sparrow: cytogenetic effects of ionizing radiations 73 



used are probably phosphorus-32, sulfur-35, carbon-14, and tritium. 

 These and others less frequently used are listed in Table 3 along with 

 their respective half-lives and radiations emitted. Of special interest 

 to geneticists are those radioactive elements or compounds which are 

 selectively concentrated in chromosomes and nuclei. (See Caldecott 

 (19) for references.) Tritiated thymidine is an example (94). Import- 

 ant but less extensive use also has been made of various precursors 

 labeled with C 14 , P 32 , S 35 , etc. (179, 181, 204). 



Tritiated thymidine was initially used by Taylor, Woods, and 

 Hughes (187) because of its localization in the nuclei and high resolu- 

 tion in autoradiography. Because of its concentration in the nucleus 

 and the short, dense, ionization tracks produced, its energy is highly 

 localized (Figure 5) and obviously can be expected to produce changes 

 fairly efficiently in nuclei (129). A number of such studies have been 

 made and some results obtained by Wimber (196) are shown in 

 Figure 6. 



Other studies in which mutations have been induced by internal 

 radioisotopes, including potentially useful ones, are too great to list 

 here. The use of labeled chemical mutagens reported by Moutschen- 

 Dahmen and co-workers (107) and by Smith (153) promises to be a 

 very interesting and valuable new technique. The genetic effect will 

 result from both chemical and physical activity. 



IV. General Survey of Cellular and Nuclear Changes 

 Induced by Ionizing Radiation 



In the interests of completeness, we shall first list here briefly 

 most of the known changes which occur in nuclei following exposure 

 to ionizing radiations. (See also Section VI.) A few are secondary 

 effects, e.g., polyploidy which results from a primary effect on the 

 spindle. Many of these are not too well-understood and the signifi- 

 cance of some with respect to genetic changes may be obscure or 

 unknown. 



1. Gene mutation 



The author will not attempt to define gene mutation but uses 

 it as a category of genetic change different from those involving some 

 kind of chromosome aberration. Gene mutation almost without 

 exception increases linearly with increasing dose. (See Muller (108) 

 and other pertinent papers in this volume.) 



