CROSSING OVER 91 



or more pairs are involved, for, obviously, unless a char- 

 acter that enters the cross comes in with some other known 

 one that is recognizable as such, there is no way of de- 

 termining whether interchange between the homologous 

 chromosomes has taken place. As will be pointed out 

 later, there is every reason to suppose, and practically 

 a demonstration of the fact, that the interchange goes on 

 irrespective of the presence of other genes by which it 

 can be observed. 



Experiments with different pairs of characters show 

 that for each two pairs there is a definite numerical ratio. 

 For instance, if a female fly with yellow wings and white 

 eyes is crossed to a fly with gray wings and red eyes 

 (wild type) the daughters will have gray wings and 

 red eyes (wild type). If the F^ female is back-crossed to 

 a male with yellow wings and white eyes,, four classes of 

 offspring will be produced in the following proportions : 



Non-crossovers Crossovers 



Yellow white Gray red Yellow red Gray white 



49.5 per cent 49.5 per cent 0.5 per cent 0.5 per cent 



99 per cent 1 per cent 



Here, crossing over takes place in only one case out of 

 a hundred. If the characters enter in a different combina- 

 tion, viz., yellow red and gray white, the crossover per- 

 centage is the same as before, viz., 



Non-crossovers Crossovers 



Yellow red Gray white Yellow white Gray red 



49.5 per cent 49.5 per cent 0.5 per cent 0.5 per cent 



99 per cent 1 per cent 



Another combination of white eyes with a different charac- 

 ter shows a different linkage. If a female fly with white 

 eyes and miniature wings is crossed to a male with red 

 eyes and long wings (wild type), the F^ daughters will 

 have red eyes and long wings. If one of these Fj females 



