SEX INHERITANCE 91 



the female. It does not follow from this that 

 there is only one sex factor involved that is car- 

 ried by the sex chromosomes. The symbols used 

 here, viz., XX-XY and WZ-ZZ, are intended 

 primarily for the chromosomes, but also for the 

 sex factors. 



These formulae for the Drosophila type and for 

 the Abraxas type raise the question as to whether 

 the postulated sex factors are identical in the two 

 cases. The employment of different letters for the 

 two types suggests, of course, that the sex factors 

 may be different. And it is true that the two sets 

 of letters are used to avoid an apparent paradox 

 that appears if we use only X and Y in both cases. 

 If this is done, XY on one scheme represents the 

 male and on the other scheme the female. Never- 

 theless, for the present the employment of different 

 letters need not necessarily mean that different 

 factors for sex are present in the two great classes, 

 for these reverse results may be due to the action of 

 the same factor-difference in a different setting. 

 For example, acid may be the color diff(^rentiator in 

 a setting of a certain solution containing it and litmus 

 (with one drop of acid the color being blue, with 

 two drops red) , but in a setting containing Congo 

 red the same differentiator may produce just the 

 opposite effects (one drop red, two droi)s blue). On 

 the other hand, it is conceivable that the setting (lit- 

 mus and acid) may remain the same and yet a reverse 

 result be produced by having a different differentiator 

 — alkali instead of acid. 



