sickle cell disease
The disease is confined mainly to blacks, especially those of W African descent, but it also occurs in persons of Mediterranean, Middle Eastern, and Indian origin. The mutation may at one time have had an advantageous effect; those afflicted with the abnormality have a higher survival rate in malaria-infested zones.
Under normal circumstances the disease occurs only in those patients who inherit the gene for the abnormal hemoglobin from both parents. This so-called homozygous form of the disease occurs in 1 in 400 African Americans. About 8% of African Americans have sickle cell trait; that is, they are heterozygotes, usually symptomless carriers who have inherited a normal hemoglobin gene from one parent and hemoglobin-S from the other. There are also intermediate forms of the disease that result when a gene for hemoglobin-S is inherited from one parent and a gene for any of several other abnormal kinds of hemoglobin is inherited from the other. Genetic screening (see genetic testing) is recommended for prospective parents at risk of passing on the disease. If both parents are carriers (i.e., have sickle cell trait), then each child has a one in four chance of having sickle cell disease.