144 MENDELISM chap. 



separated in this way depends upon the distance 

 which separates them in the chromosomes. The 

 nearer they are together, the less likely are they 

 to become separated : the further apart they are, 

 the more often is separation likely to occur. It 

 is upon the proportion of " crossover " gametes as 

 compared with "non-crossover" gametes that the 

 distances between the factors along the chromosomes 

 have been determined, always on the assumption 

 that the factors are arranged along the chromosome 

 after the fashion of beads along a string. Thus 

 in the particular instance under discussion, the 

 female DrosopJiila formed by the union between a 

 grey normal and a black vestigial gamete, the cross- 

 over gametes, viz. grey vestigial and black normal, 

 form about \ of the total. The inference is that 

 the distance between the grey factor and the normal- 

 wing factor is about -g- of the length of the chromo- 

 some. In the same way the position of other 

 factors in Chromosome II. has been determined 

 with respect to those for grey body-colour and for 

 normal wing. Using this method, the American 

 observers have constructed a map of each of the 

 four chromosomes, showing the precise position 

 along each one at which the various factors occur, 

 and it is claimed that these maps are based upon 

 a very large and fairly consistent body of experi- 

 mental facts. 



The explanation will of course serve equally well 

 to account for what we have previously termed repul- 

 sion, as well as for coupling. When the union between 

 a grey vestigial and a black normal gamete takes 

 place the F^ flies are grey normals as before. On 



