ExtractThe fetal hemoglobin of 10 infants has been examined serially from birth to several hundred days of age. In the normal child, the ratio ofG&ggr; toA&ggr; chain alters from about 7:3 at birth to about 2:3 at 150‐200 days. In children with abnormalities of hemoglobin synthesis, the ratio changes as expected from data from adults with the particular abnormality. Thus, little change in ratio was shown by two &bgr;‐thalassemia homozygotes, whereas two &bgr;‐thalassemia heterozygotes showed the same behavior as a normal child; the latter behavior is expected from one of the two classes of &bgr;‐thalassemia heterozygotes. A heterozygote from theG&ggr;A&ggr; class of hereditary persistence of fetal hemoglobin behaved much like a normal child, but two heterozygotes from theG&ggr; class showed the expected change from approximately 9:1 at birth to 10:0 postnatally. Two children with sickle cell anemia resembled a normal child in change of ratio whereas the third, not unexpectedly, did not show a change in ratio.SpeculationThe mechanisms involved in the gradual change from &ggr; chain synthesis to &bgr; and &dgr; chain synthesis are complex and ill defined. The results of analyses of the ratio between the products of theG&ggr; andA&ggr; structural genes in infants with distinct genetic disorders at birth and during the postnatal period underscore the complexity of the changeover. However, differences in the postnatal change in theG&ggr; toA&ggr; ratio in infants with an apparent identical condition may help to further define these mechanisms, and to delineate the conditions involved.