ISO RADIATION GENETICS 



Thus, their radiations are highly localized, often to the cell in which they are deposited. 



Very little mammalian genetic work has been done with these, but certain tech- 

 niques appear to be accurate enough to warrant more study. For example, death of 

 spermatogonial cells has been quantitatively measured following the simple intra- 

 peritoneal injection of tritiated thymidine into mice. 664 The results compare 

 excellently with those obtained by external gamma-radiation, 952 and offer a straight- 

 forward estimate of comparative toxicity. 



The importance of carbon- 14 to the problem of long-term genetic hazards to man 

 and animals from past nuclear weapons testing has been emphasized by Totter et a/. 1326 

 and Pauling, 993 yet only empiric estimates are available on the relative contribution of 

 the three potentially injurious consequences of radioactive decay of C 14 to N 14 : trans- 

 mutation, atomic recoil, and ionization. 



Another isotope of considerable interest is deuterium. Though not a radioactive 

 isotope of hydrogen, it is D a O or heavy water that is commonly used as a moderator 

 and coolant in nuclear reactors. Its introduction into biological systems presents 

 many intriguing problems that have been reviewed by Katz et al. 689 and Bennett 

 et al. 78 Nearly complete sterility can be induced in mice when they are provided a 

 30 per cent D 2 concentration in the drinking water. Lesser amounts of D 2 induce 

 partial sterility from which recovery will occur. Since the size of litters remains un- 

 affected, there is no evidence of genetic damage, but deuterium may well be mutagenic 

 when incorporated into the genetic materials. 



While the above summary of radiation sources is by no means all-inclusive, the 

 brief descriptions do indicate something of the type and energy of radiations that can 

 be employed and a few of the problems that exist. 



TEMPORAL FACTORS IN RADIATION 



When one begins to consider the number of permutations and combinations of 

 radiation exposure that can be employed, it soon becomes apparent that it is probably 

 impossible to account for all possibilities experimentally and even operationally 

 important. To start with, the radiation dose can be delivered in the form of a single 

 exposure (often referred to as an acute exposure, which should not be the preferred 

 terminology) , multiple exposures or a fractionated exposure, or a continuous exposure 

 (often called a chronic exposure, again not the preferred terminology). Within any 

 one of these, there can be variation in total dose and rate of dose delivered per unit 

 time, or dose rate. Obviously, for continuous exposures, dose rate and total dose are 

 positively related, although termination of the exposure can be varied to hold total 

 dose constant under different dose rates. This, then, varies the duration of exposure 

 or protraction period. 



Fractionated exposures may be even more complex. One can vary the number 

 of doses or fractions, the total dose, the dose rate, the interval between doses, the size 

 of the individual fractions, and the total protraction period. Again, a little considera- 



