298 



THE SECOND-CHROMOSOME GROUP 



plus direction from black, were those lying on the same side of black 

 as does curved, which was the first locus to be included with black in 

 the second chromosome. Likewise the loci "to the left," or in a 

 minus direction, were those lying on the opposite side of black from 

 curved. 



However, curved is too remote from black to be accurately located 

 by direct reference to black. Accordingly, the position of these inter- 



TABLE 140. Summary of available data on crossing-over in the second 



chromosome. 



mediate bases must first be mapped. Of these, purple was the first 

 to be considered as being the closest to black, and also because its 

 position with relation to black has been the subject of more investi- 

 gation, and is more accurately determined than any other second- 

 chromosome distance. The black purple cross-over value of 6.2 is 

 based on 48,931 flies, and since there is certainly no double crossing- 

 over within this distance, the value can be accepted without correction, 

 and purple can be mapped at a locus 6.2 units to the right of black. 



The third locus to be considered is vestigial, and there are open two 

 sets of data by means of which the position of vestigial can be mapped : 

 (1) the position of vestigial can be located directly by the purple 



