OF MUTANT CHARACTERS. 



171 



In order to make full use of this differentiation of purple versus not- 

 purple by vermilion, it is necessary that all flies used in the experiment 

 should be made homozygous for vermilion. This is often inconve- 

 nient, and accordingly only in the early and comparatively simple 

 experiments was this method employed. It was soon found also that 

 the separation of purple from red was not causing any trouble, so that 

 the differentiation in this case has little net advantage, though it is 

 still of interest as being the first example in Drosophila in which inten- 

 sification was recognized and deliberately made use of. 



THE RELATION OF PURPLE TO PINK. 



Some of the first purples which emerged in the F 2 were crossed to 

 pink to test whether these two eye-colors were allelomorphic or not. 

 Only wild-type FI males and females were produced (table 26), which 

 showed that the purple is not an allelomorph of pink. 



TABLE 26. F\ progeny from out-cross of purple. 



THE LINKAGE OF PURPLE AND VESTIGIAL. 



It was observed (April 2, 1912;B1) that in the F 2 from the cross of 

 the original male to wild nearly all of the flies that were purple were 

 also vestigial. This observation, following on the heels of the black- 

 curved case, furnished a second example of autosomal linkage, this 

 time one of so-called "coupling," the black curved case having been 

 "repulsion." No full counts were made of the proportion of purples 

 that were vestigial. Indeed, at this early stage the linkage relations 

 were receiving less attention than eye-color "series." 



BACK-CROSS TEST OF MALES. PURPLE VESTIGIAL "COUPLING." 



The advantages of the back-cross method of testing linkage and the 

 amount of crossing-over had only begun to be appreciated. This 

 method had been applied to a few cases in the X chromosome, and the 

 general attack upon the linkage of all autosomal mutations planned 

 by Sturtevant and Bridges (March 5, 1912) contemplated its full use. 

 Thus far only two autosomal back-crosses had been completed 

 those by which Sturtevant showed the absence of linkage between 

 the second chromosome and the third chromosome (balloon ebony, 

 May 10, 1912, and black pink, May 12, 1912). Because of the diffi- 



