1 82 T. H. MORGAN AND CLARA J. LYNCH. 



wingless flies result. The test showed, in fact, that crossing 

 over had occurred, but the result was obscured, because, neces- 

 sarily, the p2 flies would still be gray, unless the crossing over 

 had occurred simultaneously in both sexes and the cross-over 

 gametes happened to meet. The following experiments will 

 illustrate this point: 



Seven F% wingless males were paired with 15 long-winged black 

 females, and gave 133 gray females, and 96 gray males. 



But five wingless females mated to fifteen long black males 

 gave: Gray, 9 , 64; cf , 61; Black, 9,8; cT, 7. The last result 

 shows that crossing over had occurred in the gametes of one FI 

 fly, which gave rise to one fly (at least) that was heterozygous 

 for color, and from this fly came the black flies that appeared 

 in the test. The point of particular interest is that this crossing 

 over had not been apparent in the F-2 gray flies, and the reason 

 for this is not far to seek. If crossing over between wY and Wy 

 in the gametes of the FI generation had occurred, the gametes 

 would be: WYB, wYB, WyB, wyB. 



If this fly were fertilized by an ordinary FI male, whose gametes 

 would be wYB and WyB, all the wingless flies would be gray, 

 and those grays derived from the crossing-over (however rare 

 their occurrence) would be heterozygous for color. Only a back 

 cross by black flies would make evident in the next generation 

 that crossing-over had really occurred. How often it occurs 

 remains to be discovered by further experiments. The ratio 

 of 2 to i in the F 2 gray flies shows, however, that crossing-over 

 is not common, and the absence of black wingless flies in the Fa 

 generation indicates this even better. For, if crossing over were 

 common the cross-over gametes of the FI flies would occur often 

 enough for like to meet like and produce black wingless flies. 



