THE ORDER OF THE GENES 



121 



shows that such is the case, for such double crossing over 

 can be detected if enough points in the series are * in- 

 volved '^ to catch all single crossovers. Now, as sho^\^l 

 in Fig. 52, whenever double crossing over takes place 

 between y and B, the two series that result, as marked 

 by their ends alone {y and B), are still y and B. The flies 

 Avill therefore be placed in such classes, which are the non- 

 crossover classes. A numerical increase in this class 

 will decrease the calculated percentage of crossovers. 

 Thus double crossing over by increasing the number of 



y 



B 



B 



Fig. 52. — Scheme illustrating how double crossing over between two distinct genes, y and 

 B, is not recorded, when only y and B are involved. 



apparent non-crossovers, decreases the observed per- 

 centage of crossovers. When enough points are marked 

 along the series to pick up ^11 double crossovers, and 

 these are then referred to the proper single crossover 

 classes, the ^ ' piece-by-piece " per cent, estimate, and 

 the percentage obtained from the cross, are found in 

 complete agreement. 



The amount of double crossing over in Drosophila is so 

 large that the percentage of ^'crossing over'' is rarely or 



