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THE AMERICAN NATURALIST [Vol. LV 



(b) represent the absence of the factor for bar and (r) the 

 absence of the factor for producing red-eyed flies. The factorial 

 composition, then, of the red bar-eyed flies is (BR) and for 

 white round-eyed flies (br). As both the factors for bar- and 

 red-eyed flies are sex linked— that is, they occur in the X-ehromo- 

 some, there being but one in the male and two in the female, as 

 above mentioned. Then the male red bar-eyed flies are repre- 

 sented by (BR) and the female white round-eyed flies by (br — 

 br). 



Matings were made as follows: 



55 41 1 51 27 :?5 



As white bar-eyed flies and red-eyed flies are the cross-over 

 classes, then the percentage of crossing-over is equal to 660, 

 divided by 1.522. and the quotient, multiplied by 100, giving 

 43.3 per cent. 



I next extracted pure lines of white bar-eyed flies and red- 

 eyed flies, using the white bar-eyed males and red round-eyed 

 females in the iiiatiiigs. Their factorial composition being (Br) 

 for til.' itiiilcs 1111.1 (Hl)--Rb) for the females. 



(BR) \\\v) (Rb) (hi)' 



