Mutational Loads and Their Consequences 



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of increase any particular exposure would 

 produce in our spontaneous mutation rate. 

 The general impression is held that we, as a 

 species, are already adapted to the mutation 

 rate ordinarily produced by nonmanmade 

 mutagenic agents, and that if man causes 

 this rate to double through his own activity, 

 this will not pose a threat to his survival as 

 a species! Accordingly, the question be- 

 comes, how much manmade radiation would 

 be needed to produce as many mutations as 

 occur normally? A United Nations' report 

 calculated that about 30 rads (roughly equal 

 to 30 r) would be sufficient to double the 

 human spontaneous mutation rate. This is 

 called the doubling dose. It is reckoned that 

 in a population of one million people (which 

 is the approximate size of the population in 

 the St. Louis area), 1 rad delivered to the 

 gonads or sex organs of each person would 

 produce between 100 and 4.000 mutants 

 which would be transmitted to future gen- 

 erations. This 1 rad of gonadal exposure for 

 one generation would result in the birth of 

 100 to 4,000 people with new heterozygous 

 mutants, the individuals showing the effects 

 being spread out over the course of many 

 generations. Only a part of the genetic 

 deaths from these mutants would occur in 

 the first generation, and these would not be 

 evident when added to the number of genetic 

 deaths consequent to spontaneous mutation. 

 If the 1 rad gonadal exposure were repeated 

 in every generation, eventually an equilib- 

 rium would be established, in which in each 

 generation there would be 100 to 4,000 people 

 per million showing the effects of radiation- 

 induced mutants in the form of genetic death. 

 However, since the kinds of phenotypic effects 

 produced by the radiation-induced mutants 

 would be the same as those from mutants 

 which occur normally, one would not be able 

 to recognize the particular people who were 

 hurt by the radiation. 



What part of our normal load of mutants 

 comes from naturally occurring penetrating 



radiation? Since human beings receive about 

 5 rads in the course of a reproductive gen- 

 eration, that is in 30 years, it is possible that 

 as little as %q of )i of our mutations are 

 normally radiation-induced. 



How much additional radiation are we ex- 

 posed to in the course of medical treatment? 

 It has been estimated that each person in the 

 United States would receive, if medical use 

 of radiation continued at its present level, a 

 total dose to the sex cells of about 3 r per 

 generation. Of course, while some people 

 get no such amount of radiation, others get 

 considerably more. But this average radia- 

 tion dose to the germ cells from medical uses 

 alone is 60% of the amount we receive 

 spontaneously, and is raising our mutation 

 rate about 10% above the spontaneous rate. 

 With the increase, in the years to come, of the 

 use of radiation for diagnosis and therapy, 

 this amount of medical radiation might be- 

 come greatly increased. Already, in a single 

 year, radioactive materials were given in one 

 million medical treatments. 



How many germ-line mutations are being 

 produced by the radiation associated with 

 fallout following atomic explosions? This is 

 not an easy question to answer. For some 

 radiation could reach the gonads from the 

 fallout on the ground, and other radiation 

 could come from what is breathed in, or 

 from what is included with the food. In 

 the case of fallout taken in with food, the 

 distribution of particular radioactive sub- 

 stances in the body will make a large differ- 

 ence in the amount of radiation reaching the 

 sex cells. In this respect, three most impor- 

 tant radioactive substances in fallout are 

 cesium-137, strontium-90, and carbon-14. 

 Because of its distribution, cesium-137 is 

 expected to produce more gonadal radiation 

 from ingested fallout than does strontium-90. 

 This is so because cesium is distributed 

 through the tissues more or less evenly, in- 

 cluding the gonads, while strontium is pref- 

 erentially localized in bone. 



