70 BAH GENE OF DROSOPHILA 



The change in dominance as a result of the mutation is very strik- 

 ing. A cross between a 35.2 facet bar stock of the twenty-fourth 

 generation of the low selection line and an 810.6 facet full-eyed stock 

 gives 399.9 facets in the females, while a cross between the 22.0 facet 

 ultra-bar and the 810.6 facet full gives only 36.5 facets. Ultra-bar 

 thus has a much greater degree of dominance than bar. This is 

 further shown by the cross between the 22.0 facet ultra-bar stock 

 and the 61.8 facet bar stock of the second generation of the low selec- 

 tion line which gives 26.3 facets. Calculated on the basis of 10 per 

 cent class units the ultra-bar has pulled the full-eye down 85 per cent 

 of the distance between the two, while bar has pulled it only 23 per 

 cent. Likewise ultra-bar pulls bar down 85 per cent of the distance 

 between the two, while bar pulls ultra-bar up only 15 per cent. 



That the factor for the new character is located in the X chromo- 

 some is shown by the reciprocal matings between ultra-bar and bar. 

 Ultra-bar females mated with bar males give ultra-bar males, while 

 the reciprocal matings give bar males. 



In order to determine the locus of the new factor the crossing over 

 test was applied. If the new factor is at a different locus from that 

 of bar, crossing over should take place between the two. A mating 

 between ultra-bar and full-eye should then give some bar males in 

 the second hybrid generation, the percentage of such individuals 

 depending upon the distance between the locus of bar and that of 

 ultra-bar. In case, however, the new locus is identical with that 

 of bar there should be no bar males in this generation. 



Crosses between ultra-bar females and wild full males give in the 

 second hybrid generation only three males which are not ultra-bar 

 as opposed to 1,238 ultra-bar males. These three males have 88, 51, 

 and 60 facets respectively, all being within the range of bar-eye. On 

 the basis of crossing over this would mean that the locus of the new 

 factor is only 0.24 units from the bar locus on the chromosome map. 

 Unfortunately one of the three males was overetherized. Breeding 

 tests of the other two show a degree of dominance over full which 

 is different from that of bar and they therefore cannot be the result 

 of crossing over. They apparently represent a new mutation. 



The most reasonable conclusion to be drawn from these tests is 

 that the locus of ultra-bar is the same as that of bar or that it is 



