Genetic Effects of Radiations 141 



different from the stable genes, and probably there is no dis- 

 continuous range in mutation frequency. 



The stability is very little affected by ordinary environmental 

 fluctuations, temperature being the most potent of such influ- 

 ences. A 10° C. rise in temperature will increase the rate of 

 mutation about five times.'*^^ Thus the principal hereditary ma- 

 terial, the chromosomes and genes of which they are constituted, 

 is distinguished by a remarkable stability of minute structure, 

 both as regards the constituent particles, the genes, and the way 

 in which these are ordered and bound togeher to form chromo- 

 somes. 



The significant genetic effects of radiations are that gene 

 mutations and chromosome structural changes become much 

 more frequent under their influence. The order of increase over 

 spontaneous changes is a hundredfold for quite moderate doses 

 of X-rays. The chief biological interest lies in the possibilities 

 of studying the nature of the mutation process and, by extension, 

 of the gene itself, and also of the manner in which the genes 

 are tied together to form chromosomes. With the help of 

 radiations, experiments can be carried out which, if dependent 

 on spontaneous mutation alone, would be almost impossible. 



Medically, the importance lies firstly in . the fact that most 

 mutations are recessive and deleterious and, therefore, that deep 

 radiotherapy may run the risk of producing mutations in the 

 gonads. The mutations may be transmitted to the treated per- 

 son's children and spread undetected in the population in which, 

 generations later, homozygous defective individuals may arise. 

 The genetic change is immediate but the physiological conse- 

 quences are delayed. Secondly, many kinds of induced chromo- 

 some structural change are lethal to all cells in which they are 

 produced, and it is this property, among others, of radiations 

 that renders them effective in killing unwanted tissues such as 

 cancers. 



Apart from radiations, only a few agents have been found 

 capable of greatly enhancing mutation rates. The most effective 

 are certain synthetic chemicals, the naturally-occurring mustard 

 oil, allyl isothiocyanate,^ and antibodies.® 



