740 RADIATION GENETICS 



germ cells. The exposure in this experiment was protracted over a 5-week period at 

 rates of 1.64, 8.0, and 33.3 r per week. No exposures of single dose with high dose 

 rate were reported, so no comparison can be made for possible effects of dose rate. 

 This procedure would seem sufficiently straightforward to warrant the development of 

 data for analysis of effects of dose rate on the induction of recessive lethals. 



Definitive study of the mutation rate for sex-linked lethals still remains to be done. 

 One attempt was made in Charles' experiment, with equivocal results. The breeding 

 procedure is simple. Irradiated males are outcrossed, and their daughters, which may 

 now carry a sex-linked recessive lethal as a heterozygote, are mated to unrelated males. 

 If a lethal segregates, the litters will show a 2: 1 sex ratio. Due to the normal fluctua- 

 tions in sex ratios, the test system is inefficient. 



In recent years, several sex-linked genes have become recorded in the mouse. 

 Recombination percentages vary from 4 to 16 among the known loci, 492 and this is 

 close enough to permit the suggestion that Carter's technique of detecting lethals 

 between linked markers would be worth exploring. Even the use of a single marker, 

 such as tortoise (To), which is dominant for its effect on coat color and recessive for its 

 lethal effect, would provide some useful data. If an induced lethal is closely linked to 

 tortoise, test matings would have few or no male offspring, thus simplifying the test system. 



Dominant visible mutations. — Data on the induction of dominant visible mutations 

 are exceedingly unsatisfactory and for practical purposes may be considered as virtually 

 nonexistent. This is not the fault of those investigators who have made valiant 

 attempts to detect these mutations. The problem is that there is no simple quantitative 

 method available for their detection. Basically, all that can be done is to screen the 

 offspring of irradiated parents for detectable anomalies and mate these animals to 

 determine the heritability of the trait. A total gametic rate is therefore observed. 

 Since there is almost no limit to the range of dominant morphologic and physiologic 

 variants that can be measured, it is difficult to determine what the true mutation 

 rate is. 



The single greatest effort yet reported was made by Charles et a/. 198 The male 

 offspring of irradiated male parents were sacrificed, dissected, and inspected for 

 morphologic variants. Obviously, no heritability tests could follow. The female off- 

 spring were mated to unrelated stock; thus externally visible mutants could be studied 

 for transmissibility. One hundred and twenty possibilities were tested, and fourteen 

 proved to be bona fide mutations. The data provided an estimated mutation rate of 

 5.4 x 10 " 6 per roentgen per gamete. These mutations were observed among the 

 offspring of males subject to daily X irradiations that were permitted to mate con- 

 tinuously during the course of the experiment. The mutation rate thus cannot be 

 applied to any single type of germ cell but probably includes contributions from all 

 stages of gametogenesis. 



Russell 1134 reported the detection of five X-ray-induced, dominant, visible mutants 

 affecting the coat color, the ears, and the tail. These were the only morphologic 

 traits screened for dominant changes in the course of early tests on the induction of 



