genetic polymorphism

polymorphism

 [pol″e-mor´fizm]

the ability to exist in several different forms.

balanced polymorphism an equilibrium mixture of homozygotes and heterozygotes maintained by natural selection against both homozygotes.

genetic polymorphism the occurrence together in the same population of two or more genetically determined phenotypes in such proportions that the rarest of them cannot be maintained merely by recurrent mutation.

single nucleotide polymorphism (SNP) a genetic polymorphism between two genomes that is based on deletion, insertion, or exchange of a single nucleotide.

Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.

ge·net·ic pol·y·mor·phism

the occurrence in the same population of multiple discrete alletic states of which at least two have high frequency (conventionally of 1% or more).

Farlex Partner Medical Dictionary © Farlex 2012

genetic polymorphism

or

polymorphism

the presence in a population of two or more MORPHS, produced when different alleles of a gene occur in the same population and the rarest allele is not maintained merely by repeated MUTATION (i.e. has a frequency higher than, say, 0.05%). Such a definition excludes continuously variable characters such as height or skin colour in humans, but the human blood groups are classic examples, where single genes have two or more alleles, producing different antigenic phenotypes. A genetic polymorphism can be maintained by several mechanisms such as heterozygous advantage or FREQUENCY-DEPENDENT SELECTION, and can be stable over several generations (a BALANCED POLYMORPHISM) or may become ‘transient’ as when the environment changes; see, for example, SICKLE-CELL ANAEMIA.

Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005