SEX AND GENES 



205 



larger X (Fig. 117). He interprets these two as an XY 

 pair. If so, there are 48 chromosomes in each sex, but in 

 the male those of one pair, the sex-chromosomes, are 

 unequal in size. 



More recently still Oguma, who confirms de Winiwar- 

 ter's numbers, finds no Y-chromosome in the male. 



The genetic evidence for man is quite clear. The in- 

 heritance of haemophilia (or bleeding), of color blind- 



9 



Diploid Nuclei VZ 



Gametes W 



Fertilization 



Zygotes WZ 



Fig. 118. 

 Diagram illustrating the WZ-ZZ type of mechanism in sex deter- 

 mination. 



ness, and of two or three other human characters, follow 

 in their inheritance the same method of transmission seen 

 in the white-eyed flies. 



The following groups of animals belong to the XX-XY 

 type or to a modification of this type, vie., the XX-XO 

 type, in which means the absence of Y or no X. Several 

 mammals in addition to man have been reported to have 

 this mechanism — the horse and the opossum, and pos- 

 sibly the guinea pig. It is probable that the Amphibia 

 also belong here, as well as teleostean fish. Most of the 



