76 GENETIC VARIATIONS 



produces an effect on development and characteristics that is 

 different from its normal 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.* 



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

 position as a consequence of the occurrence of a chromosome 

 break close to the point where it occurs, it often so changes its 

 action as to produce what has been called a gene mutation. It 

 is conceivable that the change in relative position is not the 

 primary feature in producing the changed effect; it may 

 merely provide the evidence of a break and mark the spot 

 where it occurred. The change in effect would then be due to 

 the lesions inseparable from laceration of tissues. 



The nature of the changes in gene action that result from 

 breakage have thus far been most fully studied by Schultz 

 and Dobzhansky.* They examined thoroughly the effects on 

 the genes in a case in which the second chromosome of 

 Drosophila had been broken in two places, one at about the 

 region of the gene bristle (54.7), the other near the gene 

 brown (104.5) (see maps of the Drosophila chromosomes in 

 any good text-book of genetics). That part of the chromosome 

 between the breaks (nearly half its length) had been inverted 

 and attached anew to the two end pieces, in the way illus- 

 trated in Figure 12, at C. At one of the breaks (that near 

 brown), there was an effect on three of the genes. All three 

 were weakened, so that they became unstable; in such a way 

 that during development they went out of action in some of 

 the cells, while in others they continued to operate, thus 

 yielding mosaic individuals. In one of the three there was an 

 additional, dominant, effect, showing itself in a change of the 



