388 



AN INTRODUCTION TO MODERN GENETICS 



Q)) The fact that the mutation rate is linearly proportional to the 

 ionization, and is independent of other variables such as wave-lengths, 

 shows that the single event which causes mutation is an ionization. As 

 examples of the possibilities which can be excluded by this fact we may 

 mention the absorption of a whole quantum of energy from the in- 



1.0 



•OS 



/ 2 J V 



Fig. 157. Ionization and Rates of Visible Mutations.' 



• Somatic we -> vv mutations. 



X y, we, w, V, m, g or f mutations. 



yf Visible mutations (attached X method). 



D Visible mutations {CIS method). 



(From Timofeeff-Ressovsky.) 



coming radiation, or the passage of a single electron or ion through a 

 certain volume. 



In drawing these two conclusions, we have assumed that the events 

 (ionizations) which cause the mutations are scattered at random through 

 the tissues. For the types of X-rays usually worked with, this assump- 

 tion is justified. But quite different effects on mutation should be 

 produced by rays of a particles or neutrons, when the ionizations 

 instead of being thinly spread over a long track, occur very much 

 nearer together in a small volume. In such case the ionizations may be 

 so close to one another that it is likely that two or more would occur in 

 the neighbourhood of a single gene, and we should expect that fewer 

 mutations would be produced for a given number of ions, owing to the 



