Hundreds of mutations have been described for genes that
control production of the normal hemoglobins. Some of these
mutations impair synthesis of one or more of the normal
globin peptide chains and may result in a clinical syndrome
characterized by varying degrees of ineffective erythropoiesis,
hemolysis, and anemia. Thalassemias are classified according to
the globin chain that is deficient. The two major forms involve
impaired production or instability of α-peptide chains—causing
α-thalassemia, or of β-chains—causing β-thalassemia.