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Thalassemia is not one disease but rather a group of genetic disorders that have a single feature in common: they all have a defect in the production of hemoglobin, the protein that enables red blood cells to carry oxygen.
HEMOGLOBIN: All forms of hemoglobin are made up of two molecules: heme and globin. Globin is made up of 4 polypeptide chains. In normal adult hemoglobin (Hb A), the predominant type of hemoglobin after the first year of life, 2 of the globin polypeptide chains are identical to one another and are designated the alpha chains. The other 2 chains are also identical to one another but differ from the alpha chains and are termed the beta chains. In fetal hemoglobin (Hb F), the predominant hemoglobin during fetal development, there are 2 alpha chains and 2 different chains called gamma chains.
THE THALASSEMIAS: The problem in the thalassemias is with globin production. The thalassemias are classified according to the type of globin polypeptide chain that is underproduced. The alpha chain is involved in alpha thalassemia and the beta chain is affected in the more familiar beta thalassemia.
ALPHA THALASSEMIA: Alpha thalassemia, the heterozygous state with a single gene for alpha thalassemia, causes no symptoms or merely mild anemia because there is another gene still able to make alpha chains. But the homozygous state with both genes for alpha thalassemia is lethal before birth because no alpha chains can be made and without alpha chains there can be no Hb F or Hb A and without them there can be no life.
BETA THALASSEMIA: In beta thalassemia, the heterozygous state (called thalassemia minor) is accompanied by no symptoms or at most by very mild anemia. The homozygous form of beta thalassemia (thalassemia major) is not immediately lethal because there is some production of Hb F, which does not contain beta chains. However, there is severe anemia with complications including progressive enlargement of the liver, spleen and heart and malformation of bones.
SUMMARY: Alpha and beta thalassemia are distinct and different disorders since they diminish the production of distinct and different globin polypeptide chains and particularly in the homozygous state that has distinct and different consequences.