GENE AND CHROMOSOME MUTATION AFTER LARGE FRACTIONATED OR UNFRACTIONATED RADIATION DOSES TO MOUSE SPERMATOGONIA

Previous experiments by RUSSELL had shown that the specific locus mutation rate after a dose of 1000 R X-rays to mouse spermatogonia was below that after 600 R, whereas after a fractionated dose of 500+500 R 24 h apart the rate was above that expected by linear extrapolation. In the present experiment, using the same mouse stock (C3H × 101), the incidences of dominant lethal mutations and of chromosome translocations were shown similarly to decrease with increase of X-ray dose from 600 to 1000 rad, though proportionally the decrease in translocations was greater. After a fractionated dose of 500+500 rad 24 h apart the incidence of dominant lethals was enhanced above that expected by linear extrapolation, as were the mutation rates to a new set of specific loci a, bp, fz, ln, pa, pe) and to dominant visibles. The incidence of translocations was above that after 1000 rad but not above that expected by linear extrapolation. These results are thought to confirm Russel's view that the effects of dose fractionation were due to cell synchronization. It is suggested that the elimination of cells with unstable chromosome aberrations accounts for a large part of the decrease in observed gene and chromosome mutation rates after high radiation doses. © 1966.