Fetal hemoglobin

In fetus there is a different kind of hemoglobin,  hemoglobin  F (HbF) which comprises of two α-chains and two γ-chains (α₂γ₂), in contrast to the adult hemoglobin (HbA,  α₂β₂). HbF  is having higher  af?nity  for  O₂   under  physiological  conditions than HbA that optimizes the transfer of oxygen from the maternal to fetal circulation  across  the  placenta.  The molecular basis  for  this  difference  in  O₂ affinity is that HbF binds 2,3-bisphosphoglycerate loosely. Near birth the synthesis of γ-chain is switched off, and that of β-chain (that is present in HbA) is switched on.