Hemoglobins in thalassemia

The thalassemias are a group of genetically determined
disorders of hemoglobin synthesis.
Thalassemia occurs due to the absence or reduced
synthesis of a globin chain, which results
in unstable hemoglobin. It affects either the "
chain ("-thalassemia) or the ! chain (!-thalassemia).
Since the ! chain is a component of both
fetal and adult hemoglobins, the !-thalassemias
(!-thal) are especially severe. Hemoglobins
with four identical globin chains are
completely unstable and incompatible with life
(HbH with four " chains, Hb Bart’s with four $
chains).