This heterozygous sickle cell variant occurs when the gene for sickle hemoglobin is inherited from one parent and the gene for b thalassemia is inherited from the other parent. In US African-Americans, the gene frequency for b thalassemia is 0.8 to 1.0 percent. The frequency of sickle cell-b-thalassemia disease is 2 per 3200 births. The severity of the disease that results, including the degree of anemia and frequency of clinical complications, depends on the type of b-thalassemia gene that is inherited. Between 80 and 90 percent of affected individuals have a b-thalassemia gene that results in the production of some normal beta chains; thus some normal HbA is made. These patients have a mild hemolytic anemia with near-normal hemoglobin levels, few crises, and minimal organ damage. Those 10 to 20 percent of patients who inherit a b-thalassemia gene that produces no beta chains, and therefore no normal hemoglobin, have severe hemolytic anemia and vasoocclusive symptoms comparable with patients with SCD. Splenomegaly is found in 70 percent of patients with sickle cell-b thalassemia disease.
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