The haemoglobin of a human foetus

The haemoglobin of human foetus $(\mathrm{HbF})$ has 4 Heme group and 1 globin. In $\mathrm{HbF}$ the globin protein is made up of $2^\alpha$ and $2 \beta$ polypeptide chains. Each $\alpha$ has 141 amino acids and each $\beta$-has 146 amino acids but 37 amino acids in each $\beta$ are different from that of $\mathrm{HbA}$. It has higher affinity for $\mathrm{O}_2$ even at low $\mathrm{PO}_2$ and the oxygen $-\mathrm{Hb}$ dissociation curve remains towards the left of normal.