LEOXELL C. STRONG. 



Three-Point Back-Cross ru se X D. 



In order to place the gene more accurately, a three-point 

 back-cross was undertaken. A sepia roughoid female was 

 crossed to a dichaete male, the FI dichaete females were then 

 back-crossed to roughoid sepia males from stock (pair matings 

 being made). Results: 



ru se 



ru se 



X 



D 



ru se 



July 7, 1919. 



The results give a cross-over value for roughoid and sepia of 

 19.5 and 25.1 respectively, or a mean of 21.7; a cross-over value 

 for sepia and dichaete of 14.2 and 17.2 or a mean of 15.4. 



Four-Point Back- Cross ru se X D H. 



One other back-cross experiment was performed, using the 

 four characters roughoid, sepia, dichaete and hairless. From 

 this, 980 flies were counted, giving the cross-over value for 

 roughoid and sepia as 25.6. 



+ + 



ru se 



+ + 



ru se 



August 12, 1919. 



+ + D H 



X 



ru se 



