Thalassemia is a group of hereditary disorders associated with quantitative deficiency of the globin chains of hemoglobin. 

Depending on which gene is mutated, (typically alpha or beta), the patient will have Alpha- or Beta-Thalassemia. There are less common patterns as well, such as delta-beta thalassemia. 

Different mutations of the genes leads to a wide range of severity and phenotype, from asymptomatic carrier states, to severe transfusion-dependence.