CHANGE IN THE BAR GENE OF DROSOPHILA 323 



SUMMARY 



1. The recognition of a pronounced and regular effect of tem- 

 perature upon facet number in Drosophila has made possible a 

 more accurate analysis of the germinal changes taking place in 

 the bar-eyed races. 



2. A single male with but 19 facets appeared in the second 

 generation of downward selection in white bar, the normal range 

 of that generation being 41 to 134 facets. 



3. From this single male there was derived by crossing with 

 his sisters a new race which has been called ultra-bar. The 

 symbol for its gene is B'U. 



4. The mean facet value of the males of this race is 23 as com- 

 pared with 75.6 in the parental stock. 



5. The ranges of ultra-bar and bar do not overlap in the males 

 and barely overlap in the females. 



6. The new stock has remained unchanged for over twenty 

 months except for the appearance of a few individuals with 

 marked departures from the normal range. 



7. Among these mutations there have appeared reversals to 

 full-eye and returns to a condition resembling bar in facet num- 

 ber, but differing from it in dominance. 



8. Ultra-bar differs strikingly from bar in having a much 

 greater dominance over full, 84.8 per cent as compared with 23 

 per cent. 



9. Correspondingly ultra-bar has a dominance of 82.6 per cent 

 over bar. 



10. The ultra-bar gene is located in the sex chromosome. 



11. Crossing-over tests show that the new gene has the same 

 locus as bar or is so close to it as to be identical for all practical 

 purposes. 



12. The change involved is to be considered as a change in 

 the bar gene itself which is in the direction of the original change 

 from full to bar. 



13. The dominance relations, however, make it improbable 

 that ultra-bar can be considered merely as a quantitative in- 

 crease in the bar reaction. 



