270 



THE SECOND-CHROMOSOME GROUP 



unfavorable conditions could be obtained with streak, especially as 

 long as the calculations are restricted to the flies which actually show 

 the streak character above a certain definite grade of development. 

 Aback-cross was therefore undertaken between star and streak, between 

 streak and dachs, and between dachs and purple all in the same 

 experiment. The advantage of purple is that its locus is securely 

 mapped in the main body of loci, and its presence serves as the con- 



TABLE 122. Summary of all cross-over data involving star. 



