Genetics Test Information

The beta-like hemoglobins include the epsilon, gamma, beta, and delta globins, whose genes are present on chromosome 11 in a linked cluster (ie, the beta globin complex). The gamma genes, HBG1 (Ay) and HBG2 (Gy), contain 3 exonic coding regions and 2 intronic intervening sequences. The genes produce gamma globin chains that form tetramers with alpha globin chains to create fetal hemoglobin (Hb F). HBG1 and HBG2 differ only in which amino acid is located at position 136 (alanine or glycine). The resultant proteins are named A-gamma and G-gamma, respectively. Although G-gamma is predominant at birth, this gradually reverses during the first year of life to become the normal adult G-gamma/A-gamma ratio, which is 2:3. Some people maintain an increased G-gamma:A-gamma ratio throughout life, which has been linked to certain alterations in either gene. Additionally, some alterations in the promoter regions of the gamma globin genes are known to cause a form of hereditary persistence of fetal hemoglobin (HPFH), which is characterized by a significant but harmless elevation of Hb F into adulthood. If coinherited with sickle cell disease, HPFH has a strong modulating effect on the condition and appears to protect against some, but not all, of its complications. Some gamma genetic variations result in gamma chain hemoglobin variants, most of which are clinically insignificant; however, an incompletely studied subset causes neonatal disorders, such as hemolytic anemia, cyanosis, and methemoglobinemia.