362 GENETICS 



Their effects on the organism are in almost all cases injuri- 

 ous. 



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 de- 

 letions 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 origi- 

 nally near them. It has been suggested that possibly the 

 effect that a gene produces depends on its position with re- 

 lation 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 produces an effect on 

 development and characteristics that is different from its nor- 

 mal effect, and this changed effect is what has been called a 

 gene mutation. This is spoken of as a "position effect." That 

 the "mutations" which accompany chromosome breakage 

 are such position effects is the view to which a number of 

 recent investigators of these phenomena have come.^^ If 

 it should turn out to be correct, this would require a re- 

 interpretation of many of the accepted ideas of genetics. 



What is certain is that when a gene is brought into a new 

 position as a consequence of the occurrence of a chromo- 

 some break close to the point where it is located, it often so 

 changes its action as to give rise to what is called a gene mu- 

 tation. It is not certain whether this changed action is due 

 to the change of position, or whether it is an expression of 

 injury. 



Some questions of great interest remain. Are all gene mu- 

 tations that occur under the action of radiation the result 

 of chromosome breakage or injury? And further, are all 

 gene mutations of this type? Chromosome breakage, with 



