Question

The frequency of the sickle cell allele is high in populations at low elevations in East Africa, where mosquitos breed year-round, and lower in high elevation populations where mosquitos are much less abundant. Which, if any, evolutionary processes might contribute to these differences in allele frequencies? Explain your answer.

Answer 1

Answer: - Africa currently accounts for 85 percent of world malaria cases and 90 percent of world malaria
deaths. In the United States, about 1 in 500 African-Americans develops sickle cell anemia. In Africa, about 1 in 100 individuals develop this disease. The reason for such a huge difference is natural selection.

There are two alleles that play a role in the inheritance of sickle cell anemia: A and S. Individuals with two A alleles (AA) have a normal hemoglobin and RBCs. Those with two S alleles (SS) develop sickle cell anemia. Those who are heterozygous for the sickle cell allele (AS) produce both normal and abnormal hemoglobin. Heterozygous individuals are healthy and are known as the ‘carriers’ of the sickle cell gene. When malaria parasite invades the person with the S allele, the red cells that consists of the defective hemoglobin become sickled and die, subsequently trapping the parasites inside them and resulting in reduced infection.

When compared to AS heterozygotes, people with the AA genotype having normal hemoglobin will have a greater risk of dying from malaria whereas individuals with the AS genotype do not develop sickle cell anemia and also have less chance of getting malaria. People with AS genotype are able to survive and reproduce in malaria-infected regions. AS heterozygotes pass on A and S alleles to the next generation, so both the A and S alleles of these people remain in the population. S alleles are reduced in the population when individuals with SS genotype die in young age. In a region like Africa, where malaria is very common, the S allele gives a survival advantage to people having one copy of the allele. This is the reason why the otherwise harmful S allele is kept in the population at a relatively high frequency. This evolutionary process is a part of natural selection and thus the evolutionary process involved is natural selection.