THE INDUCTION OF CHROMOSOMAL 

 ABERRATIONS BY IONIZING RADIATIONS 

 AND CHEMICAL MUTAGENS* 



C. P. SWANSON 

 Department of Biology, The Johns Hopkins University, Baltimore 



AND 



BeNGT KlHLMANf 

 Institute of Physiological Botany, University of Uppsala 



A DECADE ago, when the late D. E. Lea's (1946) book on 

 radiobiology appeared, a physical explanation of the events 

 leading to the production of chromosomal aberrations seemed 

 eminently satisfactory. The dosage and intensity relationships, 

 the results of fractionation experiments, the spacing of ions 

 along known paths by different types of radiations, and the 

 ideas revolving around the target theory and the "breakage- 

 first" concept, fitted together sufficiently well to give a good 

 measure of confidence in a strictly physical interpretation of 

 the available data. Seven years later, however, it was possible 

 to state that "The main features of the biological experiments 

 (with ionizing radiations) make very good sense when viewed 

 from the standpoint of radiation chemistry" (Gray, 1953). 

 As it applied to aberrations induced by ionizing radiations, 

 this enlarged concept — for it was an expansion of earlier 

 ideas rather than a shift in perspective — stemmed from the 

 initial studies of Thoday and Read (1947, 1949) and their 



* Acknowledgement of financial support of the work reported here is 

 gratefully made to the U.S. Atomic Energy Commission [Contract AT (30-1) 

 1695] and to the National Science Foundation (Research Grant NSF-G2233). 

 We wish also to acknowledge our indebtedness to Dr. A. V. Beatty for permis- 

 sion to cite from his unpublished studies. 



t On leave of absence from the University of Uppsala to The Johns Hopkins 

 University under the Exchange Visitor Programme of the U.S. Information and 

 Educational Act of 1948. 



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