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.