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THE AMERICAN NATURALIST [Vol.L 



flies can be "spotted" in another way, without breeding 

 tests, and yet without making them homozygous for many 

 mutant factors and thus inviable. The method used was 

 to mate the F x females to bv males, which had been made 

 up for this special purpose. The b v daughters (F 2 ) must 

 be cross-overs, since in the F x mother b and v were in dif- 

 ferent chromosomes ; moreover, a glance at the formula of 

 the Fj females will show that these cross-over chromo- 

 somes must have been formed of the left-hand end of the 

 upper chromosome and the right-hand end of the lower. 

 Thus these bv females contain a chromosome with all nine 

 mutant factors (except in the case of the few double cross- 

 overs). Since, however, they were homozygous in only 

 two mutant factors, they could easily be bred. 



A similar scheme was used in many of the other steps 

 shown in the diagram representing combinations made in 

 group I, and was also used frequently in group II. Owing 

 to the fact that rudimentary winged females (group I) 

 are practically sterile, devices of this sort had to be used 

 in dealing with flies containing this factor from the very 

 start, and the same may be said of flies with dachs legs 

 (group II), since these also were found very hard to 

 handle. In most of the other cases, however, it was not 

 necessary to use such a method before several factors had 

 been combined together, as flies homozygous for just two 

 or three mutant factors were generally viable enough to 

 handle. There would be no object in wearying the reader 

 with a description of the exact way in which each of the 

 steps was taken; it is the author's purpose only to explain 

 the nature of methods used, giving only sufficient examples 

 to make clear the details of any devices never previously 

 employed that might be capable of application to other 



From the example of the cross involving bv, previously 

 given, we may now generalize, and establish the rule that 

 in making up, and also in keeping stocks containing many 

 linked recessive factors, if the latter cause a marked less- 

 ening of fertility or viability, it is best to follow the prac- 

 tise of keeping the stocks heterozygous, by back-crossing 

 them to stocks containing only the few recessive (or par- 



