THE MENDELIAN PRINCIPLES OF HEREDITY 83 



red eyes in these cases is typical of all sex-linked in- 

 heritance. In man, for instance, color blindness, so 

 common in males and rare in females, follows the 

 same rules. It appears that hemophilia in man and 

 night-blindness are also examples of sex-linked in- 

 heritance. These cases, as already stated, were formerly 

 included under the term ''sex-limited inheritance,'' that 

 implies that a character is limited to one sex, but we 

 now know that characters such as these may be trans- 

 ferred to the females, hence the term is misleading. 

 Their chief peculiarity is that in transmission they ap- 

 pear as though linked to the factor for sex contained in 

 the sex chromosome, hence I prefer to speak of them as 

 sex-linked characters. 



If our explanation is well founded, each sex-linked 

 character is represented by some substance — some 

 material particle that we call a factor in the sex 

 chromosome. There may be hundreds of such materials 

 present that are essential for the development of sex- 

 linked characters in the organism. 



The sex chromosomes must contain, therefore, a 

 large amount of material that has nothing whatever 

 to do with sex determination ; for the characters in 

 question are not hmited to any particular sex, although 

 in certain combinations they may appear in one sex 

 and not in the other. 



What then, have the sex chromosomes to do with sex ? 

 The answer is that sex, like any other character, is due 

 to some factor or determiner contained in these chro- 

 mosomes. It is a differential factor of such a kind 

 that when present in duplex, as when both sex chromo- 

 somes are present, it turns the scale so that a female 



