INTRODUCTORY. 



21 



The sex-linked factors of Drosophila. 



the zero, with white at i, and with bifid at 6. If we know the cross- 

 over values given by a new mutant with any two mutants of the same 

 chromosome whose positions are already determined, then we can 

 locate the new factor with accuracy, and be able to predict the cross- 

 over value which the new factor will give with any other factor whose 

 position is plotted. 



The factors are located preferably by short distances (i. e., by those 

 cases in which the amount of crossing-over is small), because when the 

 amount of crossing-over is large a correction must be made for double 

 crossing-over, and the correction can be best found through breaking up 

 the long distances into short ones, by using intermediate points. 



Conversely, when a long distance is indicated on the chromosome 

 diagram, the actual cross-over value found by experiment (i. e., the 



