Inheritance of Physical and Mental Traits 287 



to a neuropathic strain. Practically it might be well to 

 segregate such persons during the reproductive period for 

 one generation. Then the crop of defectives will be reduced 

 to practically nothing. 



I cannot close without referring to a remarkable method 

 of inheritance of human traits, namely, the sex limited. 

 As everyone knows, there are certain traits, such as facial 

 hair, which are associated with one sex; and a tendency 

 to heavy growth of beard may be transmitted by a mother's 

 germ cells to her son. In this case the determiner for heavy 

 beard does not develop JCTTL, 



in the female, but only (S/rQD 



in the male, under the |_ 



stimulus, as it were, 

 of the testicular se- 

 cretions; or perhaps c " 

 in the absence of an * * *** <* * 

 inhibiting enzyme **^ f famUy with c lor ' 

 secreted by the ovary. 



But in another class of cases the inheritance is most 

 complex. Thus usually only males are color-blind, but 

 they do not transmit their condition to their sons. On 

 the other hand, the normal women of this strain will have 

 color-blind sons (Fig. 97). 



This has been a great mystery, but thanks to the recent 

 studies in sex chromosomes by Wilson, Morgan, and others, 

 it is a mystery no longer. It is explained by one fact and 

 one hypothesis. The fact is that the male has only one 

 sex chromosome, while the female has two. The hypothesis 

 is that a factor for distinguishing colors is lacking in the 

 affected male and is lost out of the single sex chromosome 

 of such a male. Now the consequence of these two prin- 

 ciples can be seen easily. Let the striated disk (S, Fig. 98) 



