GENETICS PURPLE EYE COLOR DROSOPHILA 285 



THE VARIATION OF CROSSING OVER WITH AGE 



The reason for raising second broods in these experiments' was 

 to obtain more offspring from each female and thus secure a 

 more trustworthy index of the genetic behavior of each indi- 

 vidual. This practice was extended to all the work at this time, 

 and was continued until a comparison of the crossover values 

 of the first and second broods brought out a remarkable relation 

 in the cases involving the second chromosome. There was found 

 to be a change in the amount of crossing over so that both in the 

 totals for each experiment and in a great majority of the indi- 

 vidual cultures the crossover value had fallen significantly. 

 Equally surprising was the fact that there was no such change 

 in the case of the first chromosome, and this added another 

 proof of the distinctness of our hnkage groups, that is, of the 

 individuality of the chromosomes involved. The first case in 

 which this decrease for the second chromosome was clearly seen 

 was that of the back cross tests of the purple vestigial linkage 

 given in tables 7 and 8. Of the eight females whose tests are 

 given in table 7 seven showed a decrease in the percentage of 

 crossing over and only one (F) showed an increase, which, how- 

 ever, was smaller in amount than the smallest of the decreases. 

 In the complementary case 'repulsion' (table 8) all six females 

 showed a decided drop. The totals likewise reflected this same 

 change; the decreases were 2.9 and 4.9 units, respectively. The 

 crossover value calculated from the balanced second broods was 

 8, a decrease of 3.8 units, or, compared with the corresponding 

 crossover value (11.8) from the balanced first broods, a 32 per 

 cent decrease from the normal amount. Many other experi- 

 ments have confirmed the fact of change in crossing-over fre- 

 quency with the age of the mother, and some slight analysis has 

 been made of the mechanism behind the results (Bridges '15). 



THE LOCUS OF PURPLE— A TWO-POINT MAP 



The repetition of the purple vestigial back crosses was not 

 carried out until the summer of 1913; meanwhile considerable 

 progress had been made with the mapping of the second chromo- 



