Hemoglobin F in beta thalassemia and related conditions.

Further examination of the Hb F in beta thalassemia has substantiated earlier conclusions that the Hb F of heterozygotes has a bimodal distribution of the G gamma to A gamma ratio whereas that of homozygotes does not. Although the T gamma chain is present in the Hb F of beta thalassemia and delta beta thalassemia, it occurs also in cord blood Hb F without thalassemia and it, therefore, is not a special feature of the thalassemia condition. The T gamma chain has not been detected in the Hb F from HPFH. Because studies of genomic DNA have detected single G gamma and A gamma genes instead of pairs of both, some explanations of the observed heterogeneity and control of Hb F production must be reconsidered. On the other hand, experiments with DNA agree with the suggestion that a different extent of deletion leads to HPFH or to delta beta thalassemia. As further studies of genomic DNA are made, a search should be made for the means by which the G gamma to A gamma ratio is controlled at the newborn or adult level, for the manner in which the quantity of Hb F is regulated, and for the determinants that may influence the production of Hb F in trans.