334 GENETICS 



changes in characteristics resulting from breakage of the 

 second chromosome in Drosophila (see figure 38 for map 

 of this second chromosome). A piece of the chromosome 

 extending from about the locus 55 of the map to about the 

 locus 104.5 ^^^ been broken out and replaced in the in- 

 verted position, so that all the genes between 55 and 104.5 

 were in the reversed order from that shown on the map of 

 figure 38. At or near the two points of breakage certain of 

 the genes had become changed in their action. At the break 

 near the right or lower end of the chromosome, at about 

 104.5, three genes had become changed. One of these af- 

 fects the color of the eye. When it is in the normal or 

 unchanged condition the eye is red. After the breakage, this 

 gene became unstable in its action. In some of the cells it 

 produced its usual effect, in others it did not, so that the eye 

 showed patches of darker and lighter color. In addition, 

 the changed gene when in operation caused the eye color to 

 be darker than usual, giving a color known as "Plum." 



The other two genes that were changed in the region of 

 this break affected the growth of the bristles borne on the 

 body; one is known as "minus," the other as "abbreviated." 

 Both of these genes were made unstable, so that in some of 

 the body cells they operated, in others they did not, giving 

 bristles of different sizes in different parts of the body. 



At the other region of breakage, at about the locus 55, 

 another gene was made unstable in its action. This is the 

 gene "light," at locus 54.5, which has an effect on the eye 

 color. It was found by Schultz and Dobzhansky that this 

 gene operated normally in some of the cells, while in others 

 it did not, thus giving under some conditions a mottled ap- 

 pearance to the eyes. There was also a lethal effect at this 

 break; when both chromosomes of pair II had the break 

 at locus 55, the individuals did not develop. 



Thus by breakage of the chromosome II there were pro- 



