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| Malaria, Sickle Cell Anemia, and Balancing Selection |
Sickle cell anemia affects approximately 70,000 Americans, almost exclusively those with African ancestry. The lifespan of an individual with sickle cell anemia is currently approximately 40 years in the United States. Before the advent of modern medicine, individuals with the disease usually died before they could have offspring.
The disease iscaused by a change in a single amino acid difference in the beta chain of hemoglobin. Individuals with two copies of the sickle form of the gene have sickle cell anemia. Heterozygotes -- individuals with one normal and one mutant copy of the gene -- appear normal and do not manifest the disease except under very stressful conditions; however, they are carriers. If two carriers have a child, the child has a twenty-five percent probability of receiving two copies of the sickle form and having the anemia. Approximately ten percent of African Americans are carriers. In Africa itself, the frequencies of the disease and carriers are even higher.
If sickle cell anemia is so deadly, why are so many people heterozygous carriers of the disease? Moreover, why does the disease afflict predominantly one racial group? Surpisingly, the answer has to do with malaria. Heterozygote sickle cell carriers are much more resistant to malaria than those with just normal hemoglobin. Because heterozygotes have the best of both worlds (no sickle cell anemia and higher malaria resistance) and malaria is extremely prevalent in Africa, the sickle allele can be maintained in balance with the normal allele. Note that in the United States, where malaria is rare, the carriers possess no such advantage and may even have a small selective disadvantage. Therefore, due to the strong selection acting against those with the anemia, the frequency of sickle cell anemia should slowly decline in the United States. That the frequency of the sickle cell allele is higher in African populations than in African Americans is due to both this selection and the genetic mixing between whites and blacks in the United States.
This situation, where selection actively maintains two or more alleles at a locus, is called balancing selection. Balancing selection can arise by the heterozygotes having a selective advantage, as in the case of sickle cell anemia. It can also arise in cases where rare alleles have a selective advantage. In extreme cases, balancing selection can maintain alleles in populations long enough for speciation to occur. In such cases, one species may have alleles that are more similar to those of the other species than they are to other alleles of the same species. One case of this phenomenon occurs at loci at the major histocompatibility complex (MHC) wherein some human alleles are much more closely related to some chimpanzee alleles than they are to other human alleles (Fig. 7). MHC - also called the human leukocyte antigen (HLA) loci when referring to it in humans - encodes proteins that are used to recognize foreign invaders by cells of the immune system. Chimp-like alleles have been maintained in the human population not because they are chimp-like, but because either having rare alleles or having two different alleles has provided a selective advantage. This balancing selection is so powerful that alleles are maintained that predate the human/chimp split.