Editing Balancing selection (section)

=== Heterozygote advantage ===
[[Image:sicklecells.jpg|framed|Sickle-shaped red blood cells. This non-lethal condition in heterozygotes is maintained by balancing selection in humans of Africa and India due to its resistance to the [[malarial parasite]].]]
[[File:Malaria versus sickle-cell trait distributions.png|thumb|Malaria versus sickle-cell trait distributions]]

{{main|Heterozygote advantage}}

In [[heterozygote advantage]], or ''heterotic balancing selection'', an individual who is heterozygous at a particular gene [[locus (genetics)|locus]] has a greater [[fitness (biology)|fitness]] than a homozygous individual. Polymorphisms maintained by this mechanism are ''balanced polymorphisms''.<ref>Heredity. 2009. ''Encyclopædia Britannica''. Chicago.</ref> Due to unexpected high frequencies of heterozygotes, and an elevated level of heterozygote fitness, heterozygotic advantage may also be called "overdominance" in some literature. 
 
A well-studied case is that of [[Sickle-cell disease|sickle cell anemia]] in humans, a [[Heredity|hereditary]] disease that damages [[red blood cells]]. Sickle cell anemia is caused by the inheritance of an allele (HgbS) of the [[hemoglobin]] [[gene]] from both parents. In such individuals, the hemoglobin in red blood cells is extremely sensitive to oxygen deprivation, which results in shorter life expectancy. 
A person who inherits the sickle cell gene from one parent and a normal hemoglobin allele (HgbA) from the other, has a normal life expectancy. However, these heterozygote individuals, known as ''carriers'' of the [[sickle cell trait]], may suffer problems from time to time.

The heterozygote is resistant to the [[malarial parasite]] which kills a large number of people each year. This is an example of balancing selection between the fierce selection against homozygous sickle-cell sufferers, and the selection against the standard HgbA homozygotes by malaria. The heterozygote has a permanent advantage (a higher fitness) wherever malaria exists.<ref>Allison A.C. 1956. The sickle-cell and Haemoglobin C genes in some African populations. ''Ann. Human Genet.'' '''21''', 67-89.</ref><ref>Sickle cell anemia. 2009. Encyclopædia Britannica. Chicago.</ref> Maintenance of the HgbS allele through positive selection is supported by significant evidence that heterozygotes have decreased fitness in regions where malaria is not prevalent. In Surinam, for example, the allele is maintained in the gene pools of descendants of African slaves, as the Surinam suffers from perennial malaria outbreaks. Curacao, however, which also has a significant population of individuals descending from African slaves, lacks the presence of widespread malaria, and therefore also lacks the selective pressure to maintain the HgbS allele. In Curacao, the HgbS allele has decreased in frequency over the past 300 years, and will eventually be lost from the gene pool due to [[Underdominance|heterozygote disadvantage]].<ref>David Wool. 2006. The Driving Forces of Evolution: Genetic Processes in Populations. 80-82.</ref>