374 



GENES MODIFYING NOTCH. 



the short type carries the normal allelomorph of the"selected modifier. 

 (Fig. 97, a, 6, c, d.) The FI counts gave: 



The reciprocal cross was also made, viz, short Notch female by 

 male of selected stock, with results in FI as follows: 



It is evident from the first cross that the gene for short Notch in the 

 sex chromosome of the male does not carry a dominant allelomorph 

 for short Notch, and not even one that when heterozygous makes the 

 Notch any shorter than in the atavistic flies. It is evident from the 

 second reciprocal cross that the modifying gene for short Notch carried 

 by the other sex chromosome, viz, the one that carries Notch also, is 

 likewise not a dominant or even a modifier when in heterozygotic 

 condition. Both crosses show that the selected stock is free from 

 modifiers for making Notch shorter than the atavistic Notch. 



In four cases the cross between short Notch and black pink bent 

 was carried to the F 2 generation (instead of back-crossing as above) 

 with the following results: 



In this case the FI male gets his single X chromosome from his 

 mother, viz, an X with the shortening factor. His sisters have one 

 Notch X chromosome carrying the shortener and another from the 

 bent father without the shortening factor. Hence the expectation in 

 F 2 is that all the Notch flies should be short-Notch except for crossover 

 cases, where the gene for short-Notch carried into the X is derived from 



