206 



THE SECOND-CHROMOSOME GROUP 



It was assumed without question that arc was in the same direction 

 from black as were purple and vestigial, the only three genes previously 

 mapped. Arc and vestigial do not make a classifiable combination, 

 so that it was not advisable to test further the locus of arc by means 

 of the vestigial-arc back-cross; but purple and arc are workable and 

 accordingly the purple arc double recessive was made up. By this 

 time, however (April 1913), curved and speck had been mapped, and 

 the position of speck was seen to be close to the assumed position of arc, 

 but even farther away from black, since black speck gave about 49 

 per cent of crossing-over as against the 36 given by black arc. It was 

 resolved to run a three-point experiment, using speck as well as purple 

 and arc. The first step in this was to get arc and speck together, 

 which proved a troublesome job. The difficulty lay solely in the fact 

 that the loci of these two are so close together that only rarely was one 



Table 52. — Pi, black arc cf X wild 9 ; B. C, F^ wild-type cf X 



stock black arc 9 9 . 



of the F2 arcs or F2 specks a cross-over, and the first two attempts 

 failed. The double was finally obtained (October 1913). It was now 

 an easy matter to obtain the purple arc speck triple recessive (F3 from 

 the cross of purple arc by arc speck). 



Table 53. — Pi, black arc cf X wild 9 ; B. C, Fi wild-type 9 



stock black-arc cf cf . 



X 



The Pi for the back-cross was made by mating a purple female to an 

 arc speck male, which was considered a better type of mating, from 

 the standpoint of viabiHty, than to have all the mutants enter from 

 one parent. 



The back-cross (table 54) furnished 2,625 flies, of which 1,431 were 

 non-cross-overs, 1,038 cross-overs between purple and arc, 128 cross- 

 overs between arc and speck, and 28 cross-overs in both regions at 

 once. The distance of arc from purple was found to be greater than 

 first indicated by the black arc value (36), since now 40.6 per cent of 

 crossing-over was observed. This longer distance is in better agree- 



