208 . GENETICS 



shown in Figure 62 or in type II of Figures 59, 60 

 and 61. 



It will be seen that when a male gamete bearing 

 an X chromosome unites with a female gamete also 

 bearing an x chromosome, the outcome is a fertilized 

 egg containing xx chromosomes. Such an egg is con- 

 sequently homozygous for sex, and will develop into 

 a female individual. In the same way when a male 

 gamete lacking an x chromosome, as half the gametes 

 derived from a heterozygote do, unites with a female 

 gamete bearing an x chromosome, as all gametes 

 from a homozygote must do, then the fertilized egg 

 will be heterozygous, carrying only one x chromosome, 

 and will develop into a male indi\adual. 



The chromatin difference between the two sexes 

 may be qualitative, as Wilson holds, or quantitative, 

 as Morgan assumes, but in either case it seems cer- 

 tain that, with difference in sex, there is invariably 

 associated a definite difference in the character of 

 the chromosomes present in the germ-cells. 



These conclusions have been abundantly con- 

 firmed in various species by a large number of inde- 

 pendent w^orkers, and are now well established as 

 a part of biological science. In fact, it is not at 

 all unusual to find the technical confirmation of the 

 X chromosome theory given as a part of the routine 

 class work in university courses. 



A, 2. Various Forms of X Chromosomes 



The extra chromosome in different species may 

 assume various forms or degrees of complexity. It 



