186 



THE SECOND-CHROMOSOME GROUP 



the mating was accordingly arranged so that a cross-over anywhere 

 within this whole distance would give the required triple form. That 

 is, black purple and purple vestigial were mated together and the 



resulting purple offspring (^7^ — J were inbred. The F2 black 



Table 38. — Pi mating, black purple vestigial d^ X wild female; B. C. mating 

 Fi wild-type 9 X black purple vestigial cf . 



Table 39. — Pi, black X purple vestigial; B. C. test of Fi 9 9 singly. 



Table 40. — ^Pi, black vestigial X purple; B. C. test Pi 9 9 singly. 



purples and purple vestigials were crossed together in several mass- 

 cultures, and in Fa some triples occurred, showing that some of both 

 kinds of F2 flies used had come from cross-over eggs. A better method 

 would have been to back-cross the Fi female by a black-vestigial male. 

 In this case every black vestigial cross-over would be known to be of 



— ^' ^^ ), and these inbred would give the pure 



h -^ Vg / 



the composition 



( 



