308 CHARLES ZELENY 



Mutant D. In bottle no. 196.2 of the ultra-bar stock there 

 appeared on March 20, 1918, both males and females with a 

 facet number coming within the range of bar of the low selection 

 line. When this particular mass culture was started on February 

 2, 1918, all the individuals were ultra-bar. The number of indi- 

 viduals with the new character makes it probable that the muta- 

 tion occurred at least two generations before the first observa- 

 tion was made. A 62-facet and two 99-facet males were mated 

 separately with full-eyed wild females and gave heterozygotes 

 which differed from those of ultra-bar X full and also of bar X 

 full. The mean value in about five hundred flies as determined 

 by an estimate without counts is very distinctly between the 

 other two types of heterozygotes, and the range is extended so 

 as to overlap their ranges. This extension of the range indicates 

 the probable presence of accessory factors, but its limits make 

 it seem that the new form is a mutation of ultra-bar in the di- 

 rection of bar which is not exactly like bar. 



Other cases that may be considered as mutations are given in 

 the section on the determination of the locus of ultra-bar (p. 

 311). 



The frequency of occurrence of mutations in ultra-bar is about 

 the same as that in bar, and the stability of the two stocks may 

 be considered to be of about the same order. 



THE CHANGE IN DOMINANCE 



Even more striking than the difference in facet number be- 

 tween bar and ultra-bar is the difference in dominance. A hetero- 

 zygote between ultra-bar and full comes very close to ultra-bar, 

 while the heterozygote between bar and full is closer to full than 

 to bar (fig. 1 and table 7) . For purposes of comparison, the fol- 

 lowing method of determining the coefficient of dominance has 

 been devised. Let A and A' be the two members of an allelo- 

 morphic pair of factors, then the coefficient of dominance 



AA — AA' 



Cd = 7—: X 100 in which AA and A'A' are the mean 



AA - A'A' 



values, respectively, of the homozygous stocks in factorial units 



and AA' is the mean value of the heterozygous individuals. 



