GENETICS PUEPLE EYE COLOR DROSOPHILA 269 



THE RELATION OF PURPLE TO PINK 



Some of the first purples which emerged in the F2 were crossed 

 to pink, to test whether these two somewhat similar eye color 

 were allelomorphic or not. Four such pair matings produced 

 only wild-type males (134) and females (137), which showed 

 that purple is not an allelomorph of pink. 



THE LINKAGE OF PURPLE AND VESTIGIAL 



It was observed (April 2, 1912; Bl) that in the F2 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 appre- 

 ciated. 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 difficulty of getting 

 the necessary double recessives no back cross which in- 

 volved autosomal linkage had been possible until purple arose 

 in the vestigial stock and thereby gave the required double 

 recessive, purple vestigial, with which such a test of the amount 

 of crossing over between purple and vestigal could be conducted. 

 From the Fo described above, matings were made which gave 

 two stocks to be used in this test. One stock was the simple 



