Since sickle cell anemia is an inherited disorder, it is obviously not as common as easily spreadable diseases like malaria. In fact, only around 250,000 children are born with sickle cell anemia worldwide, although millions are carriers for the sickle cell trait. The trait is considerably prevalent in people with sub-Saharan African, Middle Eastern, Indian, or Mediterranean descent and in places where malaria is endemic.
Although this genetic disorder appears highly dangerous, it still exists today. A widely accepted theory states that the genetic mutation that produces the sickle cell trait also happens to increase the chance of survival for an individual affected with malaria. Thus, the survivors who lived in areas afflicted by malaria inevitably propagated the sickle cell trait. In terms of the theory of natural selection, carriers of the sickle cell trait were the most fit in areas where malaria was endemic. They lived long enough to reproduce and pass on the mutated HBB gene, and, in the process, those without the sickle cell trait were selected against. In this way, the sickle cell trait remained in the “gene pool” as a way to defend against malaria. Yet, oddly, people have been consistently found carrying the sickle cell trait in areas where malaria is not at all common. This is likely due to population migration, also termed gene flow. People carrying the sickle cell trait travel to places where malaria is non-endemic, like the US, resulting in the strange phenomenon where the sickle cell trait exists even though it provides no benefit or reproductive advantage.
Suppose that malaria was eradicated in a region where it was once common. One could reasonably predict that the frequency of sickle cell carriers would steadily decrease over generations, since the sickle cell trait no longer grants the fitness and reproductive success that had made it so valuable in areas where malaria was endemic. Having the sickle cell trait would ultimately make it difficult to live long enough reproduce, and so a decline in the frequency of the mutated hemoglobin would become apparent over many generations. This is likely the case in countries where carriers of the sickle cell trait had migrated to. Malaria is not common in the US, for example, and as such, carriers who once migrated into the US now no longer have that “reproductive edge” over the rest of the population. This has likely caused the frequency of the sickle cell trait and sickle cell anemia to decrease over the generations.
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