OF MUTANT CHARACTERS. 



233 



same time that the F 3 blacks were inbred, both the FI flies and the 

 double recessive necessary to test them were obtained at the same 

 time and a generation saved. 



Three back-cross cultures were raised from pairs (table 79), which 

 gave a total of 755 offspring, of which 353 or 46.8 per cent were cross- 

 overs. This was very free crossing-over, comparable with the black 

 arc and black balloon values previously found. 



TABLE 79. PI, black 9 9 



X morula cTcT; 

 cf of stock. 



mid-type 9 X black morula 



TABLE 80. PI, black arc 9 X morula cf; B.C., FI mid-type 9 X black arc 



morula d" from stock. 



From a comparison of these values and from the fact that curved 

 and morula had not crossed over readily, it was concluded that the 

 locus of morula must be close to that of arc. 



An effort was made to obtain triple-recessive black arc morula and 

 black balloon morula. This latter stock was not obtained at all, and 

 the former was secured only after repeated matings of (F 2 , F 4 , and F 6 ) 

 black arc and black morula flies. 



Triple back-crosses for the loci black, arc, and morula were made in 

 pairs in each of the four possible ways in order to balance the invia- 

 bility completely. The numbers are not equal in the different experi- 



