248 



THE SECOND-CHROMOSOME GROUP 



Pure stock of the mutation had been obtained at once through the 

 happy selection of a pure pinkish female which had been taken to be 

 simply an eosin female of somewhat lighter eye-color because of 

 being freshly hatched. 



CHROMOSOME CARRYING PINKISH. 



Since pinkish appeared in a stock of eosin black, material was on 

 hand to test the chromosome group at once. Accordingly, black 

 pinkish females were out-crossed to eosin males and the F2 eosin 

 jfemales, standard eosin in color, were back-crossed to black pinkish 

 males. In the back-cross cultures half of the flies were not-black, and 

 the not-black pinkish flies were seen to be less markedly "pinkish" 

 in eye-color than the blacks. In the absence of black the eye-color 

 was more nearly like that of the other creams, though the amount of 

 dilution is less than in any of the other creams. In the first two of 



Table 95. — Offspring given by the Fi eosin-eyed daughters from the out-cross 

 of black pinkish females to eosin males, when back-crossed to black 

 pinkish males. 



these back-cross cultures (table 95) males and females were classified 

 together. Some question having been raised in regard to the accu- 

 racy of the separation of pinkish from eosin among females, the 

 cross was repeated and the readily classifiable males (last three cul- 

 tures) gave the same result as before. It was seen that the new or 

 cross-over combinations were as numerous (5 1.4 per cent) as the original 

 classes, and this independent inheritance was taken to mean that the 

 gene for pinkish is not in the second chromosome. While this was a 

 mistaken notion — the true relation being that the gene is so far away 

 from black that in the female there is entirely free crossing-over — yet 

 it led to the device of the efficient "double-mating" method of 

 ridding a given stock of an undesired recessive. 



THE DOUBLE-MATING METHOD. 



If pinkish were in the third chromosome, then the presence of the 

 black in the pinkish stock could be of no advantage, and might be a 

 very serious handicap, since it would prevent the use of all our third- 

 chromosome stocks containing ebony or sooty. The first step in the 





