io8 MENDELISM chap. 



Hence, when mated with a white-eyed female, all his 

 daughters must be reds, since every individual re- 

 ceiving an X chromosome from the father must be 

 a female. So also all the sons from such a cross 

 must be white-eyed. For neither the X chromo- 

 some which they receive from the mother, nor the 

 Y chromosome from the father carries the red 

 factor. 



The theory that the manifestation of sex depends 

 upon definite chromosomes — that 2 X chromosomes 

 in the zygote spells a female and 1 X a male — has 

 recently been confirmed in a striking manner by 

 some beautiful experiments of Bridges. He discovered 

 in Drosophila a case where the eggs of the female 

 were formed in an abnormal manner. Though 

 most of them contained the normal 4 chromosomes, 

 of which one was an X chromosome, a few were 

 found which contained 5 chromosomes, and a few 

 in which there were only 3. The former contained 

 a representative of each of the three non-sex chromo- 

 somes together with 2 X chromosomes ; the latter 

 were without an X chromosome, containing repre- 

 sentatives of the 3 non - sex chromosomes only. 

 Since the sperms of a male are always of two kinds, » 

 X and Y, these two new classes of eggs {XX and (9) 

 led to the possibility of four new classes of zygote, 

 viz. XXX, XX y, OX, OY. Of these the two 

 kinds XXX and OY were not realised, and it is 

 presumed that flies of such constitution are incapable 

 of development. But the XX Y and the OX zygotes 

 developed into adult flies. The former were females 

 and the latter were males. The females were healthy 

 and normal and produced offspring. So long as 



