HBA1
MIM.141800 16pter-p13.3
The alpha (HBA1 and HBA2) and beta loci (HBB) determine the structure of the 2 types of polypeptide chains in the tetrameric adult hemoglobin, Hb A, alpha-2/beta-2.
The alpha locus also determines a polypeptide chain, the alpha chain, in fetal hemoglobin (alpha-2/gamma-2), in hemoglobin A2(alpha-2/delta-2), and in embryonic hemoglobin (alpha-2/epsilon-2).
The number of normal alpha genes (3, 2, 1 or none) in Asian cases of alpha-thalassemia results in 4 different alpha-thalassemia syndromes:
Three normal alpha genes gives a silent carrier state.
Two normal alpha genes results in microcytosis (so-called heterozygous alpha-thalassemia).
One normal alpha gene results in microcytosis and hemolysis (so-called Hb H disease).
No normal alpha gene results in ’homozygous alpha-thalassemia’ manifested as fatal hydrops fetalis.