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Sickle1 - "balanced polymorphism" In turns out...

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Sickle-Cell Case First example concerns classic case of sickle-cell anemia , a genetically transmitted blood disease common in some sub-Saharan African and So. Asian populations Sickle-cell gene involves change in molecular structure of hemoglobin, which alters shape of red blood cells, especially when oxygen supply drops (e.g., during strenuous exertion) Homozygotes (H s H s ) suffer from sickle-cell anemia and other complications, and thus have very low fitness (usually die before reproducing) Heterozygotes (H n H s ) have "sickle cell trait " -- a much less severe impairment, primarily evident under physiological stress of some kind Yet H s allele frequency is up to 30-40% in many W. African populations (mostly in heterozygotes) Such high gene frequencies could only be maintained by strong, consistent, and long- term selection in favor of heterozygotes, creating what population geneticists call a
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Unformatted text preview: "balanced polymorphism" In turns out that the "balance" is due to fact that heterozygotes have lower rates of 1) malarial infection, and 2) malarial mortality if infected, compared to "normals" (H n H n ) In fact, studies have revealed that, because heterozygotes have higher reproductive success than normals in malaria-infested areas, the distribution of H s gene closely matches region of endemic malaria = broad band across Sub-Saharan Africa (as well as areas in E. Mediterranean, parts of So. Asia, etc.) In W. Africa, malaria is transmitted by Anopheles mosquitoes, which breed best in cleared areas (i.e., in sunlit pools, which are common in rainy season) Swidden agriculture increases cleared areas, and thus the breeding habitat of Anopheles, thereby indirectly increasing the incidence of malaria...
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