218 



THE SECOND-CHROMOSOME GROUP 



Three F2 mass-cultures were raised (table 62), from which a rough 

 calculation of the amount of crossing-over was made, as follows: The 

 class dachs black is a non-cross-over simple class. Each of the two 

 complementary classes dachs and black is a cross-over class. The wild- 

 type class is composite, including the wild-type non-cross-over class 

 complementary to the dachs black class, and also all the flies coming 



& 



from that half of the sperm which carried the wild-type chromosome, 

 for which reason it can not be used in a simple calculation. The aver- 

 age of the dachs and the black classes ( — — — = 82 ] was used as the 



cross-over class to compare with the 256 dachs black non-cross-overs. 

 The percentage of crossing-over thus calculated was 24.3, and this was 

 temporarily taken to be the dachs black distance. The position of 

 dachs was assumed to be to the right of black, since all the other 

 mutants thus far found were in that direction. 



A male test was made by back-crossing an Fi male by a dachs black 

 female (table 62). There were no cross-overs in a total of 170 flies. 



