The two diseases have been studied together that provide evidence of evolution in humans is sickle cell anemia and malaria.
What is sickle cell mutation?
The sickle cell mutation is relevant to malaria because infection of a red blood cell with the malaria parasite leads to hypoxia. In individuals of the AS genotype such blood cells sickle and are then eliminated by macrophage cells of the body’s immune system, lessening the burden of infection (Luzzatto, 2012). Carriers of the sickle cell trait are particularly resistant to severe malarial episodes; they are less resistant to mild cases. The mechanism by which carriers are protected from malaria is different than the acquired immunity that both AA and AS individuals achieve following repeated exposure to the disease.
The benefit that possessing a single copy of the sickle cell gene conveys counterbalances the biological cost incurred when homozygous SS children are stricken with sickle cell disease. An individual of the AS genotype is more likely to reach adulthood than is an individual of the AA genotype, but the former is also more likely to see his/her child die of sickle cell disease. This is known as a heterozygote advantage or balanced polymorphism. As shown more formally below, the stronger is the pressure of malaria on survival, the more advantaged are individuals who carry the S gene, and in equilibrium, the higher the percentage of the population who will be carriers. Indeed, it was the correlation of high prevalence of the sickle cell gene and the presence of malaria that first led scientists to understand the protective role of the sickle cell mutation (Kwaitkowski, 2005). As will be seen in the next section, the underlying genetic mechanism by which the sickle cell trait is transmitted provides a means of mapping sickle cell prevalence into an estimate of the mortality burden of malaria.
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