Normal adult hemoglobin, known as hemoglobin A (Hb A), comprises four polypeptide globin chains, four heme rings inside each globin chain, and an iron molecule at the center of each heme ring. Two globin chains in hemoglobin A are designated alpha and two as beta. This differs from other types of hemoglobin molecules, such as fetal hemoglobin (hemoglobin F (Hb F)), composed of two alpha chains and two gamma chains.
Alpha thalassemia results from a diminishment in alpha chain production, leading to an overall decrease in hemoglobin levels. Each person has four genes coding for alpha globin—two inherited from their mother and two from their father. Physiologically, anywhere from one to four gene loci that code for the alpha chain may be deleted from chromosome 16. The greater the number of loci deleted or inactivated, the greater the severity of the anemia that develops.
Many different mutations result from partial deletions of alpha genes. This unit of study deals only with forms of alpha thalassemia that have entire gene loci deletions.
