REPORT OF THE 

 COMMITTEE ON GENETIC EFFECTS 



I. Our Present Position 



During the period elapsed since the 1956 report of this Committee there have been a number 

 of significant developments in genetics and radiobiology. New insight has been gained into the 

 nature of the genetic material, the characteristics of the mutation process, the manner in which 

 genes control the processes of development, and the ways in which all of these are affected by 

 various kinds of radiation. Yet, in some respects, the estimation of human radiation 

 hazards is more difficult than it appeared to be in 1956. For one thing, the assumed 

 constancy of the total genetic effect irrespective of dose rate, for which there seemed to be 

 good evidence at that time, has turned out not to apply to spermatogonia and oocytes, which 

 are the most important cell stages as far as human hazards are concerned. 



Among the reported new findings that have a bearing on the assessment of the genetic 

 effects of radiation and that have been considered by the Committee are the following: 



1. In mice, fewer mutations are produced in spermatogonia and oocytes by chronic 

 irradiation (i.e., a low dose rate) than by the same amount of acute irradiation (i.e., a high 

 dose rate ) when the total dose is the same. However, the data are not yet sufficient to establish 

 the precise quantitative relations between dose and effect at low doses for either acute or for 

 chronic irradiation. A similar dose-rate effect has been reported for sex-linked lethals induced 

 in oogonia of Drosophila. 



2. At the time of the previous report there was little information on the results of irradia- 

 tion of female mice. Data now available indicate that late oocytes are not widely different 

 from spermatogonia in their sensitivity to induction of mutations by acute irradiation. If any- 

 thing, they suggest greater sensitivity. 



3. There is some shortening of life in the progeny of irradiated male mice, as well as in 

 the irradiated mice themselves. 



4. Studies of human cells grown in tissue culture have shown that doses as low as 25 r 

 will cause detectable chromosome breakage in a significant proportion of the cells. 



5. Additional studies on children of survivors of the atomic bombings at Hiroshima and 

 Nagasaki, and on children elsewhere whose parents received radiation for medical or other 

 reasons, suggest that the sex-ratio in these children has been slightly but significantly altered 

 as a result of radiation-induced mutations affecting prenatal viability. The fact that the sex- 

 ratio may be influenced by many factors indicates the need for conservatism in interpreting 

 this finding. 



In view of the recent increase in fallout, which to a large extent comes from the 1958 

 tests and which of course will be reduced gradually if atmospheric tests are not resumed, and 

 of the fact that the contribution of carbon- 14 was not considered in the earlier report, esti- 

 mates of the amounts of radiation from fallout are increased. On the other hand, the fact that 



