Thalassemia: A group of genetic disorders that involve underproduction of hemoglobin, the indispensable molecule in red blood cells that transports oxygen and carbon dioxide. All forms of hemoglobin are made up of two molecules: heme and globin. The globin part of hemoglobin is made up of four polypeptide chains. In normal adult hemoglobin (Hb A), the predominant type of hemoglobin after the first year of life, two of the globin chains are identical to each other and are called the alpha chains. The other two chains, which are also identical to each other but are different from the alpha chains, are called the beta chains. In fetal hemoglobin (Hb F), the predominant hemoglobin during fetal development, there are two alpha chains and two different chains called gamma chains. In thalassemia, there is a mutation (change) in one or both of the alpha or beta globin chains. Depending on which globin chain is affected, the mutation leads to underproduction or absence of that globin chain, a deficiency of hemoglobin, and anemia. The carriers of heterozygous forms of alpha and beta thalassemia have red cell anomalies that range from very mild to severe.