:10.1093/nar/gkm744. PMC 2190721. PMID 17942417.
- Pál, C. et al. (2001). Highly expressed genes in yeast evolve slowly. Genetics 158:927–931.
- Camps, M. et al. (2007). Genetic constraints on protein evolution. Crit. Rev. Biochem. Mol. 42:313-326.
- Razeto-Barry, P. et al. (2011). Molecular Evolution, Mutation Size and Gene Pleiotropy: a Geometric Reexamination. Genetics 187(3):877-885. http://www.genetics.org/content/187/3/877.abstract