Basis of Sex Determination 205 



periment had been carried out with them, instead of 

 with white-eyed flies, the same proportions of normal 

 and colour-blind would have been found: namely, nor- 

 mal colour vision in the F x generation, in both males and 

 females, and half of the males of the F 2 generation 

 colour-blind, the other half and all the females with 

 normal vision. Of course, in man, intermarriage 

 between two different F x strains would have been re- 

 quired in place of the inbreeding of the F! generation, 

 which took place in Morgan's experiments. Morgan 

 interprets his experiments as follows. The normal red- 

 eyed Drosophila has one kind of eggs, each possessing 

 one X chromosome. This X chromosome has also the 

 factor for the development of red-eye pigment. The 

 white-eyed male has two kinds of spermatozoa, one 

 with an X chromosome, the other with a Y chromosome, 

 both lacking the factor for red-eye pigment. If we 

 designate the X chromosome with the factor for red- 

 eye pigment by X and the X and Y chromosomes 

 lacking the factor for redness with X and Y the follow- 

 ing combinations must result if we cross a normal red- 

 eyed female with a white-eyed male: 



Eggs Sperm Result 



X X XX red-eyed female 



X Y XY red-eyed male 



It is obvious that all the offspring of the first genera- 

 tion (the F j generation) must be red eyed, since all the 



