CROSSINGOVER IN DROSOPHILA 



157 



factors, while that for the purple-vestigial region is about that 

 usually found. The rise of slightly more than 5 units in each 

 case is about what we should expect on the basis of the data in 

 the previous experiment. 



Heat effect apparent for short regions only. The third experi- 

 ment is instructive. It was made with the characters star — S' — 

 black and curved. Since star is a dominant and the homozy- 

 gous flies do not survive, it was necessary to make the test by 

 using Pi star females crossed to Pi black-curved males. The 

 star Fi females we're then selected and backcrossed to the double 

 recessive black-curved males, and the crossing over percentages 

 among their offspring calculated. From the data of other work- 

 ers in this laboratory who have tested the star-black region 

 with several intermediate factors, it has been shown that the 

 two are separated by approximately 45 units. As the control 

 value shows this percentage is not realized when no intermediate 

 points are used in the experiment because of the obscuring of 

 the result through unobserved double crossingover. For the 

 same reason the black-curved percentage is not as large as the 

 sum of the black-purple and purple-curved percentages in the 

 first experiment would lead one to expect. The temperature 

 used in this experiment was again 31.5°C. and the test gave the 

 following results: 



TABLE 7 



S' 



The second brood data are of interest but will be discussed 

 in another section in connection with other similar data. The 

 first brood data show very clearly that while the black-curved 



