1. an autosomal recessive anemia characterized by crescent- or sickle-shaped erythrocytes and accelerated hemolysis, due to substitution of a single amino acid (valine for glutamic acid) in the sixth position of the β-chain of hemoglobin, the gene of which is on chromosome 11; affected homozygotes have 85–95% hemoglobin (Hb) S and severe anemia, whereas heterozygotes (said to have sickle cell trait) have 40–45% Hb S, the rest being normal Hb A; low oxygen tension causes polymerization of the abnormal β-chains, thus distorting the shape of the red blood cells to the sickle form. Homozygotes develop “crisis” episodes of severe pain due to microvascular occlusions, bone infarcts, leg ulcers, and atrophy of the spleen associated with increased susceptibility to bacterial infections, especially streptococcal pneumonia. Occurs most commonly in people of African descent.
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