The gamma globin genes which are HBG1 and HBG2， usually expressed in the fetal liver， spleen and bone marrow. 2 gamma chains together along with 2 alpha chains comprise fetal hemoglobin （HbF） which is normally replaced by mature hemoglobin （HbA） at birth. Gamma chain production continues into adulthood in several beta-thalassemias and linked situations. The two types of gamma chains differ at residue 136 where glycine is found in the G-gamma product （HBG2） and alanine is found in the A-gamma product （HBG1）. The former is predominant at birth.
