GENETIC VARIATIONS 36 1 



have a greater resistance to high temperatures than the nor- 

 mal unmutated Individuals with red eyes. 



Other cases of mutations held to be beneficial could doubt- 

 less be found, but they are very rare. 



In the preceding chapter it was seen that gene mutations 

 frequently result from breakage of the chromosomes. Cer- 

 tain of the genes at or near the points of breakage change 

 their action, resulting in changes In the inherited characters. 

 Both chromosome breakage and the production of gene mu- 

 tations occur under radiation. Not all the mutations oc- 

 curring under radiation are known to be the result of chro- 

 mosome breakage or other Injury, though It Is not known 

 that they are not. There are certain other relations between 

 chromosome breakage and the occurrence of mutations that 

 suggest a connection between the two. The number of mu- 

 tations produced Is proportional to the energy of the radia- 

 tion (the Intensity multiplied by the time of action), and 

 this also Is true for the number of cases of breakage. The 

 actual numbers of mutations produced under radiation is 

 nearly the same as the actual number of cases of breakage. 

 These facts, taken in connection with the known cases in 

 which chromosome breakage induces mutation, suggests that 

 the mutations produced under radiation are indeed the re- 

 sult of chromosome breakage. Certainly this is true in many 

 cases if not for all. 



Just what is it in the breakage of chromosomes that 

 brings about the changes known as gene mutations? On the 

 one hand there is the fact of breakage, the tearing apart of 

 structures that were together, with the injuries to tissues 

 that commonly accompany it. The changed action of the 

 genes may be the expression of small scale injuries, induced 

 at the same time as the greater Injuries seen as breaks, dele- 

 tions and deficiencies. The nature of the changes induced in 

 the genes agrees with this interpretation. The genes are 

 often made unstable. They are as a rule partly Inactivated. 



