Gene polymorphism

Genes which control hair colour are polymorphic.

A gene is said to be polymorphic if more than one allele occupies that gene's locus within a population.[1] In addition to having more than one allele at a specific locus, each allele must also occur in the population at a rate of at least 1% to generally be considered polymorphic.[2]

Gene polymorphisms can occur in any region of the genome. The majority of polymorphisms are silent, meaning they do not alter the function or expression of a gene.[3] Some polymorphisms are visible. For example, in dogs the E locus can have any of five different alleles, known as E, Em, Eg, Eh, and e.[4] Varying combinations of these alleles contribute to the pigmentation and patterns seen in dog coats.[5]

A polymorphic variant of a gene can lead to the abnormal expression or to the production of an abnormal form of the protein; this abnormality may cause or be associated with disease. For example, a polymorphic variant of the gene encoding the enzyme CYP4A11, in which thymidine replaces cytosine at the gene's nucleotide 8590 position encodes a CYP4A11 protein that substitutes phenylalanine with serine at the protein's amino acid position 434.[6] This variant protein has reduced enzyme activity in metabolizing arachidonic acid to the blood pressure-regulating eicosanoid, 20-hydroxyeicosatetraenoic acid. A study has shown that humans bearing this variant in one or both of their CYP4A11 genes have an increased incidence of hypertension, ischemic stroke, and coronary artery disease.[6]

Most notably, the genes coding for the major histocompatibility complex (MHC) are in fact the most polymorphic genes known. MHC molecules are involved in the immune system and interact with T-cells. There are more than 32,000 different alleles of human MHC class I and II genes, and it has been estimated that there are 200 variants at the HLA-B HLA-DRB1 loci alone.[7]

Some polymorphism may be maintained by balancing selection.

  1. ^ "Genetic polymorphism - Biology-Online Dictionary | Biology-Online Dictionary". September 2020.
  2. ^ "Genetic Testing Report-Glossary". National Human Genome Research Institute (NHGRI). Retrieved 2017-11-08.
  3. ^ Chanock, Stephen (2017-05-22). "Technologic Issues in GWAS and Follow-up Studies" (PDF). Genome.gov. Archived from the original (PDF) on 2018-08-22. Retrieved 2017-11-30.
  4. ^ "Dog Coat Colour Genetics".
  5. ^ "E-Locus (Recessive Yellow, Melanistic Mask Allele)". www.animalgenetics.us. Archived from the original on 2017-10-30. Retrieved 2017-11-08.
  6. ^ a b Wu CC, Gupta T, Garcia V, Ding Y, Schwartzman ML (2014). "20-HETE and blood pressure regulation: clinical implications". Cardiology in Review. 22 (1): 1–12. doi:10.1097/CRD.0b013e3182961659. PMC 4292790. PMID 23584425.
  7. ^ Bodmer, J. G.; Marsh, S. G. E.; Albert, E. D.; Bodmer, W. F.; Bontrop, R. E.; Dupont, B.; Erlich, H. A.; Hansen, J. A.; Mach, B. (1999-04-01). "Nomenclature for factors of the HLA system, 1998". European Journal of Immunogenetics. 26 (2–3): 81–116. doi:10.1046/j.1365-2370.1999.00159.x. ISSN 1365-2370. PMID 10331156.