132 



PHYSIOLOGIC \l. GEh ETICS 



The question remains whether or not some of the detailed data 

 available point in the same direction. The most extensive set 



of tacts known thus far relates to the eye-pigmen1 series in 

 Drosophila at the white locus of the X-chromosome. In Table 

 13, the phenotypes are arranged serially from dark to light for 

 t he female. 



Table 13 



These data show the presence of all three possibilities of simplex 

 effects in one series of alleles. It is remarkable that a chromosome 

 rearrangement exists, the so-called pale translocation, which 

 affects the eye colors of this series. Those that are lighter in the 

 male are still lighter, and those that are darker are still darker. 

 The pale translocation has therefore the same effect as the sexual 

 difference of the system of development. Other eye-color modi- 

 fying genes with comparable effect are known, e.g., pinkish, 

 which dilutes male more than female eosin (Stern, 1929a). 

 These facts point to the same type of explanation as before: the 

 effect of the modifiers certainly works through a set of reactions 

 different from that controlled by the white series. Here a change 

 in the interplay of different developmental reactions is patent. 

 As the effect is of the same type as the sex effect, we might 

 reasonably attribute it to the same causes. It must be agreed, 

 though, that it will be difficult to devise a detailed form of such 

 a system to cover also the irregularity of behavior within the 

 series. 



