SEX AND GENES 



201 



site poles (Fig. 113). One daughter cell gets the X, the 

 other the Y. At the other maturation division each splits 

 into daughter chromosomes. The outcome is four cells 

 that later become spermatozoa; two contain an X-chro- 

 mosome, two contain a Y-chromosome. 



Any egg fertilized by an X-sperm (Fig. 114) gives rise 

 to a female that has two X's. Any egg that is fertilized 



9 



cf 



Diploid Nuclei XX 



Gametes 

 Fertilization 



XY 



Zugotes 



Fig. 114. 



Diagram illustrating the XX-XY type of mechanism in sex deter- 

 mination. 



by a Y-sperm gives rise to a male. Since the chances are 

 equal that an egg will be fertilized by one or the other 

 kind of sperm, the expectation is that half the offspring 

 will be female and half will be male. 



Given such a mechanism, certain kinds of inheritance 

 are explicable, some of them including ratios that do not 

 appear, at first sight, to conform to Mendel 's 3 to 1 ratio. 

 On closer scrutiny, however, the apparently exceptional 

 ratios are found to furnish confirmation of Mendel 's first 

 law. For instance, if a white-eyed female of Drosophila 

 is bred to a red-eyed male, the female offspring are 

 red-eyed and the sons are white-eyed (Fig. 115). The 



