Sex Determination {II) 



107 



FIGURE 14-6. Head shapes in human sperm. Round-headed sperm are reported 

 to be smaller and more numerous than oval-headed sperm, suggesting these carry 

 the Y and X chromosomes, respectively. {Courtesy of L. B. Shettles.) 



ratio." Because of this gene, almost all Y 

 chromosomes degenerate during meiosis and 

 almost all sperm carry an X. In the second 

 case, the females were the important factor, 

 for it was found that these females transmit 

 a spirochaete microorganism to their offspring 

 through their eggs. Such females, when mated 



to normal males, produce zygotes which in- 

 itiate development. But very early in devel- 

 opment the spirochaete kills XY individuals, 

 so that almost all survivors are female. 



Finally, when one talks about sex deter- 

 mination the question often comes up: Can 

 sex be predetermined? In theory the answer 



