A Change in the Bar Gene of Drosophila Involving Further Decrease in Facet Number and Increase in Dominance

As part of a study of the factors controUingeye facet number in Drosophila melanogaster, selection has been carried on with a view to the determination of the germinal changes appearing during the course of selection. One of the most interesting of these changes occurred during the second generation of downward selection in the white bar stock. An exceptionally "low" male of this generation was shown to possess a changed gene with a marked increase in dominance. This individual appeared on November 20, 1917, and had but 19 facets, while the mean of its 62 brothers was 82.6 facets with a range of 41 to 134. With class sizes such that the range of a class is 10 per cent of its mean this represents a departure of -16.80 10 per cent class units from the mean of the males of the unselected bar stock, while the departure of the brothers of the mutant is on ly-3 .44 units. The new stock derived from the third hybrid generation of a mating of the mutant with a sister possesses the characteristics of the original male, and at the present writing (May 10, 1919) has remained without change for nearly 18 months except for the appearance of a few additional mutants. The name ultra-bar has been given to the new character. Its symbol, Bu, indicates that ultra-bar is a dominant allelomorph of bar. Full-eyed (wild-type) females at 27°C. have an average facet number of 810.6, bar females of the second low selected generation have 61.8 facets, and ultra-bar females 22.0 facets. The corresponding numbers for males are respectively 849.8, 75.6, and 23.0.