CHANGE IN THE BAE GENE OF DROSOPHILA 321 



practicable number of individuals. Besides there may be a 'no- 

 crossover ' accessory factor. 



The reasonable conclusion to be drawn from the evidence is 

 that ultra-bar has the same locus as bar or a locus so close to it 

 that the two are a single unit in all demonstrable cases. It 

 follows that ultra-bar has been produced by a change in the bar 

 gene, and not by a change at some other locus. 



CONCLUSIONS 



Eye-facet number in Drosophila furnishes an excellent material 

 for the quantitative study of both germinal and envu'onmental 

 factors. The demonstration of the striking and regular temper- 

 ature effect makes possible the recognition and analysis of ger- 

 minal factors. As outlined in preliminary reports (see bibliog- 

 raphy) selection for facet number in bar-eye has shown that the 

 effect of selection here is the result, first, of a sorting out of ger- 

 minal diversities in the original stock and, second, of the isolation 

 of new diversities as they arise. Besides occasional reverse mu- 

 tations to full, the original diversities as well as the new changes 

 have befen due to accessory factors. The mutation described 

 in the present paper, however, resembles the reverse mutations 

 in that it involves change in the bar gene. The effect here, how- 

 ever, is essentially an intensification of that produced by bar. 

 There is further decrease in facet number and a great increase 

 in dominance. 



It is interesting to note also that the change occurred in a 

 line in which low selection was being carried on. In the light 

 of all the evidence concerning the direction of mutations, no 

 special significance can be attached to this fact at present. For 

 instance, reverse mutations to full occur in the low selection lines 

 of bar and in ultra-bar as well as in the high selection lines. 

 While, therefore, a connection between the direction of selection 

 and the direction of mutations in general seems improbable, the 

 matter is of sufficient importance to warrant careful record of 

 all instances. 



THE JOUBNAL OF EXPERIMENTAL ZOOLOGY, VOL. 30, NO. 3 



