142 MENDELISM chap. 



be more easily followed by means of a definite 

 example, and for this purpose we may take the 

 behaviour of the factors for grey body - colour 

 and for normal, as opposed to vestigial wings, 

 in the F^ 9 DrosopJiila discussed above. The 

 factors are found in Chromosome II., and it is 

 supposed that they are both located towards the 

 middle of the chromosome, and separated from one 

 another by about \ of the length of the chromosome 

 as indicated in Fig. 42. Let us suppose that 

 at a certain stage preceding the formation of the 

 gametes, when the two members of the pair 

 come to be in intimate contact along their whole 

 length, they tend to cross over one another as in 

 Fig. 42. Further, let us suppose that they fuse 

 at the point where they cross over one another, and 

 that when they subsequently separate they exchange 

 that portion of the chromosome which lies beyond 

 the point of junction. The result of this will be that 

 our pair of chromosomes will now be constituted in 

 the manner indicated in Fig. 42. 



But as the point of junction is here beyond that 

 portion of the chromosome which lies between G 

 and TV, the association between these two factors 

 will not be disturbed. If, however, the point of 

 junction is between G and A^, as in Fig. 42 C, 

 the separation of the chromosomes will bring about a 

 separation between G and N. Instead of being left 

 with a grey normal and a black vestigial chromo- 

 some, we shall now have a grey vestigial and a 

 black normal one. The process of " crossing- 

 over," as it is called, results in the production of 

 gametes unlike those of the two parental types which 



