58 LINKAGE 



show in a most striking way that in the previous ex- 

 periment with yellow and white, these factors hold 

 together so strongly from generation to generation, 

 not because of any innate relation between these 

 characters, but simply because they started together 

 in the same chromosome. 



In the case of yellow and white just given the 

 linkage between the two factors is very strong in the 

 sense just defined, that is, they tend in a high degree 

 to preserve whichever combination they have. Other 

 factors show a different strength of linkage. For 

 example, if a female with white eyes and miniature 

 wings is bred to a wild male, and then the Fi females 

 (red, long) are backcrossed to white miniature males 

 they will give the following classes of offspring. 



Non-crossovers Crossovers 



White miniature Red long White long Red miniature 



33.5 per cent. 33.5 per cent. 16-5 per cent. 16.5 per cent. 



67 per cent. 33 per cent. 



The two large classes, white miniature and red long, 

 correspond to the combinations that entered. The 

 two smaller classes are the crossover combinations. 

 Crossing over, therefore, takes place in 33 per cent, 

 of cases. 



Another combination gives a still greater amount 

 of crossing over : the linkage may be said to be weaker. 

 If a white eyed female is bred to a bar male (bar is 

 a dominant mutation), and if the Fi females (red 

 bar eyed) are bred to the double recessive (white 

 round eyed) sons, the following classes appear: 



