CHAPTER XIV 



SEX-CHROMOSOMES AND SEX-LINKED 

 INHERITANCE 



The discovery that the female in certain species of 

 animals has two X-chromosomes and the male has only one 

 X-chromosome, either with a Z-chromosome in addition 

 (Stevens) or without the Y (Wilson), established a view 

 first suggested by McClung that the difference between 

 the sexes is connected with the distribution of particular 

 chromosomes. Two interpretations of the facts have been 

 proposed: The first, and most obvious one, was that the 

 presence of two sex-chromosomes (XX), in connection 

 with the rest of the cell complex, causes a female to 

 develop ; while only one sex-chromosome (X) in connection 

 with the rest of the cell causes a male to develop ; the sec- 

 ond interpretation was that of XX and X are merely 

 indices of sex, i.e., that the sex-chromosomes follow sex 

 and do not determine sex. 



It is now possible to show that sex follows the chromo- 

 somes and not the reverse, because if a "female produc- 

 ing'' sperm (X) fertilizes an egg without an X (as excep- 

 tionally occurs) an XO individual is produced that is a 

 male, whereas if this same sperm had fertilized an egg 

 with an X, giving an XX individual, a female would be 

 the result. Conversely when a "male producing" Y- 

 sperm fertilizes an egg with two X's (as exceptionally 

 occurs) an individual is produced that is a female, despite 

 the presence in her of a Z-chromosome. 



The Sex-Chromosome 

 It will be convenient to treat the XX-XF type of com- 

 bination first. I shall follow the usual custom of calling 

 both X and Y sex-chromosomes. 



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