GENETICS PURPLE EYE COLOR DROSOPHILA 297 



SPECIAL PROBLEMS INVOLVING PURPLE— AGE VARIATIONS, COIN- 

 CIDENCE, TEMPERATURE VARIATIONS, CROSSOVER MUTATIONS 

 PROGENY TEST FOR CROSSING OVER 



We have already seen how the study of the age variation in 

 crossing over for the second chromosome began with the purple 

 vestigial back cross (p. 283) and was continued and confirmed 

 by the black purple curved triple back cross (p. 294). Some 

 of the early data suggested that the drop in the second broods 

 was followed by a recovery and perhaps even by a rise in later 

 broods (Bridges' '15). 



To gain further light on the course of the variation through- 

 out the life of the fly, a special and extensive experiment was 

 continued through four broods. The entire length of the chromo- 

 some was covered by the loci chosen { 



\+ pr c sp, 



This experiment showed the normal crossover values for the 

 first broods, the usual drop for the second broods, and a slight 

 continued drop for the third and fourth broods. However, the 

 experiment proved inconclusive because of two ill adaptations: 

 the chromosome distances involved were so long (e.g., S pr = 

 52.7) that real changes could be concealed by a concomitant 

 change in double crossing over, and further because the ten-day 

 broods gave only four points on the curve of age variation, each 

 point representing only the net change for a ten-day period, 

 while the real underlying curve may have changed its course 

 so that an unknown part of each ten-day period may have been 

 a fall and the rest a rise. 



The original black purple curved experiment had avoided one 

 of these difficulties in that the black purple distance is so short 

 that there is probably no double crossing over whatever within 

 it. The second difficulty was met by Plough in his similar 

 studies on the temperature variation of linkage by transferring 

 his parents every two days instead of every ten (Plough, '17). 

 As one of his control experiments Plough ran a black purple 

 curved back cross of thirteen pairs, transferring each pair to a 

 fresh culture tube every two days as long as the female lived 

 (table 14, Plough, '17). The plotted curve of the percentages 



THE JOCBNAL OF EXPERIMENTAL ZOOLOGY, VOL. 28, NO. 2 



