EYE COLOR IN DROSOPHILA MELANOGASTER 367 



they are eosin-eyed if from the eosin mother and red-eyed if 

 from the black mother. The offspring from the black mother 

 constitute a test of whether the father were free from black, for 

 in this case none of the red-eyed offspring hatching in the double 

 mating culture should be black, while if the father were hetero- 

 zygous for black half of the red-eyed offspring should be black. 

 Only one of the four cultures gave black offspring, and this 

 culture was then discarded. The eosin-ej^ed flies of the other 

 three cultures were all heterozygous for pinkish and at the same 

 time free from black. By mating together some of these eosin- 

 eyed flies pure pinkish offspring should be obtained as a quarter 

 of the offspring. A more efficient method, and the one actually 

 followed, was to save the fathers and mate them to their eosin- 

 eyed daughters, since in this case half, rather than a quarter, of 

 the progeny should be pure pinkish. 



In order to show by an actual test that the gene for pinkish 

 is in the third chromosome, it was decided to take advantage of 

 the fact of no crossing over in the male and to run a back-cross 

 test of a male heterozygous for pinkish and for the dominant 

 third-chromosome character dichaete. It was now realized that 

 the back-cross tests of females heterozygous for pinkish and 

 black had not excluded the possibiUty of pinkish being in the 

 second chromosome, though they had shown that, if so, it could 

 be only in one or the other end-region and not near black. Ac- 

 cordingly, exactly the same procedure was followed as in the 

 tests for the location of cream b, that is, a pinkish female was 

 outcrossed to a male which had the dominant second-chromo- 

 some star as well as dichaete. The Fi eosin star dichaete males 

 were then back-crossed to pinkish females. The result showed 

 (table 23) that the gene for pinkish is in the second and not the 

 third chromosome; for, as well as could be judged, none of the 

 star flies were pinkish, while all the not-stars seemed to be pink- 

 ish, and dichaete was present in half of both the star and the 

 pinkish classes. 



In the Ught of this test, and from the fact that there was about 

 50 per cent of crossing over between black and pinkish, we could 

 place pinkish in either the extreme left or the extreme right end- 



THE JOURNAL OP KXPERIMENTAI, ZOOLOGY, VOL. 28, NO. 3 



