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 ch-romosomes 

 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 Drosophila 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 \ 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 Fj flies are grey normals as befor'e. On 



