334 RADIATION STANDARDS, INCLUDING FALLOUT that has been offset by other evidence of greater exposure or additional types of genetic damage which were previously not reckoned with. One could refer, for example, to the data from the twosizable populations living today which were subjected to intensive radiation; namely, the survivors of Hiroshima and Nagasaki. Whatever information is extractable from the two Japanese populations has been carefully collected and analyzed, in particular, possible changes in the sex ratio of infants born to irradiated parents. 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 morelight 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 diffculty in finding a truly comparable control group to provide a baseline, to tell us what should be expected in the absenceof irradiation of the propulation. 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 knownto 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, andthey evoke their effects in the same way, by controlling the synthesis of the proteins made in the cells. Thus, when ionizing radiation strikes ERY esaSeesgnbnaapeedicpadaeepehgeeosicreatch