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 z (zeta) and e (epsilon) chains.

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

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