172 



THE SECOND-CHROMOSOME GROUP 



culty of getting the necessary double recessives no back-cross which 

 involved autosomal Unkage 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 vestigial could be conducted. From the F2 

 described above, matings were made which gave two stocks to be 

 used in this test. One stock was the simple purple vestigial and the 

 other was purple vestigial pure for vermiHon. The special advantage 

 of this latter stock lay in the fact that the presence of vermiUon 

 accentuates the difference in eye-color between the flies that are 

 purple and those that are not; that is, vermilion purple is easier to 

 separate from vermilion than is the case in the equivalent separation 

 of purple from red. 



Table 27. — B. C. offspring given hy the Fi (vermilion) sons, from out-cross of 

 (vermilion) purple vestigial males to vermilion females, when back-crossed 

 to (vermilion) purple vestigial females. 



This latter stock was accordingly used in the Pi mating for the first 

 back-cross test. VermiUon purple vestigial males were out-crossed to 

 females of vermilion stock (May 25, 1912). Both parents were homo- 

 zygous for vermilion, and the Fi flies were all vermilion, as expected. 

 Both purple and vestigial are recessive. When the back-cross matings 

 came to be made the culture bottle happened to contain no virgin Fi 

 females, since the Pi mating had been niade at Columbia and the Fi 

 progenj' used had hatched en route to Wood's Hole. The back-cross 

 was therefore made in only one way — by mating the Fi males to virgin 

 vermilion purple vestigial females of the stock kept for that purpose. 

 Five back-crosses were started by mating in each case a single Fi ver- 

 milion male by two or three stock vermilion purple vestigial females. 

 At the end of 10 days the parents were removed from the culture 

 bottles and put in fresh bottles in which second broods were raised. 

 In one case a third brood was raised (table 27). 



