The alpha (HBA) and beta (HBB) loci determine the
structure of the 2 types of polypeptide chains in adult hemoglobin,
Hb A. The normal adult hemoglobin tetramer consists of two alpha
chains and two beta chains. Mutant beta globin causes sickle cell
anemia. Absence of beta chain causes beta-zero-thalassemia. Reduced
amounts of detectable beta globin causes beta-plus-thalassemia. The
order of the genes in the beta-globin cluster is 5'-epsilon --
gamma-G -- gamma-A -- delta -- beta--3'.
