No. 594] THE MECHANISM OF CROSSING-OVER 



301 



tially recessive) factors necessary to show which offspring 

 contain the desired cross-over or non-cross-over chromo- 

 some. In the example, this method was used in combining 

 two stocks to make up a recombination stock. The same 

 means is employed in maintaining the multiple stock after 

 it has been synthesized. Thus, in the case of group I, the 

 females containing in one chromosome the combination 

 ywAbvmsrf (the "F 2 " obtained above), were crossed to 

 cciBr males. In this way some daughters (F 3 ) are pro- 

 duced (which these were was determined by breeding 

 tests) that received from their mother ywAbvmsrf, and 

 from their father c^Br. These F 3 females having the com- 

 position - — — -g-, were then back-crossed to cCjBr 



males again, in order to maintain the stock. Since all the 

 daughters (F 4 ) received cciBr from their father, those 

 which do not show these characters fully developed must 

 have received from their mother factors near both ends 

 of the chromosome containing the nine mutant factors. 

 Therefore, except for the very few flies in which crossing- 

 over occurred between w and y, which is at the very end, 

 or in which double crossing-over occurred, all the light 

 cherry, normal winged, partially bar eyed flies will have a 

 composition like that of their mother, and may be bred in 

 the same way, again to the c^Br males, which now hatch 

 from the same bottle. This then is a cross exactly like the 

 preceding one, except for the few cross-over flies above 

 mentioned. The latter may be detected, however, and 

 their offspring discarded if the females are bred in sepa- 

 rate bottles. This same cycle may be repeated generation 

 after generation. Thus a continual supply is maintained 

 of flies heterozygous for all these factors. 



In making the linkage determinations, such flies are bred 

 to normal or to bar males, and the female offspring, which 

 are all alike in appearance except in respect to the par- 

 tially dominant factors A and Br, and which should, there- 

 fore, have had approximately equal chances for surviving, 

 are individually tested for their contained characters. 

 For the tests, the female need not be virgin, since, what- 

 ever kind of male is employed, the sons will show only 



