22 HEREDITY IN RELATION TO EUGENICS | 



atozoon, this odd chromosome goes in toio 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 i 

 characters or ''sex-limited" characters. Consequently in j 

 respect to each and every such character the primordial 

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

 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 fertiU- 

 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 i 

 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 on 

 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 



