This serious blood disease is one of several hereditary anemias. Recent figures indicate the frequency of sickle cell anemia among North American Negroes varies considerably. One survey indicates that 8 1/2 percent of the Negro population carry the trait, but do not have the disease, and 0.3 to 1.3 percent have the disease. Sickle cell anemia disease is even more widespread among natives of central and western Africa, where it is present in as much as 4 percent of the Negro population (Figure 5).
Sickle cell anemia was first observed in 1910, when the blood of patients was found to contain abnormal sickle-shaped red corpuscles. However, it was not until 1949 that the corpuscles were found to contain an abnormal hemoglobin. The chemical structure of this abnormal hemoglobin was determined in 1957. In the protein chain containing more than 500 amino acids, one amino acid, valine, was substituted for glutamic acid in normal hemoglobin. This slight variation alters the chemical properties of the hemoglobin and causes the red corpuscles to change from the normal disk shape and become sickle-shaped when the blood circulates through tissues where the oxygen supply is low. The abnormal corpuscles disintegrate in the blood stream. Thus, loss of red corpuscles causes anemia.
Sickle cell anemia usually appears during the latter months of the first year of life. The gene associated with sickle cell anemia is recessive. Therefore, the disease appears only in homozygous individuals. Heterozygous individuals, with a normal allele, may transmit the disease to offspring but do not develop it.
Figure 5. A distribution of sickle cell anemia.
(figure available in print form)