184 



THE SECOND-CHROMOSOME GROUP 



of the amount of crossing-over in the female between the loci purple 

 and vestigial (table 29) had given a cross-over value of 9.1 units. The 

 next cross-over value to be worked out was that of black vestigial as 

 about 20 imits (Morgan, 1912). With these two values alone it was 

 not possible to determine the relative order within the chromosome of 

 the three loci involved; it was apparent that black was farther away 

 from vestigial than from purple, but it could not be told whether it lay 



Table 35. — Pi mating, 'purple cf X black 9 ; Fi mating, 

 wild-type 9 9 and cf cf . 



on the same or on the other side of vestigial from purple. The black 

 purple value should be one of two values depending on the order of the 

 genes; it should be an approximation to either the sum (20 -|- 9 = 29) or 

 the difference (20 — 9 = 11) between the black vestigial and the purple 

 vestigial values. To carry out a back-cross experiment for black and 

 purple it was first necessary to make up the double recessive. No 

 easy task was anticipated in this, for it had just become known that 

 on account of no crossing-over in the male no double recessive could be 



Table 36. — Pi mating, purple cf X black 9; B.C., Pi 9 X black purple cf . 



obtained in F2, as in fact none was (table 35). As expected, the F2 

 ratio approximated 2:1:1:0. Three sorts of F3 mass-culture 

 matings were made: black X bla,ck, purple X purple, and black X 

 purple. Of these matings the last type is by far the most valuable, 

 since in case one of the flies happened to come from a black purple 

 cross-over egg X a black sperm it would give some purple offspring 

 when crossed to purple; and these, inbred, would give the required 

 black purples as a quarter of the next generation. Likewise, if one 

 of the purples had come from a cross-over black purple egg, the black 

 X purple cross would produce some blacks that would give the required 



