OF MUTANT CHARACTERS. 



203 



When the Fi flies from the cross of star by purple curved speck began 

 to hatch, a surprise was met with in that half of the flies were speck in 

 two cultures (1806, 1807), but not in the third (1808, table 112). It 

 had not been noticed before tliat there was any speck in the star stock; 

 and it is not clear how speck came to be there, since nf) cross to speck 

 had been made, and so far as known none of the stocks concerned in 

 the history of star had contained speck even as an impurity. However, 

 this circumstance gave an immediate test of the linkage reliition of 

 star and speck, since these two cultures constituted a star speck back- 

 cross test of crossing-over in the female. The two cultures gave 369 

 flies, of which 184, or 49.9 per cent, were cross-overs. \Vhile this 

 value is that corresponding to any locus as far from speck as black, or 

 any other to the left of black, it is also the result one would obtain if 

 star were not in the second chromosome at all. This possibility 

 caused such concern for the experiment already planned that imme- 



Table 113. — Fi, star speck d* X purple curved speck 9 . 



diately a black-cross test of crossing-over in the male between star 

 and purple curved speck was carried out. This male test proved that 

 star is actually in the second chromosome, since of the 134 flies (cul- 

 ture 1913, table 113) 133 were non-cross-overs, as opposed to the very 

 free crossing-over in the female test. 



CROSSING-OVER IN THE MALE. 



But one fly was a cross-over, since it was distinctly a star curivd 

 speck male, while all other flies were either star speck or purple curvet! 

 speck. This cross-over fly occurred on the sixth day of the counts of 

 a pair culture, so that there is no possibility of overlap of generations; 

 and no opportunity for contamination, since no possibility of star 

 curved flies had ever existed in any other or previous culture. Tliat 

 classification and pedigree were both as recorded was proved by a t-est 



Table 114. — Cross-over star curved speck cf X purple curved speck 9. 



made with the cross-over male. He was out-crossed to a purple 

 curved speck female and produced star curved speck offspring in 

 equal numbers (culture 2048, table 114). There can be little doubt, 



