Dictionary of Human Evolution and Biology

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Sickle Cell Disease

Heritable autosomal condition caused by homozygosity for the mutant hemoglobin S allele (HbS). SCD is found in populations where malaria is endemic, and is characterized by anemia caused by a culling of red blood cells (RBC) possessing an altered morphology due to defective hemoglobin in the RBC wall that crystallizes under conditions of dehydration or low oxygen tension, deforming them into a characteristic sickle shape; the deformed corpuscles block capillary blood flow (thrombosis). Symptoms of a sickling crisis include external manifestations of thromboses and ulceration of limb tissues (internal lesions also occur), bone and joint pain, infections and anemia. A substitutional mutation of the glutamic acid for the normal valine amino acid at position six in the beta chain of the more common form (HbS) of the adult hemoglobin molecule is the cause of clinical problems. The marrow and spleen are stressed to produce more RBCs; too few RBCs causes anemia and too few white blood cells (WBCs) impairs immunity. Sickle cell disease usually limits life to 10 years or less in countries where incidence is high. SCD-affected individuals are homozygous (SS), with an estimated direct fitness lowered to 10% or less. Carriers of the allele (AS) enjoy the maximum relative fitness (100%) due to heterozygote advantage, and are said to carry the sickle cell trait. Originally treated as a recessive genetic condition, since heterozygotes can now be identified electrophoretically, SCD is treated as a codominant. Although the cause of SCD is known, prophylaxis had been limited to transfusions until recently. The stimulation of fetal hemoglobin synthesis by administration of hydroxyurea has been successful. Worldwide, the HbS allele is found in Africa, South America, in the Australasian and Indian subcontinents, in the Mediterranean region, and in the Caribbean.

See hemoglobin alleles. Aka Dresbach’s anemia, Herrick’s anemia, sicklemia, and sickle cell anemia, SCA.

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