Population Genetics in Humans-2.3

Population Genetics in Humans-2.3 - Population Genetics in...

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Population Genetics in Humans The following is taken directly from: http://www.pbs.org/wgbh/evolution/educators/course/session7/explain_b_pop1.html Human Genetics: Concepts and Application Because of natural selection, different alleles are more likely to confer a survival advantage in different environments. Cycles of infectious disease prevalence and virulence often reflect natural selection. Balanced Polymorphism If natural selection eliminates individuals with detrimental phenotypes from a population, then why do harmful mutant alleles persist in a gene pool? A disease can remain prevalent when heterozygotes have some other advantage over individuals who have two copies of the wild type allele. When carriers have advantages that allow a detrimental allele to persist in a population, balanced polymorphism is at work. This form of polymorphism often entails heterozygosity for an inherited illness that protects against an infectious illness. Examples are fascinating. Sickle Cell Disease Sickle Cell disease is an autosomal recessive disorder that causes anemia, joint pain, a swollen spleen, and frequent, severe infections. It illustrates balanced polymorphism because carriers are resistant to malaria, an infection by the parasite Plasmodium falciparum that causes cycles of chills and fever. The parasite spends the first stage of its life cycle in the salivary glands of the mosquito Anopheles gambiae. When an infected mosquito bites a human, the malaria parasite enters the red blood cells, which transport it to the liver. The red blood cells burst, releasing the parasite throughout the body. In 1949, British geneticist Anthony Allison found that the frequency of sickle cell carriers in tropical Africa was higher in regions where malaria raged all year long. Blood tests from children hospitalized with malaria found that nearly all were homozygous for the wild type of sickle cell allele. The few sickle cell carriers among them had the mildest cases of malaria. Was the presence of malaria somehow selecting for the sickle cell allele by felling people who did not inherit it? The fact that sickle cell disease is far less common in the United States, where malaria is rare, supports the idea that sickle cell heterozygosity provides a protective effect.
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Population Genetics in Humans-2.3 - Population Genetics in...

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