72 



SEX-LINKED INHERITANCE IN DROSOPHILA. 



LINKAGE OF CHERRY, CLUB, AND VERMILION. 



The need for a readily workable character whose gen should lie in 

 the long space between cherry and vermilion has long been felt. 

 Cherry and vermilion are so far apart that there must be considerable 

 double crossing-over between them. But with no favorably placed 

 character which is at the same time viable and clearly and rapidly 

 distinguishable, we were unable to find the exact amount of double 

 crossing-over, and hence could not make a proper correction in plotting 

 the chromosome. Club occupies just this favorable position nearly 

 midway between cherry and vermilion. The distances from cherry 

 to club and from club to vermilion are short enough so that no error 

 would be introduced if we ignored the small amount of double cross- 

 ing-over within each of these distances. 



It thus becomes important to know very exactly the cross-over 

 values for cherry club and club vermilion. The experiment has the 

 form of the yellow club vermilion cross of table 52, except that cherry 

 is used instead of yellow. Cherry is better than yellow because it is 

 slightly nearer club than is yellow and because the bristles of yellow 

 flies are very inconspicuous. In yellow flies the bristles on the side of 

 the thorax are yellowish brown against a yellow background, while in 

 gray-bodied flies the bristles are very black against a light yellowish- 

 gray background. 



For the time being we are able to present only incomplete results 

 upon this cross. In the first experiment cherry females were crossed to 

 club vermilion males and the wild-type daughters were back-crossed to 

 cherry club vermilion, which triple recessive had been secured for this 

 purpose. Table 53 gives the results. 



Table 53. — Pi cherry 9 9 X club vermilion cfcf . B. C. Fx wildr-type 9 

 X cherry club vermilion c? cf . 



