Evidence of Prenatal Heterosis Relating to X-Ray Induced Congenital Anomalies. ∗

Summary and Conclusions

1. Embryonic mice at 8.5 days gestation from pure C57 and CF1 strains, and their hybrids, were x-irradiated to 200 r to determine whether prenatal heterosis was manifest. Criteria were the number of normal, resorbed, dead, or anomalous mice seen at 18.5 days in the hybrids as compared with the pure lines. 2. The hybrids at 8.5 days exhibited some degree of heterosis based upon the higher percentage of normal fetuses, and the lower percentage of resorptions when comparisons are made with the pure strain parental stocks. 3. For heterosis to be manifest, the hybrids had to overcome the severely deleterious effect of the presence of C57 genes as indicated by the 90% resorptions among this parental stock and only 30% in the hybrids, while the CF1 showed 39%. Thus, there is evidence of more than mere dominance of the more resistant strain genes, hence heterosis at least by the time of organogenesis. 4. The heterosis for the embryo results in better survival, hence better survival also of the x-irradiation-produced congenital anomalies.