Genetic Effects of Radiations 153 



ionized quarter-micron.^^ It has been estimated that 15 to 20 

 ionizations represent the minimum amount of energy which, dis- 

 sipated in a chromosome, is sufficient for the probabihty of 

 breakage to approach unity. It should be emphasized that these 

 numerical values refer to Trade scantia chromosomes, and that 

 quite different values may characterize the chromosomes of other 

 organisms. 



From a genetical point of view, the use for therapeutic pur- 

 poses of neutrons and similar radiations with densely ionized 

 paths instead of gamma rays and X-rays, is to be favored, for 

 the following reasons. For a given dose, neutrons are more 

 efficient in the production of chromosome structural changes 

 that will lead to the death of the cells and tissues, while they 

 are less efficient in the production of gene mutations which, 

 produced in gonads, could be harmful to future generations. 



Finally, reference should be made to ultraviolet radiations. 

 These can cause excitation but not ionization, i.e., they can 

 introduce into genes or chromosomes at one time only a small 

 amount of energy compared with that which may be introduced 

 by X-rays. Ultraviolet radiations produce the usual range ,of 

 gene mutations,^^- ^^ the rate being directly proportional to the 

 dose. The shorter wave lengths, notably those between 2,500 

 and 3,000 A approximately, are considerably more effective than 

 slightly longer wave lengths. The ultraviolet is also able to 

 produce chromosome breaks, although with a remarkably low 

 efficiency ; ^^ however, there is no certain evidence that inter- 

 changes or other two-break aberrations can be produced. From 

 a genetic point of view, the ultraviolet can be extremely useful 

 in providing mutations free from chromosome structural changes, 

 always provided of course that the objects to be treated are small 

 enough to be capable of penetration by the rays. 



References 



1 Auerbach, C. and J. M. Robson (1944) Nature, Lond. 154, 81. 



2 Catcheside, D. G. (1945) Biol Rev. 20, 14. 



3 Catcheside, D. G. and D. E. Lea (1945) /. Genet. 47, 1. 



4 Catcheside, D. G., D. E. Lea and J. M. Thoday (1946) /. Genet, [in 



press]. 



