In thalassemia the genetic defect, which could be either mutation or deletion, results in reduced rate of synthesis or no synthesis of one of the globin chains that make up hemoglobin. This can cause the formation of abnormal hemoglobin molecules, thus causing anemia, the characteristic presenting symptom of the thalassemias. Thalassemia is a quantitative problem of too few globins synthesized, whereas sickle-cell disease (a hemoglobinopathy) is a qualitative problem of synthesis of an incorrectly functioning globin. Thalassemias usually result in underproduction of normal globin proteins, often through mutations in regulatory genes. Hemoglobinopathies imply structural abnormalities in the globin proteins themselves. The two conditions may overlap, however, since some conditions that cause abnormalities in globin proteins (hemoglobinopathy) also affect their production (thalassemia). Thus, some thalassemias are hemoglobinopathies, but most are not. Either or both of these conditions may cause anemia. The two major forms of the disease, alpha- and beta- (see below), are prevalent in discrete geographical clusters around the world - it is presumed associated with malarial endemicity in ancient times. Alpha is prevalent in peoples of Western African and South Asian descent. It is nowadays found in populations living in Africa and in the Americas. It is also found in Tharu in the Terai region of Nepal and India. and is believed to account for much lower incidence of morbidity and mortality, accounting for the historic ability of Tharus to survive in heavily malarial areas where others could not. Source
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