HERITAGE OF THE INDIVIDUAL 295 



Color-blindness thus serves to illustrate the association 

 of genes of different characters on the same chromosome and 

 the association later of their respective characters in the 

 adult. But the presence of separate genes on the same 

 chromosome by no means indicates that the genes must 

 always be distributed together, for there is considerable 

 evidence that during synapsis genes may reciprocally cross- 



XO XX 



X K9 



XX XO 



9 d 1 



HI 



XX XX XO XO 



9 9 tf d 



FIG. 150. Diagram to show the inheritance of color-blindness from the female. 

 4 color-blind female (shown in black) transmits the character to all of her sons, and to 

 half of her grandsons, and to half of her granddaughters. (After Morgan.) 



over from one synaptic mate to the other and thus become 

 separated from their former gene associates on the same 

 chromosome. This CROSSING-OVER removes the limitations 

 which, at first glance, would seern to confine the possible 

 number of characters capable of independent segregation in 

 Mendelian inheritance to that of the chromosome number, 

 and renders invalid any objections to the universality of 

 Mendelism which are based on the chromosome mechanism 

 as at present understood. And further, the crossing-over 

 gives an opportunity to determine the relative positions of 

 different genes on a chromosome if it is assumed that the 



