221 



THE THEORY OF THE GENE 



chromosomes, and that crossing-over between these chro- 

 mosomes may take place. 



For example, the character, white body color, is sex- 

 linked in inheritance. Its allelomorph is red body color. 

 When a pure white female was mated to a pnre red male, 

 the sons and daughters (F ± ) were all red. When these 

 were inbred they gave, 



Red 2 Red $ White ? White $ 

 41 76 43 



Assuming that the genes for white are carried in the two 

 X-chromosomes of the female and designating such a 

 chromosome by X w (Fig. 129), and assuming that the 



white Q 



x^x" 



b 



H 



re 



dc^ 



X Y 



germ cells 



F a 



Fig. 129. 



Diagram showing the inheritance of a character whose gene is 

 carried by the ' ' Y-chromosome, ' ' as well as by the X-chromosome. 



gene for red is carried both by the X and the Y of the 

 male, and designating these by X r and Y r , the cross given 

 above works out on the XX-XY formula as shown in Fig. 

 129. If red (r) dominates white (w), both sons and daugh- 



