Final answer:
The Hb^S allele is associated with sickle cell anemia, causing production of abnormal hemoglobin S and crescent-shaped red blood cells. It's a recessive trait where homozygous individuals experience the disease, while heterozygous carriers may have malaria resistance.
Step-by-step explanation:
The Allele for Sickle Cell Anemia and Hemoglobin
The allele Hb^S is responsible for sickle cell anemia, a genetic disorder characterized by the production of abnormal hemoglobin called hemoglobin S. This form of hemoglobin causes red blood cells to warp into a sickle or crescent shape. The Hb^S allele is autosomal recessive and is the result of a mutation in the beta-globin gene where glutamic acid is replaced by valine at the sixth position. This substitution leads to a sticky patch on the hemoglobin surface, making it prone to abnormal aggregations especially when oxygen levels are low, causing the cells to become crescent-shaped. Individuals with two copies of the Hb^S allele (homozygous recessive, ss) will have the disease, while carriers (heterozygous, AS) may have some protective advantage against malaria. The sickle cell trait originated from areas with high malaria prevalence, such as sub-Saharan Africa, and has spread globally. These sickle cells can block capillaries and impair oxygen delivery, leading to the symptoms of sickle cell anemia.