Normal haemoglobin is always composed of a tetramer of globin chains.
Two of the chains are alpha globin.
In the adult the alpha chains pair with two beta chains (alpha2 beta2). This haemoglobin is called HbA.
Approximately 2% of adult haemoglobin is termed HbA2 and is formed by the pairing of alpha and delta chains (alpha2 delta2).
Less than 1% of adult haemoglobin is the foetal form. Foetal haemoglobin, HbF, has an alpha2 gamma2 structure.
Embryonic haemoglobin has an epsilon2, gamma2 structure.
The expression of the haemoglobin types changes throughout prenatal life and infancy.
Reference
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