Malaria, Sickle-Cell Anemia, and Natural Selection

This paper explores the genetics behind malaria and sickle-cell anemia, a fascinating case where the presence of an allele for sickle-cell anemia prevents individuals from getting malaria. This effect explains the presence of some natural selection in favor of the sickle-cell anemia allele.

Alternative versions of a gene are alleles. Each gene resides at a specific chromosome locus. The DNA at that locus, however, can vary somewhat in sequence of nucleotides and information content. Alleles are these possible DNA variations.

Individuals who are homozygous for an allele have both alleles of the same sort, one on each pertinent locus of two homologous chromosomes. Individuals who are heterozygous for an allele have two different alleles, one on each of the homologous chromosomes.

Natural selection through differential reproductive success can cause allele frequencies in a population to change. Disasters or dramatic changes in the environment can also bring about a bottleneck effect whereby the small quantity of individuals remaining does not statistically represent the former population. Thus, the available gene pool has been altered dramatically.

Malaria is a tropical disease transmitted through the bite of a mosquito. The malarial protozoa infect the liver and reproduce, subsequently infecting the victim's red blood cells and becoming available for transfer to other individuals via another mosquito.

People in Africa or of African descent often carry the sickle-cell anemia allele because heterozygotes for the allele can be protected from malaria while not exhibiting considerable symptoms of sickle-cell anemia. They can survive to reproductive age and transfer the allele to offspring, thus perpetuating the allele's occurrence in the gene pool.