CHAPTER XXIX 



SEX DETERMINATION 



Certain facts presented in an earlier chapter show that there 

 is a close connection between sex-linked inheritance and sex 

 determination, since only male-determining gametes or only 

 female-determining gametes are able to transmit sex-linked 

 characters in particular crosses. We must now consider more 

 fully the facts and theories of sex determination. In all the 

 higher animals and plants a discontinuous variation occurs 

 as regards sex, every individual being either male or female. 

 The distribution of males and females in successive genera- 

 tions presents many analogies with Mendelian inheritance. 

 This idea occurred to Mendel himself, as is shown in his post- 

 humously published letters. Bateson suggested it independ- 

 ently in 1902, and this idea was more fully elaborated by 

 Castle (1903). The view is now generally accepted that a 

 factor concerned in sex determination is in all the higher 

 animals and plants inherited in accordance with Mendel's 

 law. What in such cases is the distinction between male and 

 female individuals? 



The essential difiference between a female and a male indi- 

 vidual is that one produces eggs, the other sperm. All other 

 differences are secondary and dependent largely upon the 

 differences mentioned. If in the higher animals (birds and 

 mammals) the sex glands {i. e.y the egg-producing and sperm- 

 producing tissues) are removed from the body, the superficial 

 differences between the sexes largely disappear. In insects, 

 however, the secondary sex characters seem to be for the 

 most part uninfluenced by presence or absence of the sex 

 glands. Their differentiation occurs independently, though 

 simultaneously, with that of the sex glands, evidently de- 

 pending on the genetic (chromosome) constitution of the 

 cells in each part of the body. When the constitution of cells 



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