INHERITANCE 307 



periodically as in the Oyster. Such marked departures from typical 

 sex differentiation serve to emphasize that the final establishment 

 of sex is indeed a resultant of many complex factors. (Fig. 192.) 



2. Linkage 



Since the basic mechanism that regulates sex is the same 

 as that which determines the distribution of other characters 



<@> XY XX 



? 6 S 



F, <@> <@> w wy 



f, <@> @ <©^> <§> XX XX XY XY 



$ 9 d d 2 5 cf <? 



Fig. 194. — Diagram to show the inheritance of color-blindness from the 

 male. A color-blind man (shown in black) transmits the character through his 

 daughters (carriers) to half of his grandsons. X indicates the X chromosome 

 with the gene for color-blindness. 



in inheritance, it might be supposed that the genes of other 

 characters as well are carried in the X chromosome. As 

 a matter of fact the behavior in inheritance of certain characters is 

 such that it can only reasonably be explained on this assumption. 

 Accordingly such characters are known as sex-linked. This brings 

 us again to the point at which we digressed to consider sex — the 

 discussion of genes associated in the same chromosome. 



The best known examples of sex-linked characters in Man are 

 color-blindness in which the affected individual is unable to 

 distinguish red from green, and hemophilia in which the individ- 

 ual's blood has little tendency to clot so that bleeding from even 

 a slight wound may be serious. Both abnormalities have long been 

 known to be inheritable, and in the same peculiar criss-cross way. 

 Thus color-blindness is usually transmitted from a color-blind man 



