Normally, the majority of adult hemoglobin (HbA) is composed of four protein chains, two a and two b globin chains arranged into a heterotetramer. In thalassemia, patients have defects in either the a or b globin chain, causing production of abnormal red blood cells (In sickle-cell disease, the mutation is specific to b globin). The thalassemias are classified according to which chain of the hemoglobin molecule is affected. In a-thalassemias, production of the a globin chain is affected, while in b-thalassemia, production of the b globin chain is affected. The b globin chains are encoded by a single gene on chromosome 11; a globin chains are encoded by two closely linked genes on chromosome 16 Thus, in a normal person with two copies of each chromosome, two loci encode the b chain, and four loci encode the a chain.Thalassemias are genetic disorders inherited from a person's parents. There are two main types, alpha thalassemia and beta thalassemia.] The severity of alpha and beta thalassemia depends on how many of the four genes for alpha globin or two genes for beta globin are missing.