24 THE THEORY OF THE GENE 



In the preceding illustrations of linkage and crossing- 

 over, that have been given, the genes are represented as 

 lying in a line — like beads on a string. The numerical data 

 from crossing-over show, in fact, that this arrangement is 

 the only one that is consistent with the results obtained, 

 as the following example will serve to illustrate. 



Fig. 20. 



Diagram illustrating the linear order of three sex-linked genes, 



viz., yellow wings, white eyes, bifid wings. 



Suppose that crossing-over between yellow mngs and 

 white eyes occurs in 1.2 per cent of cases. If we then test 

 white with a third member of the same series, such as 

 bifid wings, we find 3.5 per cent of crossing-over (Fig. 

 20). If bifid is in line and on one side of white it is ex- 

 pected to give with yellow 4.7 per cent crossing-over, if on 

 the other side of white it is expected to give 2.3 per cent 

 of crossing-over with yellow. In fact, it gives one of these 

 values, namely, 4.7. We place it, therefore, below white in 

 the diagram. This sort of result is obtained whenever a 

 new character is compared with two other members of 

 the same linkage group. The crossing-over of a new 

 character is found to give, in relation to two other known 

 factors, either the sum or the difference of their respec- 

 tive cross-over values. This is the known relation of 

 points on a line, and is the proof of the linear order of the 

 genes; for no other spatial relation has yet been found 

 that fulfills these conditions. 



