342 RADIATION STANDARDS, INCLUDING FALLOUT The 1960 report clearly indicates that the National Academy of Sciences Committee saw no reason to modify its basic conclusions or to alter its chief recommendations, The geneticists still hold that any amount of ionizing radiation, however small, increases the risk of harmful mutations arising in the reproductive cells. It still recommends that all exposures to ionizing radiations be avoided whenever possible, except for necessary exposures for medical or dental diagnostie or therapeutic reasons.. It further holds to the recommendation that the average dose received by the reproductive organs of any person duringthe first 30 years of life “should not exceed 10 roentgens of manmade radiation, and should be kept as far below this as is practicable.” This recommendation is in essential agreement with that of the International Commission of Radiological Protection. From the nature of sex determination in the humanspecies, it is to be expected that an irradiated male parent will on the average produce fewer daughters than sons, while an irradiated female parent will on the average produce fewer sons than daughters. Precisely these deviations in the sex ratio have indeed been found, indicating that radiation-induced mutations have slightly but significantly altered the prenatal viability of the young. However, the determination of the doses to which the individual parents were exposed is still very problematical; and there are many other conditions which may influence the sex ratio. It may be that studies of the mortality of the children born to irradiated persons would throw more light on the production of mutations that lower the viability or fertility of the affected children; but such studies must be prolonged for many years to come before death or sterility might become evident, and there is also an almost insuperable difficulty in finding a truly comparable control group to provide a baseline, to tell us what should be expected in the absence of irradiation of the population. Surveys have been started of the populations living on the radioactive sands and soils of Kerala Province in India and in interior Brazil. Beyond preliminary radiation measurements and preparation of plans for the medical and genetic analysis of these populations, little progress has been made, however, A major difficulty will be that of finding comparable populations living on unradioactive soils to serve as controls and provide a baseline for evaluation of effects. To the geneticist, however, there is nothing very discouraging about the prospect of having to turn to experimentally controlled organisms in order to get answers to questions such as we are posing. During the past 6 years a truly stupendous breakthrough has occurred in our knowledge of the chemical nature of the hereditary material. It has been shown not to be protein but to consist, in all organisms, of nucleic acid. In most living things, except for a few viruses, it is even the same kind of nucleic acid, known as deoxyribose nucleic acid, or DNA. The chemical structure of this material is now well known, its chemical behavior outside the living system has been studied extensively, and it is known to behave qualitatively, if not quantitatively, alike in the face of radiation or chemical mutagenic agents, no matter whether it comes.from microbe, man, or mouse. it is no longer relevant to say, as so many have said in the past when faced with unpleasant facts about the production of harmful mutations by radiation, “Well, a man is not a mouse, or a fruit fly, or a microbe.” The genes produce different effects in the course of the life of different kinds of organisms, but the chemical nature of the genes is the same, their responses to radiation are of the same kind, and they evoke their effects in the same way, by controlling the synthesis of the proteins made in the cells. Thus, when ionizing radiation strikes the DNA molecules of a cell, it can be demonstrated to have a comparable action in all cases. It fractures and disrupts the chromosomes which contain the DNA; it blasts the genes, usually with destructive effects ; and it causes chromosomes to stick together and fail to enter the daughter cells properly, at the time whenthe parentcell is dividing. We may expect to find differences, mainly quantitative ones, in the responses to radiation of different species. For that matter, it is now quite clear that the DNA and the chromosomes of the fruit fly’s spermatozoa are not at the same level of susceptibility as those in the oocytes of the female fruit fly during her maturity. It is also true that the DNA and the chromosomes in the immature germ cells are far less susceptible to radiation than those in the mature germ cells. So it is not surprising to find that the reproductive cells of a mouse are more susceptible than those of a fruit fly, as indeed they are. What we need is more information of this sort from a variety of animal species; but the longer the animals live, and the fewer offspring each female can produce, the more laborious and expensive the experiments must be. The Atomic Energy Commis- ARRARMIpAT Teasats