Equality of Sexes. 



Experiments oh sex-linked inheritance seem to indi- 

 cate that the X chromosomes are the vehicles for the dis- 

 tribution of the sex factors. In both types where fertiliza- 

 tion occurs the chances are that approximately an equal 

 number of males and females are produced, as shown by the 

 following diagrams: — 



First type. 



O = egg cell with X chromosome 

 = sperm cell with an X chromosome 

 = sperm cell without the X chromosome 

 = Zygote with 2 X chromosomes (female) 

 = Zygote with XY chromosomes (male). 

 S s = male gametes 



S 



s 



A 



a 



Female 

 Gametes 



A 



= 2A and 2a, an equal number of 

 males and females. 



Second Type. 



O = egg cell with a W chromosome 

 o = egg cell with a Z chromosome 

 S = sperm cell with a W chromosome 

 A = Zygote with 2 W chromosomes (male) 

 a = Zygote with WZ chromosomes (female) 

 S S = male gametes 



Female 

 Gametes 



O 



= 2A and 2a, an equal number of 

 females and males. 



This conclusion agrees with statistical evidence which 

 shows an approximately equal number of males and females 

 of human birth. This and other facts already presented 

 in chapter 15 lend support to the assumption that sex 

 is a character dependent upon determiners or factors that 

 act in a Mendelian manner. (See page 111). 



Discuss influence of lethal factors. 



It should be borne in mind that the chromosome theory 

 of sex-determination has not been accepted by many geneti- 

 cists and physiologists. The sex-chromosome has not yet 

 been found in plants, although certain facts recently as- 

 certained are suggestive of its presence. Recent investiga- 

 tions by Hertwig (1912) on frogs' eggs, by Miss King (1912) 

 on toads' eggs, and by Riddle ( 1 9 1 7 ) on pigeons showed that 

 sex can be changed, even controlled, by varying and regula- 

 ting the physiological conditions. Many physiologists 



143 



