GENETIC VARIATIONS 347 



What are the agents that produce mutation? For many 

 years after mutations were found to occur, nothing was 

 known as to the agents that cause them. A few genes mu- 

 tated, as a few of the atoms of a radiant metal disintegrate; 

 in both cases no outside agent appeared to be at work. 

 There seemed to be no relation between the environment 

 and the production of gene mutations. But of late this situ- 

 ation has changed. In 1927, H. J. MuUer discovered that 

 by subjecting developing organisms to a powerful dose of 

 X-rays, the genetic system is altered in many ways, and 

 among the rest, the method of action of single known genes 

 Is altered: that is, gene mutations are produced.^ The X-rays 

 affect other parts of the organisms also. They kill many of 

 them, they leave many others sterile, they cause injuries of 

 many kinds. They cause the chromosomes to break and re- 

 unite, in the ways described in the preceding chapter. And 

 they cause numerous gene mutations, which produce in the 

 offspring of the radiated individuals changes in the inherited 

 characteristics, mainly abnormalities, weaknesses and other 

 aberrations from the normal. Such changed characteristics 

 are inherited in later generations by the autosomal or sex- 

 linked methods. The number of visible mutations produced 

 under the action of radiation is about seventy-five times as 

 great as occurs without radiation. 



This effect of radiations in producing mutations was con- 

 firmed by others and extended to other organisms. Radia- 

 tions from radium have the same sort of effect as X-rays; 

 they greatly increase the number of gene mutations. Gene 

 mutations have been produced by radiations in maize, bar- 

 ley, tobacco, cotton, various animals, and many other or- 

 ganisms. 



By radiation the same types of mutations are produced 

 as occur spontaneously. In many cases the mutations due to 

 the radiations are identical with others that had occurred 

 without radiation. Others induced by radiation differ from 



