1 84 



Heredity and Environment 



Eyes 



<o> <o> <o 



c? 



Chromosomes 

 X© IX Parents 



I XI I Gametes 



XX 9 J 



2X1 



9 



X v X 



2S © 



XX 



*1 



.Fi 



Gametes 



6 



XO F* 



Fig. 63. Diagram of Inheritance of Color Blindness through the 

 Male. A color blind male (here black) transmits his defect to his grand- 

 sons only. The corresponding distribution of the sex chromosomes is 

 shown on the right, the one carrying the factor for color blindness being 

 black. (After Morgan.) 



panying diagrams (Figs. 63, 64) illustrate the method of inheri- 

 tance of color blindness. As in the previous diagrams jf. repre- 

 sents the sex-determiner, © its absence, and X the sex-determiner 

 which carries the factor for color blindness. 



It will be seen that a color blind father and a normal mother 

 have only normal children, but the father transmits to his daugh- 

 ters and not to his sons the sex-determiner which carries the 

 factor for color blindness. But since color blindness does not 

 develop in females unless it is duplex (i.e. comes from both 

 father and mother) whereas it develops in males if it is simplex 

 (i.e. comes from either parent) all the daughters of a color blind 

 father and normal mother will appear normal although carrying 

 one determiner for color blindness, while all the sons will be nor- 

 mal because they carry no determiner for color blindness. But 

 these daughters transmit to one-half of their children the single 



