SEX-LINKED FACTORS IN DROSOPHILA 



51 



M, and the fly hatched from the resulting gamete will be placed 

 in the non-cross-over class, though in reality he represents two 

 cross-overs. In order to see if double crossing over really does 

 occur it is necessary to use three or more sex-linked allelomorphic 

 pairs in the same experiment. Such cases have been reported 

 by Alorgan ('11 d) and JNIorgan and Cattell ('12) for the factors 

 B, CO, and R. They made such crosses as long gray red by 

 miniature yellow white, and long yellow red by miniature gray 

 white, etc. The details and analyses are given in the original 

 papers, and for our present purpose it is only the flies that are 

 available for observations on double crossing over that are of 

 interest. Table 4 gives a graphical representation of what hap- 

 pened in the 10495 cases. 



Double crossing over does then occur, but it is to be noted that 

 the occurrence of the break between B and CO tends to prevent 

 that between CO and R (or vice versa). Thus where B and CO 

 did not separate, the gametic ratio for CO and R was about 

 1 to 2, but in the cases where B and CO did separate it was 

 about 1 to 6.5. 



Three similar cases from my own results, though done on a 

 smaller scale, are given in the table at the end of this paper. 

 The results are represented iij tables 5, 6 and 7. 



TABLE 4 



TABLE 5 



