Flexi Says: The allele (S) for sickle cell anemia is a harmful, autosomal recessive allele. It is caused by a mutation in the normal allele (A) for hemoglobin (the oxygen-carrying protein on red blood cells). Malaria is a deadly tropical disease that is common in many African populations. Heterozygotes (AS) with the sickle cell allele are resistant to malaria. Therefore, they are more likely to survive and reproduce. This keeps the S allele in the gene pool. This mutation is assumed to evolve in the countries where malaria was prevalent. The majority of people who carry one or both S alleles are found in Nigeria, India, and the Democratic Republic of Congo.