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 F, 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 F, females (red 
bar eyed) are bred to the double recessive (white 
round eyed) sons, the following classes appear: 
