Pleiotropy

Pleiotropy (from Greek πλείων pleion, "more", and τρόπος tropos, "way") occurs when one gene influences two or more seemingly unrelated phenotypic traits. Therefore, a mutation in a pleiotropic gene may have an effect on several traits simultaneously due to the gene coding for a product used by a myriad of cells or different targets that have the same signaling function.

There are three types of pleiotropy: molecular gene pleiotropy, developmental pleiotropy, and selectional pleiotropy. The different types are used to help categorize the different types of pleiotropy that can occur, but they are not exclusive to one another since the different types do overlap. Molecular gene pleiotropy is seeing how many traits are affected by a molecular gene. Developmental pleiotropy is a mutational pleiotropy observing the relationships between mutations and different phenotypes, excluding fitness. Selectional pleiotropy is evaluating how a mutation affects fitness.

An example of pleiotropy is phenylketonuria, which is an inherited disorder that affects the level of phenylalanine in the body. Phenylalanine is an amino acid that can be obtained from food. Phenylketonuria causes this amino acid to increase in amount in the body, which can be very dangerous. The human disease is caused by a defect in a single gene on chromosome 12 that affects multiple systems, such as the nervous and integumentary system. Other examples of pleiotropy are albinism, sickle cell anemia, and certain forms of autism and schizophrenia. Pleiotropy not only affects humans, but also animals, such as chickens and laboratory house mice, where the laboratory house mice have found to exhibit the "mini-muscle" allele.

Pleiotropic gene action can limit the rate of multivariate evolution when natural selection, sexual selection or artificial selection on one trait favors one specific version of the gene (allele), while selection on other traits favors a different allele, which shows how evolution is negatively related with pleiotropy in models. Although some of the evolution in genes can be beneficial, some gene evolution is harmful to an organism. Genetic correlations and responses to selection most often exemplify pleiotropy.

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