362 



GENES MODIFYING NOTCH. 



male gets is the eosin-ruby-bearing X chromosome of the mother (the 

 other males die). In other words, their composition is that represented 

 in the third line to the left in figure 93. 



RedNotch . X 

 eosinruby 



n m 



Star Dichaete X ' S> 



ptarDiehaete 



S' 



s' 



s' 



FIG. 93. 



If this male is now back-crossed to a selected Notch female (see figure 

 93) any red-eyed Notch daughter that is also Star-Dichsete (upper line 

 to right; No. 1) must have gotten the Star (II) and the Dichaete (III) 

 chromosomes from her father (neither of which bears the modifiers 

 sought for) and an X chromosome also from the father with genes for 

 eosin ruby eyes and normal wings. She must also have gotten the 

 second and third chromosomes that may carry in one or in both the 

 modifiers sought for (which are recessive) from her mother, as well as an 

 X chromosome bearing the genes for red-eye and Notch wing. Hence 

 such a female should be atavistic Notch, because either the S' or D' 

 genes will bring in the normal allelomorph of the postulated modifiers 

 in II and III. Conversely, females that are not Star and not Dichaete 

 (No. 2) should be of the selected type, since their second and third 

 chromosomes, one or both, contain the modifiers. 



Continuing the analysis, it is evident that if the modifier (one or more) 

 is in the second chromosome, then all Star Notch daughters (No. 3) 

 should be atavistic, and all not-Stars (No. 4) the selected type of 

 Notch; and if the modifier is in the third chromosome, then all Dichsete 

 Notch daughters should be atavistic (No. 4) and all not-Dichsete (No. 3) 

 selected type of Notch. 



The ability to pick out atavistic flies from selected-type flies is essen- 

 tial to this test. In general, this can be done successfully, with, how- 

 ever, a margin of error, but the error is expected from the information 

 at hand to be so small as not to effect the main result. Moreover, the 

 occurrence of red-eyed females with normal wings (flies that are known 

 from the linkage relations of the experiment to have the Notch gene) 

 in any of the classes named above is an almost certain index of the 

 occurrance of the modifier. 



