322 CHARLES ZELENY 



The constancy of the new gene seems to be of about the same 

 order as that of bar. Reverse mutations to full occur in both. 

 In ultra-bar there is also a reverse mutation in the direction of 

 bar, but apparently not to the same point because, while the 

 mutant comes within the facet range of bar, it does not have the 

 same dominance over full. 



The locus of the new gene is the same as that of bar or is so 

 close to it that our methods of analysis are not capable of sepa- 

 rating the two. The crossing-over test is the only one that can 

 be applied, and it has the obvious difficulties due to the improba- 

 bility of any crossing over between two genes which are very 

 close together, and it is also subject to the criticism that the 

 crossing over may itself be inhibited by an accessory factor. 

 However, it is our only test and its findings must be taken at 

 their face value. 



In the particular test applied, the presence or absence of 

 crossing over in F2 of crosses between ultra-bar and full, a few 

 individuals appeared which came within the facet range of bar. 

 The untested ones might of course have been bar. It is un- 

 fortunate that absolute certainty was not obtained because of 

 the interest in progressive changes of the genes. Most of the 

 progressive changes in characters observed in Drosophila are 

 due to accessory factors and not to changes in a single gene. 



The presence of three genes at the same locus made possible 

 an attempt to see if ultra-bar bears a direct quantitative relation 

 to bar. If this is true, it might be considered as merely two or 

 more bar units closely held together. The values for full, full 

 X bar, bar, ultra-bar X full, ultra-bar X bar, and ultra-bar fe- 

 males could in that case be represented as resulting from differ- 

 ent amounts of a common inhibitor of facet number and there 

 should be a consistent relation in the facet values. This, how- 

 ever, is not true, and it seems probable therefore that the change 

 ■from bar to ultra-bar is specific and not merely a quantitative 

 intensification of the bar factor. 



