GENE MUTATIONS AND EVOLUTION 75 



that have been subjected to radiation or heat. Under natural 

 conditions there occur translocations, inversions, and the 

 other phenomena resulting from chromosome breakage, 

 though much less frequently than under radiation. Variations 

 in the physical state of the chromosomes, giving rise to 

 adhesions and breakage, occurs at times under natural 

 conditions. 



And now come back to the mutations. Mutations, as before 

 remarked, commonly accompany breaks, occurring at or near 

 the point of breakage. The mutations must in some way result 

 from the breaks, or from something that accompanies the 

 breaks. 



Just what is it in the breakage of chromosomes that brings 

 about the changes called gene mutations ? On the one hand 

 there is the fact of breakage, the tearing apart of structures 

 that were together, with its usual connotation of injury. The 

 changed action of the genes might well be the expression of 

 small scale injuries, induced at the same time as the greater 

 injuries that are seen as breaks, deletions and deficiencies. The 

 usually harmful or reductional effect of the mutations agrees 

 well with this interpretation. 



Another change that usually results from the breakage is 

 an alteration in the relation of the genes to each other. When 

 a chromosome breaks, the pieces usually reunite in a changed 

 position, as we have seen, giving inversions or deletions or 

 translocations. The result is that genes near the points of 

 breakage are torn apart from those with which they are 

 normally in close contact and are brought into close relations 

 with other genes than those that are originally near them. It 

 is suggested that possibly the effect that a gene produces 

 depends on its position with relation to other genes; that is, 

 presumably on the interaction of genes that are close together. 

 Thus a gene moved to a new position with relation to others 



