Evolutionary adaptation
The thalassemia trait may confer a degree of protection against malaria, which is or was prevalent in the regions where the trait is common, thus conferring a selective survival advantage on carriers (known as heterozygous advantage), thus perpetuating the mutation. In that respect, the various thalassemias resemble another genetic disorder affecting hemoglobin, sickle-cell disease. Incidence
The disorder is more prevalent in certain ethnicities and age groups. Beta thalassemia is most prevalent in the "thalassemia belt" which includes areas in Sub-Saharan Africa, the Mediterranean extending into the Middle East and Southeast Asia. This geographical distribution is thought to be due to beta-thalassemia carrier state (beta thalassemia minor) conferring a resistance to malaria. In the United States, thalassemias prevalence is approximately 1 in 272,000 or 1,000 people. There have been 4,000 hospitalized cases in England in 2002 and 9,233 consultant episodes for thalassemia. Men accounted for 53% of hospital consultant episodes and women accounted for 47%. The mean patient age is 23, with only 1% of consultants being older than 75, and 69% being 15–59. It is estimated that 1.5% of the worlds population are carriers and 40,000 affected infants are born with the disease annually. Beta thalassemia major is usually fatal in infancy if blood transfusions are not initiated immediately. See also
Alpha-thalassemia
Anisopoikilocytosis (variance in red blood cell size, usually as a result of beta thalassemia)
Delta-thalassemia
Hemoglobinopathy

References
Further reading
Cao, Antonio; Galanello, Renzo (2010). "Beta-Thalassemia". In Pagon, Roberta A; Bird, Thomas D; Dolan, Cynthia R; Stephens, Karen; Adam, Margaret P (eds.). GeneReviews. University of Washington, Seattle. PMID 20301599.
Ineffective erythropoiesis (red blood cell production) can also lead to compensatory bone marrow expansion which can then lead to bony changes/deformities, bone pain and craniofacial abnormalities. Extramedullary organs such as the liver and spleen that can also undergo erythropoiesis become activated leading to hepatosplenomegaly (enlargement of the liver and spleen). Other tissues in the body can also become sites of erythropoiesis, leading to extramedullary hematopoietic pseudotumors which may cause compressive symptoms if they occur in the thoracic cavity or spinal canal. Cause
Mutations
Two major groups of mutations can be distinguished:

Nondeletion forms: These defects, in general, involve a single base substitution or small insertions near or upstream of the β globin gene. Most often, mutations occur in the promoter regions preceding the beta-globin genes. Less often, abnormal splice variants are believed to contribute to the disease. Deletion forms: Deletions of different sizes involving the β globin gene produce different syndromes such as (βo) or hereditary persistence of fetal hemoglobin syndromes.Mutations are characterized as (βo) if they prevent any formation of β globin chains, mutations are characterized as (β+) if they allow some β globin chain formation to occur. mRNA assembly
Beta thalassemia is a hereditary disease affecting hemoglobin. As with about half of all hereditary diseases, an inherited mutation damages the assembly of the messenger-type RNA (mRNA) that is transcribed from a chromosome.