566 



GENERAL CONSIDERATIONS 



toes; length of fingers; tapering fingers (Fig. 389); ear lobes; few hairs on 

 the middle segment of the fingers; mathematical, musical, and artistic 

 abilities; probably memory, and reasoning ability; and many others. 



Some characters seem very unimportant but are inherited as con- 

 stantly as our valuable characters: for instance, the ear lobes or lack of 

 ear lobes; the few hairs or their absence on the middle segment of the 

 fingers; ear pits; a white lock of hair; and numerous others. 



610. Sex Determination. — In all that has hitherto been said of 

 chromosomes reference has been made only to what are called ordinary 

 chromosomes, or autosomes, which exist in pairs. There are, however, 



Fig. 389. — Tapering fingers in father and son. The fingers of mother are nontapering. 



{Photographs by Whitney.) 



other chromosomes, which, because of their connection with the deter- 

 mination of sex, are known as sex chromosomes. 



In the males of certain ipsects there are two sex chromosomes, recog- 

 nized as the rc-chromosome and the ^/-chromosome, the latter often being 

 the smaller; but in the males of other animals there may be only one sex 

 chromosome and this an x-chromosome. In both cases in the female 

 there are two a;-chromosomes. Experiments have shown that it is the 

 a;-chromosome which in some way determines sex. 



Since in chromosome reduction these sex chromosomes must go 

 to either one sex cell or the other (Fig. 390), it follows that in cases in 

 which the male has one a:-chromosome, only half the sperm cells will 

 contain such a chromosome, while, since the female has two, all of the egg 

 cells will contain one. If a sperm cell which contains an x-chromosome 

 unites with the egg cell, then the zygote will contain two x-chromosomes 

 and from it will develop a female; if, on the other hand, the sperm cell 



