372 



GENES MODIFYING NOTCH. 



SHORT NOTCH BY STAR DICH&TE. 



There were four crosses of this combination that gave in FI Notch 

 females with intermediate wings shorter, on the average, than the 

 atavistic type, and therefore more on the order of the short type. 

 The FI records are as follows : 



In these results the condition of the wings was the same in the 

 Dichaete female and in those not-Dichsete, showing that the gene of 

 the latter does not itself act as a modifier. This information is of 

 value in working out the location of the modifier by means of the S' D' 

 test. The further fact that the FI females were " intermediate" be- 

 tween atavistic and short-Notch is, then, more probably due to some 

 other modifier of the Star Dichsete short Notch. This complication 

 makes it more difficult to interpret the results when Star Dichsete is 

 used to locate the modifier of short Notch, but still leaves such a test 

 possible, as the following experiments show. 



SHORT NOTCH BY EOSIN RUBY FORKED. 



When short-Notch females were crossed to eosin ruby forked males 

 of stock the FI Notch females had in nearly all cases shorter wings than 

 the atavistic notch flies (or ordinary Notch) obtained in other cases 

 (four are drawn in fig. 96, a, 6, c, d). In these instances the eosin 

 ruby forked males may carry not only the normal allelomorph of the 

 selected modifier, but also another gene that carries the Notch towards 

 the short-Notch direction, which might be the modifier in the X 

 chromosome of the father. The F 2 generation came from the short- 

 like Notch FI female by her brother. The records are as follows : 



The failure to sharply distinguish between the types of Notch hi 

 these F 2 counts shows that without first purifying the eosin ruby 



