WHITE-OCELLI. 



233 



The usefulness of the mutant white-ocelli was not appreciated 

 for some years after its discovery. This neglect was due, in 

 large part, to the fact that the regular examination of flies during 

 this period was carried out by aid of a hand lens only, and the 

 separation of the white from the normal ocelli was difficult 

 because of the small size of the region affected. The later work 

 has been done with a binocular microscope, with special attention 

 to proper illumination and magnification; and under these condi- 

 tions the separation is complete and entirely accurate, though 

 still somewhat slow. 



THE LINKAGE RELATIONS OF WHITE-OCELLI. 



An accurate localization of the gene for white-ocelli was made 

 easy by the use of the two excellent dominant characters dichsete 

 and hairless. The locus of dichaete was near the left end of the 

 map of the third chromosome as then known, being about 13 

 units to the right of sepia, while the locus of hairless was some- 

 what to the right of the middle (about 42 units to the right of 

 sepia and 21 to the left of rough). Spineless white-ocelli males 

 were outcrossed to dichaete hairless females; and the FI dichsete 

 hairless females were back-crossed by spineless white-ocelli males, 

 with the results shown below: 



* Crossovers in the first region, that between dichsete and spineless, are headed 

 by " i," etc. 



The results of this experiment showed that the locus of white- 

 ocelli is to the right of that of hairless by about 6.8 units (a total 

 of 114 crossovers involving region three). The locus of white- 

 ocelli, as thus established, is in what had been the longest un- 

 occupied region of the third chromosome. There had been no 

 workable mutant in the entire distance of about 20 units from 

 ebony (1.5 units to the right of hairless) to rough (21.2 units to 



