220 



THE SECOND-CHROMOSOME GROUP 



As soon as the first flies from the triple back-cross had begun to hatch 

 it became evident that the locus of dachs is to the left of black and not 

 to the right. The discovery that streak, a dominant, was so far to the 

 left of black that it gave very free crossing-over had just been made 

 and had led to the most extensive recasting that any of our maps had 

 been subjected to. The location of dachs in the middle of this long 

 gap was therefore very welcome. 



Table 65. — Pi, dachs black cf cf X vestigial 9 9. Fi wild type 9 dachs 



black-vestigial cf cf from stock. 



The total amount of crossing-over between dachs and black as calcu- 

 lated from this balanced experiment was 17.9 units, which agreed with 

 the 17.7 units found in the simple dachs black back-cross. 



Table 66. — Linkage of dachs black and vestigial with balanced inviability. 



Table 67 gives the summary of the crossing-over data including 

 dachs. The calculation of the locus of dachs on the basis of all the 

 data is 17.5 units to the left of black, or referred to star as the zero- 

 point at 29.0. 



VALUATION OF DACHS. 



The usefulness of dachs is limited only by its rather poor and erratic 

 viability. In many experiments the viability of dachs is up to par, but 

 in combination with certain other characters it has been unsatisfactory; 

 thus the stock called "tt" (dbp^cp^Sp) is poorly viable and almost 

 useless, while the ''tt — " stock, which differs only in the omission of 



