436 MAMMALIAN RADIATION GENETICS 



sons are difficult, but it appears that neutrons may be more effective 

 than x-rays in the production of both dominant lethals and reciprocal 

 translocations. 



Estimates of the Genetic Hazard of Radiation 



In spite of the lack of knowledge regarding radiation-induced mutation 

 rates in mammals, several estimates of the genetic hazards of radiation 

 in man have been published. These are based primarily on the induced 

 rates in Drosophila. A common scale for measuring the hazard is the 

 dose of roentgens that would produce a rate as high as the spontaneous 

 rate. For this, a knowledge of the spontaneous rate in man is required 

 also. Here, again, little information is available. For these reasons, 

 most, if not all, geneticists would agree that current estimates of even 

 the basic factor in the human genetic hazard, namely, the increase in 

 mutation rate, are little better than guesses. Nevertheless, such guesses 

 have to be made, whether explicitly or implicitly, because of the neces- 

 sity for practical decisions, involving perhaps serious consequences, 

 about the protection of people against exposure to radiation. 



Several groups concerned with the safeguarding of personnel have, 

 for some time, assumed that the mutation rate in man would be doubled 

 by a dose of about 30-50 r. Two estimates differing widely from this 

 figure in opposite directions have been suggested by Evans (4) and 

 Wright (23). A brief consideration of these estimates will serve to bring 

 out a few more of the problems involved in calculating the genetic haz- 

 ard. 



Evans takes 10~^/3 X 10~^ = 300, in round figures, as the dose of 

 roentgens required to double the natural rate of gene mutations per 

 generation in man. The denominator in this expression is the induced 

 rate for sex-linked lethal mutations in Drosophila, per roentgen per locus, 

 averaged over all loci in the X-chromosome. Evans accepts this as ap- 

 plying to all chromosomes in man. The numerator, 10~^, is assumed 

 by Evans to be the rate for comparable spontaneous mutations per locus 

 and generation in man. He bases this figure on the rates for hemophilia 

 and epiloia. 



Wright criticizes Evans on the following grounds. If the spontane- 

 ous rate of lethal mutation per locus per generation in Drosophila is 

 10""*^, and if Drosophila has only one-tenth as many loci as man, both of 

 which figures are accepted by Evans, then the spontaneous rate per 

 gamete per generation in Drosophila is only one-hundredth of what is 

 assumed by Evans for man. Wright argues that it is likely that Dro- 

 sophila and man would have acquired about the same spontaneous mu- 



