GENETICS PURPLE EYE COLOR DROSOPHILA 303 



does 'white' in the X chromosome; nor, conversely, is there any- 

 other second chromosome eye color so dilute (or morphological 

 change so extreme) as to interfere with the classification of 

 purple on flies possessing both characters. 



The recessiveness of purple seems to be complete and constant, 

 so that there is no chance of confusion between it and the 

 heterozygote. 



The locus of purple on the basis of very extensive data is 6.2 

 units to the right of black, or, referred to star as a base, at 52.7. 

 Purple is therefore practically at the middle of the second 

 chromosome, and is thus useful in combination with mutations 

 whose genes are located anywhere throughout the chromosome. 

 Its closeness to black, the locus which is the primary base in the 

 mapping of the second chromosome and another of the very 

 best characters, furnishes a working distance which is short 

 enough to exclude double crossing over and long enough to avoid 

 the excessive probable errors incident to very small percentages 

 of crossovers. Outside this black purple section the second 

 chromosome is as yet mostly mapped in distances too great — or 

 too small — to handle satisfactorily in special tests. Furthermore, 

 it appears that this purple region is pecuharly sensitive, as is 

 proved by its exceptionally high double crossing over (this 

 paper) by its greater disturbance by age (Bridges, '15; Plough, 

 '17) and by its unique reaction to genetic variations in crossing 

 over (Sturtevant, in press). The explanation of this sensitive- 

 ness is probably that this region is actually near the middle of the 

 chromosome with the spindle fiber attachment, and that this 

 middle region is the last part to undergo synapsis. 



The number of subjects in the genetics of Drosophila toward 

 whose early and continued development pui:ple has contributed 

 is surprisingly large. 



In the field of mutation it gave with vermilion the first case 

 in which 'intensification' or 'disproportionate modification' was 

 recognized and made use of. It was the first of the class of 

 ' dark' eye-color mutations. It has been one of the most popular 

 models in Drosophila for 'mimic' mutations. The most striking 

 'epidemic of mutation' or 'mutating period' was that inaugurated 

 by purple. 



