GENES MODIFYING NOTCH. 369 



FOURTH TEST (FOR FOURTH-CHROMOSOME MODIFIERS). 



The following method of finding out whether a modifying gene is 

 present in a particular chromosome was suggested by Dr. A. H. Sturte- 

 vant. A short-Notch female is first crossed to a black pink bent male. 

 The FI males 1 are then crossed (in pairs) to stock short-Notch females 

 (see scheme below). 



Eladk pink "ben 



\iy Shoi-i Notch O 



by ShortHotch o 



F 2 Notch O byBlackpink'bent o 



Since the FI male had one fourth chromosome from black pink bent 

 (and since there is no crossing -over in the male), half of his Notch (F 2 ) 

 daughters will have this chromosome (only one each), and half will not 

 have it. If they are of two sorts (so classified in table 12), such as 

 intermediate and short Notch, their differences might depend on the 

 presence of the bent fourth chromosome in half of the Notch females. 

 If now we separate as far as possible the females into two classes, and 



TABLE 12. F t cf (out of short Notch 9 by bent cf) by short Notch 9 . 



test each female separately to find out if she has or has not a bent 

 fourth chromosome, we should get an answer to our question. Each 

 female was bred to a black pink bent stock male. The presence|of 

 black, of pink, and of bent (separate or combined) in the offspring was 

 recorded. Table 13 gives the end-results; the first column records the 

 kind of F 2 female tested, the next three columns the kind of notch, and 

 the next three columns indicate (by X) whether black, pink, or bent 

 was present; and the last column the total number of flies. 



1 Their sisters that were not recorded here were all atavistic Notch. 



