People with sickle-cell anemia have hemoglobin S and are homozygous recessive. Sickle-cell anemia is an autosomal recessive genetic disorder caused by a mutation in the HBB gene that produces hemoglobin S. When an individual inherits two copies of the mutated gene (one from each parent), they have sickle-cell anemia. Individuals who are heterozygous (carrying one normal and one mutated gene) are carriers of the disorder but do not have the disease itself.