genetic polymorphism The occurrence in a population of two or more genotypes in frequencies that cannot be accounted for by recurrent mutation. Such occurrences are generally long-term. Genetic polymorphisms may be balanced (such that allele frequencies are in equilibrium with one another at a given locus), or transient (such that a mutation is spreading through the population in a constant direction). In the former case, the different alleles may be maintained by different environmental conditions (in space or time), one being favoured under one set of circumstances and another under another set; or a heterozygous genotype may be in some way superior to the genotypes that are homozygous at that locus (called a ‘heterozygous advantage’). It has been pointed out, however, that in many populations polymorphism is so high that to account for it all by natural selection would entail an impossible genetic load; thus, a good deal of polymorphism would be due to chance increase or decrease of neutral alleles.

genetic polymorphism An occurrence in a population of two or more genotypes in frequencies that cannot be accounted for by recurrent mutation. Such occurrences are generally long-term. Genetic polymorphism may be balanced (such that allele frequencies are in equilibrium with one another at a given locus) or transient (such that a mutation is spreading through the population in a constant direction). In the former case, the different alleles may be maintained by different environmental conditions (in space or time), one being favoured under one set of circumstances and another under another set; or a heterozygous genotype may be in some way superior to the genotypes that are homozygous at that locus; this is termed a ‘heterozygous advantage’. It has been pointed out, however, that in many populations polymorphism is so high that to account for it all by natural selection would entail an impossible genetic load; thus, a good deal of polymorphism would be caused by chance increase or decrease of neutral alleles.

genetic polymorphism An occurrence in a population of 2 or more genotypes in frequencies that cannot be accounted for by recurrent mutation. Such occurrences are generally long-term. Genetic polymorphism may be balanced (such that allele frequencies are in equilibrium with one another at a given locus), or transient (such that a mutation is spreading through the population in a constant direction). In the former case, the different alleles may be maintained by different environmental conditions (in space or time), one being favoured under one set of circumstances and another under another set; or a heterozygous genotype may be in some way superior to the genotypes that are homozygous at that locus; this is termed a ‘heterozygous advantage’.

genetic polymorphism See polymorphism.