1. Hybrid (C3H♀ × 101♂) male mice were given two doses of 600 r. acute x-irradiation eight weeks apart and outcrossed at the end of their sterile period. Their fully fertile sons were outcrossed and daughters of these sons were backcrossed to them, in order to study the rates of induction in spermatogonia of dominant and recessive lethal and visible mutations, as well as dominant semi-sterility.
2. F1 litter-size decreased by 15·2% at birth and 15·8% at weaning age, as compared with controls. This decrease was very largely due to dominant lethality acting at about the time of embryonic implantation. There was also a highly significant increase in the incidence of heritable semi-sterility, the estimated rate of induction of reciprocal translocations being 3·3% per gamete.
3. The dominant lethality was shown to be partly a secondary consequence of induced translocation heterozygosis. The estimated overall rate of dominant lethal induction was 10·6% per gamete, with ‘primary’ dominant lethals induced at a rate of 4·0% per gamete.
4. The estimated mutation rate to dominant visible mutations was 4·6 × 10-7/gamete/r., but this was based on only two mutations in the irradiated series.
5. In the backcross generation there was again significantly more embryonic death in the irradiated series, mainly at the small mole stage and this was attributed to induced lethal mutation. The survival in the irradiated series was 96·80% of that in the controls and from this the rate of induction of recessive lethals was calculated to be 2·5 × 10-4/gamete/r. There was no evidence of the induction of lethals acting later than 14 days' gestation.
6. The estimated rate of induction of recessive visible mutations was 1·8 × 10-5/gamete/r., but the results showed heterogeneity, probably due to personal factors.
7. No significant sex-ratio differences were found.
8. The results were compared with those of specific locus and other relevant experiments. The rates of induction of recessive lethal mutations and of recessive visibles were both lower than might have been expected. On these results the mouse was only 4–5 times more sensitive than Drosophila to the mutagenic effects of radiation.