Sickle Cell Anemia

Sickle cell anemia is a hereditary disease that affects the hemoglobin molecules in the blood. The disease occurs because one amino acid changes in the hemoglobin molecule. The result is a sickle shaped red blood cell.

The change happens close to the oxygen binding site on the hemoglobin molecule. If there is oxygen in the blood things go all right but when there is not enough oxygen the changed hemoglobin molecules stick together in clumps.

Individuals who have one normal (s) allele and one sick (S') allele are heterozygous (sS') carriers of the disease. Since neither allele is dominant in this case, they produce both normal and sickle shaped blood cells. People who have two alleles with the (S'S') sick allele rarely reproduce.

Sickle cell anemia is common in the African-American population in the United States and Canada. Out of 100 African-Americans 10 are carriers but will not be sick because they are heterozygous (Ss) for the allele. One out of 100 African-Americans is homozygous and will become sick with sickle cell anemia. In some African populations, especially in Central Africa almost 45 % of the population (45 people out of 100) carry one allele for the sickle cell trait. Why is it that the heterozygous form of sickle cell anemia is so common in Central Africa even though the homozygous individuals die without reproducing? The answer has to do with environmental conditions and the presence of malaria in Central Africa.

Since the heterozygous individuals rarely get malaria they survive better than those individuals who have the "wild" type (ss) genotype and also survive better than those who have the homozygous dominant (SS) genotype since these individuals die young. The presence of the malaria parasite in the environment causes natural selection and maintains the heterozygous (Ss) population.