CRETINISM 613 



daughters the sex-chromosome containing the recessive 

 factor for color-blindness, but the daughters are not 

 color-blind because they have also received a sex-chromo- 

 some with the dominant normal factor from the mother. 

 Since the human female has two sex-chromosomes all 

 eggs will be alike and always have a sex-chromosome. 

 But on the other hand the male has only one sex-chromo- 

 some and, therefore, produces two kinds of sperms, one 

 kind with the sex-chromosome and the other lacking it. 

 The sons in the family just mentioned receive their 

 single sex-chromosome from their mother and are con- 

 sequently free from any trait of color-blindness. Should 

 the normal-appearing daughters marry normal husbands 

 all their daughters would be able to distinguish colors, but 

 half of them would be hybrid like their mother. Half 

 the sons would receive a " color-blind " sex-chromosome 

 and half a normal one. The first would, of course, be 

 color-blind, like their grandfather, and the second would 

 be strictly normal. 



Cretinism is an abnormal state of the human species 

 due in some cases, at least, to a deficiency of the hormone 

 produced in the thyroid gland in the neck. The individ- 

 ual affected is likely to be dwarfed in stature and dis- 

 torted in features and to have a very low grade intelli- 

 gence. It is known that some forms, at least, of 

 cretinism are heritable. Presumably the hereditary 

 factors directly control the development of the thyroid 

 gland and through it produce the observed effects. It 

 has recently been claimed by a competent biologist that 

 there is evidence indicating that many racial characters 

 depend on hormone action. If this should be substan- 

 tiated by further investigation, these hereditary traits 

 would also be controlled in the same indirect fashion as 

 cretinism. 



