aay rIerg® Oy ves 344 RADIATION STANDARDS, INCLUDING FALLOUT newly fertilized egg, but only about 0.2 percent when the sperms are irradiated before fertilizing the egg. Losses of other chromosomes than the sex chromosomes are fatal in early development in the mouse, and presumably in humans losses of most chromosomes except the sex chromosomes and the smaller chromosomes of other sorts are likewise fatal (dominant lethals). Many are now known to cause multiple congenital defects resulting in neonatal death. In any event, to the previous estimates of detrimental mutations produced by ionizing radiations we must now add something for this novel: and unsuspected type of human damage. At the present time one cannot say how many mongoloid idiots and sexual aberrant types have been produced by radiation. It may indeed be a small proportion of the total. Yet until we know more about the relation of these conditions to radiation dosage, we must be exceedingly cautious, for there is no reason to doubt that radiation will cause such defects, particularly if administered to the female or her just-fertilized egg cell. Until quite recent years nearly all radiation exposures in genetic investigations were for some reason restricted to males. Gradually evidence began to accumulate from fruit fly experiments to show that just as there are differences in sensitivity between immature and mature reproductive cells in a single sex, so too there are differences in sensitivity to radiation between the male and female reproductive cells. W. L. Russell has now obtained information bearing on the production of mutations in the oocytes of female mice. Although the data are still insufficient, it appears at present that at a high dose rate (80 to 96 roentgens per minute) the female germ cells are more sensitive than spermatozoa, where as at a low dose rate (90 roentgens per week) they yield even fewer mutations than the male germ cells do. Moreover, there ig some indication that the mutations in the female germ cells differ qualitatively from those in the male germ cells. The frequencies at different gene loci, among the seven tested loci, are different in the male and female data. Clearly, if an adequate idea of the sensitivity of a population to radiation is to be achieved, we will need to have sufficient data on both sexes and for the entire lifespan. We are far from reaching any such goal at present, even for that most intensely studied species, the fruit fly Drosophila Melanogaster. The most unexpected and most discussed development during these last years in the study of mutations induced by radiation has been the demonstration by W. L. Russell and his Oak Ridge colleagues that in mice there is definitely a difference in the frequency of mutations induced at high dose rates and at low dose rates. The difference, as might already be gathered from the foregoing account, is greater in the case of the female germ cells, the oocytes, than in the case of the male germ cells, the spermatogonia. In the latter, for which which mutations induced at a high dose rate as at a low dose rate. For both sexes taken together, the yield at high dose rate is about six times as great as at a low dose rate. The most recent experiments have refined the definition of “high dose rate” and “low dose rate’ and reveal a situation of increasing complexity. Initially, 90 roentgens per minute and 0.009 roentgen per minute were used for the respective high and low rates. The total dose administered was of course adjusted to be the same. The quality of the radiation was different, since in the high dose rate experiments 250-kilovolt X-rays were used while in the low dose rate experiments cobalt 60 gamma radiation was used; but in experiments in which a high dose rate and a low dose rate were both administered from the same source, any significant difference owing to the quality of the rays was ruled out. Now tests have been conducted at intermediate dose rates of 9 roentgens per minute and about 0.8 roentgen per minute. These prove to be in the critical range for the dose rate effect. For the male germ cells, the mutation rate at 9 roentgens per minute is intermediate between the results at high and low dose rates. At 0.8 roentgen per minute the mutation frequency is already that characteristic for a low dose rate. In the case of the female germ cells, however, the 0.8 roentgens per minute rate still yields an intermediate frequency of mutations. It should perhaps be emphasized that in terms of human exposure 0.8 roentgen per minute is not what one would consider a very low dose rate, since at that rate it would require only 12 to 13 minutes to equal the level recommended by the NAS Genetics Committee as an upper limit for the average gonadal dose. Russell’s data indicate that for low to moderate dose rates there is some sort of recovery process such that potential mutations are restored to normal. This is very encouraging, from the point of view of the exposure of human populations of radiation, since most exposures are likely to be at moderate fo low dose rates if the exposed person survives at all. It must be very strongly emphasized, se oP PE Ba rr ragasergnenta sts + TER OS Taya e Slghatnmtons Speehpeashas atedaterthe RSIS ATER