386 AN INTRODUCTION TO MODERN GENETICS 



"r-imits," has been found by many authors, both for germinal and 

 somatic mutations. It applies to individual mutations, or to small groups 

 of mutations as well as to the total rate. 



(2) The mutation rate produced by a given ionization dose is inde- 

 pendent of the wave-length of the rays used. This result holds for all 

 wave-lengths from gamma rays to very soft X-rays (10 KV) and also 



.0 



to 



^ 



1000 2000 3000 



/onisatlon produced 



HOOO SOOO 6000 



Fig. 155. The Relation between Ionization and Mutation Rate. — The muta- 

 tion rate is directly proportional to the ionization produced, and is not affected 

 by the nature of the ionization-producing rays (whether /3 particles, soft X-rays 

 (with long wave length), hard X-rays, or gamnna-rays). 



(From Zimmer, Griffith, and Tihnofeeff-Ressovsky.) 



for P rays. It probably does not hold for ultra-violet, and it may not 

 hold for rays of a particles or neutrons, but data for the latter, whose 

 importance we shall discuss later (p. 388), are not yet available. 



(3) The mutation rate produced by a given dose is independent of 

 the time over which the dose is given. A short exposure to intense 

 radiation is no more effective than a longer exposure to weaker radia- 

 tion; the only important variable is the total dosage received. 



These three results clearly amount to the single datum that the 

 mutation rate is linearly proportional to the ionization produced and is 

 independent of any other characteristic of the radiation. From this 

 two conclusions can be dravm : {a) The fact that the relation is linear 

 shows that a mutation is produced by a single event, and not by the 



