No. 592] 



THE MECHANISM OF CROSSING-OVER 



219 



We may next consider the disposition of the factors of 

 group III in a diagram based upon data from flies hetero- 

 zygous for C. The separation frequencies which I have 

 obtained are given below : 



A diagram based upon these data would show sepia at 

 0, pink at 19.5, spineless and kidney at 20.9, sooty and 

 rough at 30, and the total length would thus be 30. The 

 tenth of a unit of distance between kidney and sooty is 

 based upon one fly, in which separation had taken place 

 between these factors. Tests of the fly (which was a 

 sooty rough, resulting from a backcross of a female con- 

 taining p sps k so r from one parent and spread C from 

 the other) showed that the factor C had remained with 

 spread, and that no recombination had taken place be- 

 tween the positions of spread and sooty. This one fly, 

 therefore, proved that both spread and C were to the 

 right of kidney. The factor C is thus seen to lie right in 

 the heart of the region where it exerts its maximum effect, 

 as Sturtevant has also found in the case of the similar 

 factors in group II. 



Sturtevant has obtained slightly higher frequencies of 

 separation between pink and ebony in some of his crosses 

 heterozygous for C. The lower value here recorded may 

 then be due to the flies being heterozygous for another 

 factor besides C which disturbs separation frequency, 

 and which is also met with in the crosses not involving C. 



The following values have been obtained in crosses in 

 which the factor C was homozygous : 



