What is Heterozygote advantage?
'''Heterozygote advantage''' is the situation where, at a [[gene]] [[Locus (genetics)|locus]], the [[heterozygote]] has greater [[fitness]] than either [[homozygote]].<ref>King R.C. Stansfield W.D. & Mulligan P.K. 2006. ''A dictionary of genetics''. 7th ed, Oxford University Press.</ref><sup>p205</sup> It is a type of [[balancing selection]]. == Sickle cell anaemia as an example == Hb<sup>+</sup> = gene for normal haemoglobin molecules; Hb<sup>s</sup> = [[Mutation|mutant]] gene. Three [[genotype]]s are possible in a population: Hb<sup>+</sup>Hb<sup>+</sup> producing a normal [[phenotype]]; Hb<sup>+</sup>Hb<sup>s</sup> the heterozygote; and Hb<sup>s</sup>Hb<sup>s</sup> homozygous for [[sickle cell anaemia]]. === Fitness === The fitness of these three genotypes varies according to the [[environment]]. In the [[temperate]] zone, the normal homozygotes and the heterozygotes will leave most offspring. This is because the homozygote for sickle cell has less chance of surviving to leave children, and if they do survive, they will have many health problems. In a [[tropical]] climate, such as equatorial [[Africa]], [[malaria]] is a big killer. There, the heterozygote has the advantage in fitness. They do not get malaria so badly, and they do not suffer the earlier deaths of the homozygote sickle cell sufferers. Not surprisingly, the Hb<sup>s</sup> gene is present in about a quarter of Africans from the countries near the [[equator]]. In [[Europe]]ans it is a rare gene. == Polymorphism == Heterozygote advantage is probably the main cause of [[polymorphism]] in biological [[species]]. Polymorphism, alternate genetic types in a population (for example different [[blood group]]s), is extremely common. The fitness of the heterozygotes keeps a wide range of alternate gene alleles in a population. This, in turn, gives the population a wide range of genetic 'resources' and this gives a better chance of survival. == Related pages == *[[Hybrid vigour]] == References == {{Reflist}} [[Category:Classical genetics]]