What causes sickle-hemoglobin C (Hb S-C) disease?

Sickle-hemoglobin C (Hb S-C) disease occurs in individuals who have one gene for sickle cell anemia and one gene for hemoglobin C disease. The mutated genes affect the production of hemoglobin, or the material that makes blood red. Hb S-C disease is more common than other hemoglobin disorders because it is heterozygous. This means that both parents do not have to have a mutated gene in order to produce a child with the disorder. Instead, the disorder occurs when one parent passes on the gene for sickle cell anemia and the other passes on the gene for hemoglobin C.