THE MENDELIAN PRINCIPLES OF HEREDITY 81 



mental principle of segregation applies to a certain class 

 of characters that in the last chapter I called " sex- 

 linked" characters. 



Diagram 35 (page 64) will recall the mode of trans- 

 mission of one of these characters, viz. white eyes. 



Let us suppose that the determiner for white eyes 

 is carried by the sex chromosome. *The white-.eyed 

 male has one sex chromosome of this kind. This sex 

 chromosome passes into the female-producing spermato- 

 zoon. 



Such a spermatozoon fertilizing an egg of the red- 

 eyed fly gives a female with two sex chromosomes 

 one capable of producing red, one capable of producing 

 white. The presence of one red-producing chromosome 

 suffices to produce a red-eyed individual. 



When the FI female produces her eggs, the two sex 

 chromosomes separate at one of the two maturation 

 divisions. Half of the eggs on an average will contain 

 the "white" sex chromosome, half the "red." There 

 are, then, two classes of eggs. 



When the FI male produces his sperm, there are 

 also two classes of sperm one with the "red" 

 sex chromosome (the female-producing sperm), and 

 one without a sex chromosome (the male-producing 

 sperm) . 



Chance meeting between eggs and sperm will give 

 the classes of individuals that appear in the second filial 

 generation (F 2 ) . It will be observed that the Mendelian 

 ratio of 3 red to 1 white appears here also. Segregation 

 gives this result. 



The explanation that has just been given rests on 

 the assumption that the mechanism that brings about 



