358 



GENES MODIFYING NOTCH. 



Had different modifying factors been present, the atavistic type of 

 Notch should have been shown by the daughters, but if both Hues had 

 been changed through the isolation of the same modifying gene, the 

 results are expected to be the same as when the male comes from the 

 same line as the Notch female. The cross showed that the Notch 

 modifier was the same in both lines. 



LOCALIZATION OF THE GENE FOR NOTCH. 



Earlier evidence had shown that the gene for Notch lies in the X- 

 chromosome somewhere in the region between eosin and ruby. The 

 following "three point" experiment was devised to furnish more precise 

 data. The red-eyed Notch female was bred to an eosin ruby forked 

 male. Her Notch daughters are expected to contain one X chromosome 

 with the Notch locus and the other X chromosome to contain the eosin 

 ruby forked loci. The approximate location of these loci is that shown 

 in figure 92. 



The figure also indicates that three possible regions of crossing over 

 occur between the three pairs of genes involved. There are sixteen 

 possible classes: two non-cross-overs, six single cross-overs, six double 

 cross-overs, and two triple cross-overs. The characters shown by each 

 of these classes are the following : 



Non-cross-overs 



1. eosin ruby forked 



2. Notch 



Single cross-overs Double cross-overs Triple cross-overs 



1. eosin Notch ruby forked. 



2. wild tj-pe. 



1. eosin Notch ruby. 



2. forked. 



3. eosin Notch forked. 



4. ruby. 



5. eosin ruby. 



6. Notch forked. 



5. eosin forked. 



6. Notch ruby. 



When an Fi Notch of this composition is crossed to an eosin ruby 

 forked male (the multiple recessive) all the classes of gametes produced 

 by such a female will be revealed both in the female and in the male 

 offspring, except that there \Adll be only half as many classes of males 

 as of females, since all those males that get the gene for Notch will die. 

 In the table, the male classes are entered separately from the female 

 classes. It was anticipated that calculations based on the males alone 

 would be more accurate than those based on the females alone, because 

 in the latter sex there is some difficulty in separating the eosin from 

 the ruby females, while in the males no such confusion is possible. The 

 computations show, however, that the differences between the two 

 sets of data are as near as is to be expected for the numbers involved. 

 Therefore the estimates based on the total figures are probably to be 

 preferred. 



ff 



