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THE AMERICAN NATURALIST [Vol. L 



those sex-linked characters that their mother contained 

 and they may therefore be used to determine the composi- 

 tion of their mother. As a matter of fact, however, males 

 containing v were generally employed, so that v, if it had 

 been present in the tested female, would appear in her 

 daughters as well as her sons. This additional test for v 

 was desirable because it is a factor which in a white, 

 cherry, or bar eye it is difficult or impossible to detect. 



Stock of the second chromosome was obtained, and is 

 maintained, in an essentially similar way. Here the at- 

 tempt was not made to put most of the mutant factors into 

 one of the two chromosomes of the heterozygous females 

 to be tested. This was partly because an experiment of 

 this sort with one chromosome would seem sufficient. 

 Moreover, it was harder to make up multiple stocks of the 

 second chromosome, since the order of certain factors had 

 not at first been well determined, and since, besides, it takes 

 a greater number of generations to put non-sex-linked 

 factors together into the same stock than it does to put 

 sex-linked factors together. For, if two recessive stocks 

 of chromosomes II are crossed, the F x males, in which 

 crossing-over never occurs, transmit the recessive factors 

 of only one stock to each son and daughter. The latter 

 then can not be homozygous for both sets of recessive fac- 

 tors, and so it is impossible to pick out, except by further 

 breeding, those that received both sets from the mother. 

 But as in the case of the bv illustration given, if a male 

 with C D is available to cross with the " F, " hvbrid female 

 ABC 



jy^p, the "F 2 " individuals showing both characters 



C and D must have the composition AB ™ EF , and so the 

 desired cross-overs may be picked out immediately in F 2 . 



A somewhat similar scheme, often especially useful for 

 obtaining desired combinations in a non-sex-linked group, 

 involved making use of cross-overs that break combina- 

 tions already obtained. This too may be shown by an 

 example. It was desired to obtain a stock containing the 

 second chromosome factors dachs legs, jaunty wings, 

 curved wings, and balloon wings. Dachs black stock al- 



