150 



THE SECOND-CHROMOSOME GROUP 



during which time the balloon gene was constantly maintained in 

 heterozygous condition. If the effect of the not-balloon gene always 

 present hi the homologous chromosome were to render the balloon 

 gene less characteristically balloon-producing, then the balloon stock 

 finally extracted from this long-continued heterozygosis should exhibit 

 a lower grade of the balloon character than that shown by the regular 

 stock of balloon which for some 5 years had been kept homozygous by 

 inbreeding. When the average grade of the individuals of a stock 

 freshly extracted from this heterozygous condition was determined and 

 compared with the like grade determined for the homozygous stock, 

 it was found that the difference from normal of the outcrossed type 

 was not less than the difference of the inbred stock. A comparison 

 of the standard deviations of these two stocks showed that there had 

 been no increase in variability on account of the continued heterozy- 

 gosis. These facts together showed that, in an adequately tested case 

 of character variability, contamination of genes was not operative to 

 a detectable degree. 



A summary of the linkage data involving balloon is given in table 16. 



TABLE 16. Summary of data upon linkage of balloon with other second- 

 chromosome loci. 



VESTIGIAL (v g ). 



(Plate 7, figure 2.) 



ORIGIN AND DESCRIPTION OF VESTIGIAL. 



The mutant wing-character now called vestigial was found by Mor- 

 gan (December 1910) in a stock culture of truncate flies (Morgan, 1911). 

 A few flies of both sexes were found which seemed to have tiny scales 

 in place of wings. The size of the vestigial wing in relation to the size 

 of the body, and the characteristic manner in which these wings are 

 held out at right angles to the body instead of lying back above the 

 abdomen, are shown by the figure. The character was at first called 

 "wingless," and this name appeared in the first few publications 



