THE MECHANISM OF HEREDITY 13 



and the parts have reunited at some point between the factors in 

 question. This is shown in the next diagram (Fig. 5) where the 

 crossing over is represented. The number of times that this occurs 

 will be measured by the number of the two other classes of males, the 

 gray-red and the yellow-white. The actual results are : 



Expressed in percentages the crossing over takes place in 1.3 per 

 cent, of the total number of males produced. 



The hypothesis of independent assortment for two pairs of characters 

 calls for equal numbers in each of the four classes of males in the cross 

 just given. The numbers show how far the actual results depart from 

 this expectation. 



There is one further point here that demands consideration. If the 

 factors lie in a linear order in the chromosome as the hypothesis requires 

 it is evident that the nearer together two factors lie the smaller will be 

 the chance that a twist occurs between them. Consequently the fre- 

 quency of crossing over can be taken as a measure of the distance of the 

 factors from each other in the chromosome. On this basis the position 

 of these factors in the chromosomes has been calculated. I shall return 

 to this point later. 



When three pairs of sex-linked characters are involved the result is 

 essentially the same, but the possibility of another class of individuals, 

 viz., those produced by double crossing over offers certain relations of 

 peculiar interest. If a female fly with the characters yellow body color, 

 white eyes and miniature wings is mated to a wild male with gray body 

 color, red eyes, and long wings the daughters are like the father and the 



YWM 1361 Y W M. 



GRL 2089 q r l~ 



YRL 23 Y ^W __M 



W M 



* 



GWM 17 

 YWL 817 



GRM 887 



YRM Y W M 



owl 6 — g^ r 



Fig. 6. Diagram illustrating the, two sex chromosomes in the F x female derived 

 from a YWM female and a GRL male. The first (upper) pair of lines show the 

 chromosomes without crossing over. To the left the number of the F 2 flies of the 

 composition of the two chromosomes respectively are represented. The second pair of 

 lines illustrate the results of "single crossing-over' between YW and GR. The 

 third pair of lines represent the " single crossing-over " between WM and RL. The 

 fourth pair of lines represent the results of " double crossing-over." 



