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 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).
In alpha thalassemia, the heterozygous state (with a single gene for alpha thalassemia) is innocuous or harmless. There are no symptoms or at most mild anemia, because there is another gene still able to make alpha chains.
However, 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 Hb F or Hb A, the inevitable result is an unsuccessful pregnancy.
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