Sickle Cell Anemia

Sickle cell disease and malaria are both potentially lethal diseases. Though malaria is an infectious disease and sickle cell disease is inherited, both can cause life-threatening conditions.

He hypothesized that there was a connection between malaria and sickle cell disease. On the basis of this hypothesis, Dr. Allison predicted high frequencies of sickle cell disease only in areas where malaria is common.

He expanded his study area beyond Kenya to the rest of East Africa to see if malaria and sickle disease were connected. He evaluated blood samples for malaria parasites and the presence of sickle cells. Dr. Allison gathered blood samples from more than 5,000 children in East Africa. He analyzed the samples to identify malaria parasites and sickle cells. He found that children carrying the sickle cell character (or trait) had a lower parasite count, as if they were partially protected against malaria.

The person is susceptible to malaria. The person is homozygous at the hemoglobin locus. A person with two copies of any allele is homozygous. A person with two normal copies of the hemoglobin allele is more susceptible to malaria than someone with a sickle cell hemoglobin allele.

Individuals with one sickle cell allele are protected from malaria and do not have sickle cell disease, thus keeping the allele in the population. People with one sickle cell allele are protected from malaria, but do not have sickle cell disease. Protection from malaria comes at the cost of more sickle cell disease in the population.

In areas with malaria, individuals with one sickle cell allele reproduced at higher rates than those with no sickle cell alleles. In areas without malaria, individuals with two sickle cell alleles reproduced at lower rates than those without sickle cell disease. In different environments, natural selection favors different characteristics. In areas with malaria, the reproductive advantages of having one sickle cell allele (and some protection from malaria) kept the allele at high frequencies in the population. In areas without malaria, the reproductive disadvantages from sickle cell disease reduced the allele in populations.

The sickle cell allele will decrease in frequency. Without malaria, selection for the sickle cell allele decreases. As a result, the frequency will likely decrease.