198 THE SECOND-CHROMOSOME GROUP 



without confusion between them. Purple is not so dilute that it would 

 interfere with the classification of other eye-colors, as does "white" 

 in the X chromosome; nor conversely is there any other second-chro- 

 mosome eye-color so dilute, or morphological change so extreme, as to 

 interfere with the classification of purple in flies possessing both 

 characters. The recessiveness of purple seems to be complete and 

 constant, so that the chance of confusion between it and the heterozy- 

 gote is nil. 



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 

 not far from the middle of the second chromosome as mapped, and is 

 thus within striking distance of mutant loci near either end or anywhere 

 throughout the chromosome. Its closeness to black (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 

 cross-overs. 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 peculiarly sensitive, as is proved by its exceptionally high 

 double crossing-over (this paper), by its greater disturbance by age 

 (Bridges, 1915; Plough, 1917), by temperature (Plough, 1917), and by 

 its unique reaction to genetic variations in crossing-over (Sturtevant, 

 Part III of this volume) . The explanation of this sensitiveness is prob- 

 ably 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 purple 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. 



In the early experiments involving purple several other mutations 

 arose, probably the most interesting of which was "extra bristles," 

 which led to the study made by MacDowell on the effect of selection 

 on bristle number. 



In the attack upon the problem of "inviability" purple entered into 

 the first experiment planned to include the balancing of inviability by 

 complementary crosses. This practice was extended to involve three 



