164 



Heredity and Environment 



a developed character which is the result of many intrinsic and 

 extrinsic causes. The X chromosome is only one factor in the 

 development of sex but if it is a factor which differs in the case 

 of the two sexes, it is a "sex-determining" factor. 



P 



9 



®o 



Gametes 



? 



6 



Fi 



Fig. 57. Diagram Showing Sex as a Mendfxian Character, the female 

 being homozygous, the male heterozygous for sex. The female forms 

 gametes all of which contain the X chromosome; the male forms two sorts 

 of gametes one-half of which contain the X chromosome and the other 

 half lack it. All possible combinations of these gametes give 2:'2or 1:1 

 ratio of females to males. 



2. Environmental Influence. Alteration of Sex Ratios. — On 

 the other hand there are many observations which seem to indi- 

 cate that the sex ratio may be changed by environmental condi- 

 tions acting before or after fertilization and therefore it has 

 been concluded that sex is determined by extrinsic rather than by 

 intrinsic causes. Many of these observations, as already remarked, 

 are now known to be erroneous or misleading, since they do not 

 prove what they were once supposed to demonstrate. But there 



