206 Basis of Sex Determination 



eggs have one X chromosome with the factor for red. 

 According to the results obtained from cytological 

 studies which will be explained in the next chapter, the 

 females with the chromatin constitution XX will form 

 two types of eggs in equal numbers: namely, eggs with 

 an X and eggs with an X, i. e., all eggs have one X 

 chromosome, but in fifty per cent, of the eggs the X has 

 the factor for red, in fifty per cent, this factor is lack- 

 ing (X). The males having the chromosome constitu- 

 tion XY form two types of spermatozoa, one with an 

 X possessing the factor for red pigment and one, the Y 

 chromosomes, lacking this factor. If inbred the next 

 F 2 generation will give rise to the following four types 

 of offspring: (i) XX, (2) XX, (3) XY, (4) XY, all four 

 types in equal numbers. 



(1) and (2) give females, both red eyed, since both 

 contain a red-factored X chromosome. (3) and (4) 

 give males, (3) giving rise to red-eyed males, since it 

 contains a red-factored X chromosome, (4) producing 

 males with white eyes since this X chromosome is 

 lacking the factor for red eyes. Since all four combina- 

 tions must appear in equal numbers (provided the 

 experimental material is ample enough, which was the 

 case in these experiments), in the Fj generation both 

 males and females should have red eyes and in the F a 

 generation all the females should have red eyes and 

 half of the males should have red, half white eyes. 

 These results were obtained. 



