22 HEREDITY IN RELATION TO EUGENICS 



atoisoon, this odd chromosome goes in toto to one of the two 

 daughter cells (Fig. 5). The X chromosomes are commonly 

 regarded as the ''sex-chromosomes," With them are asso- 

 ciated various characters that are either secondary sex 

 characters or *' sex-limited'' characters. Consequently in 

 respect to each and every such character the primordial 

 egg cells are duplex and all the ripe eggs have one sex de- 

 terminer and its associated characters. The primordial 

 male cells are simplex and consequently, after segregation 

 has occurred, the spermatozoa are of two equally numerous 

 kinds — with and without the Sex-determiner, The fertili- 

 zation of a number of eggs by a number of sperm will result, 

 in two equally common conditions — namely -a. fertilized 

 egg, called zygote, that contains two sex determiners — such 

 develops into a female; and a zygote that contains only one 

 sex determiner — such develops into a male. The nature of 

 the germ cells in the germ gland of the future child and of 

 the associated secondary sex-characters thus depend oij 

 which of the two sorts of sperm cells go into the make-up 

 of the zygote. 



Whenever the male parent is characterized by the absence 

 of some character of which the determiner is typically 

 lodged in the sex chromosome a remarkable sort of inherit- 

 ance is to be expected. This is called sex-limited inherit-* 

 ance. The striking feature of this sort of heredity is that 

 the trait appears only in males of the family, is not trans- 

 mitted by them, but is transmitted through normal females 

 of the family. Striking examples of this sort of heredity 

 are considered later in the cases of multiple sclerosis (Fig. 

 64) ; atrophy of optic nerve (Fig. 77) ; color blindness (Fig. 

 88); myopia (Figs, 90, 91); ichthyosis (Figs. 106, 108); 

 muscular atrophy (Fig, 125); and haemophilia (Fig. 134). 



The explanation is the same in all cases. The abnormal 

 condition is due to the absence of a determiner from the 



