CHANGE IN THE BAR GENE OF DROSOPHILA 311 



cent of the distance between the two, while ultra-bar pulls full 

 down 84.6 per cent. Correspondingly, ultra-bar pulls bar 82.6 

 per cent of the distance between them. The difference in dom- 

 inance is shown also in the tabulations of the facet values of the 

 daughters of a cross between a heterozygous female and a bar 

 male as compared with the daughters of a heterozygous female 

 with an ultra-bar male (tables 7 and 8 and figs. 6 and 8). In 

 the first case (fig. 6) the distribution of values is distinctly 

 bimodal, because the mean values of heterozygous and bar 

 females are far apart. In the second class the distribution is 

 essentially unimodal, because the values of heterozygous and 

 ultra-bar females are close together. 



THE LOCATION OF THE ULTRA-BAR GENE 



That the new factor is located in the sex chromosome is indi- 

 cated by the results of numerous matings as given in tables 8 and 

 9. Bar females mated with ultra-bar males give intermediate 

 females and bar males (figs. 4 and 5). Heterozygous females 

 mated with bar males give bar and intermediate females and bar 

 and ultra-bar males (figs. 6 and 7). Heterozygous females mated 

 with ultra-bar males give bar and ultra-bar males (fig. 9) and 

 presumably ultra-bar and heterozygous females, though the two 

 populations are so close together that the overlapping gives an 

 approach to a normal variation curve (fig. 8). The compound 

 character of the curve is indicated by its greater variability as 

 compared with either ultra-bar alone or heterozygotes alone, 

 as shown in table 8. 



If ultra-bar is the result of an accessory factor acting in con- 

 junction with an unchanged bar gene, it should be possible by 

 crossing over to separate the two and thus obtain the original 

 bar. 



B'U 



Ultra-bar females crossed with full males give in Fi — X — 



"D/TT 



females and males. If there is no crossing over Fo should 



B'U 



T)/TT V. 



give or ultra-bar males, or full males, 



—X— ' ' — X— 



B'U 



