HERITAGE OP THE INDIVIDUAL 



293 



shows that the sex of an individual is usually determined 

 at the time of fertilization, and not subsequently as most of 

 the well-known theories contend. But obviously we must 

 guard against thinking of either the X chromosome or the 

 'sex gene' as 'producing' sex. Sex is a complex character 

 whose full development is undoubtedly conditioned by 'sex 



MatureEgg Oogonium 



sSpermatogon ium 



FIG. 148. Diagram to show the relation of the two classes of sperm in fertilization. 

 The formation of gametes in the male is shown at the left, in the female at the right; 

 fertilization, producing the male or female zygote, in the center. X chromosome in 

 black (After Wilson.) 



hormones/ etc., but since the X chromosome is the differen- 

 tial in the sexes, it is to that extent 'sex-determining.' 



2. Linkage 



Since sex is regulated by an internal mechanism which 

 appears to be the same as that which determines the dis- 

 tribution of characters in Mendelian inheritance, it might be 

 supposed that the genes of other characters as well are 

 carried by 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. Ac- 

 cordingly such characters are known as SEX-LINKED. This 



