Hemoglobin SC occurs as a double heterozygous condition in less than .5% of African Americans but can be as high as 25% in West Africa. Although HbSC disease may produce a less severe anemia, the chance for retinal hemorrhage and renal and bone necrosis is greater due to increased viscosity. Most persons with HbSC disease have splenomegaly, but significant symptoms usually do not show up until the teenage years.
The double heterozygote for HbSD is quite rare and produces an anemia midway in severity between sickle cell disease and sickle cell trait.
The rare double heterozygote for HbE produces an anemia similar to Hb S beta-thalassemia.
