G gamma:A gamma chain ratios were determined in homozygous beta thalassemia and cord blood samples using triton-urea polyacrylamide gel electrophoresis. The mean G gamma/G gamma + A gamma proportion in the two groups were 0.62 +/- 0.10 and 0.72 +/- 0.03, respectively. There was no significant correlation of the gamma chain composition in either fetal hemoglobin or total hemoglobin levels; this suggests that these two factors do not influence gamma chain ratios. There was also no marked variation in the G gamma:A gamma ratios in beta thalassemia patients when they were divided into higher fetal hemoglobin (greater than 50%) and lower fetal hemoglobin (less than 50%) groups. These observations are consistent with the finding of a selective advantage of G gamma chains over A gamma chains in disorders where the erythropoietic stress is higher than normal, and may be inherited as a specific genetic entity in haplotypic polymorphism.