30 



SEX-LINKED INHERITANCE IN DROSOPHILA. 



In the crosses between white and bifid there were 1,127 cross-overs 

 in a total of 20,800 available individuals, which gives a cross-over value 

 of 5.3. In the crosses between yellow and bifid there were 182 cross- 

 overs in a total of 3,175 available individuals, which gives a cross-over 

 value of 5.8. In crosses between bifid and vermilion there were 806 

 cross-overs in a total of 2,509, which gives a cross-over value of 32.1. 

 On the basis of all the data summarized in table 65, bifid is located at 

 6.3 to the right of yellow. 



LINKAGE OF CHERRY, BIFID, AND VERMILION. 



In a small experiment of our own, three factors were involved — 

 cherry, bifid, and vermilion. A cherry vermilion female was crossed 

 to a bifid male. Two daughters were back-crossed singly to white 

 bifid males. The female offspring will then give data for the Hnkage of 

 cherry white with bifid, while the sons will show the hnkage of the 

 three gens, cherry, bifid, and vermilion. The results are shown in 

 table 3. 



Table 3. — Pi cherry vermilion 9 9 X bifid cfcT- 



white bifid cf c? • 



B. C* Fi wild-type 9 X 



*B. C. here and throughout stands for back-cross. 



Both males and females give a cross-over value of 5 units for cherry 

 bifid, which is the value determined by Chambers. The order of the 

 factors, viz, cherry, bifid, vermilion, is established by taking advantage 

 of the double cross-over classes in the males. The male classes give 

 a cross-over value of 20 for bifid vermilion and 24 for cherry vermilion, 

 which are low compared with values given by other experiments. The 

 locus of bifid at 6.3 is convenient for many linkage problems, but this 

 advantage is largely offset by the liability of the bifid flies to become 

 stuck in the food and against the sides of the bottle. Bifid flies can be 

 separated from the normal with certainty and with great ease. 



