A variety of hemoglobin molecules are produced in humans at different stages of life.

All are tetramers consisting of seven distinct polypeptide chains, each encoded by a separate gene.

Embryonic hemoglobin has ζ (zeta) and ε (epsilon) chains.

By eight weeks gestation, these are replaced by fetal hemoglobin with alpha chains and two types of γ (gamma) chains.

98% of adult hemoglobin are HbA molecules consisting of alpha and beta chains; 2% are HbA2, composed of alpha and δ (delta) chains.