164 ROSCOE R. HYDE 



The figures given in this experiment are very small and the 

 source of the sterility doubtful since the controls throw no light 

 upon it. It is to be noted, however, that both classes of females 

 are affected in family No. 2 b. In the case of the white-eyed 

 females more than 50 per cent are affected. 



To analyze the evidence • from the foregoing experiments let 

 it be assumed that a gene responsible for the functioning of the 

 oviduct is carried by the X-chromosome and that in the sterile 

 individuals this gene has changed so that the oviduct fails to 

 function. In analogous cases when the female is affected both 

 X's are affected but she remains normal so long as only one X 

 is affected. When the affected X goes into the males they should 

 not be sterile because the male has no oviduct. But such a male 

 will be able to transmit the defect. To follow the argument, let 

 us make three assumptions. 



a. If the defect in this experiment came from the white-eyed 

 grandmother then her germinal make-up would be expressed by 

 the formula wwXx, in which the x represents the affected cromo- 

 some. Since the red-eyed male did not contain the defect his 

 formula is RXO. Crossing these two individuals gives: 



w X — wX 

 / RX — O Gametes 



wR.Xx — wRXX — wxO — wXO Fi Red-eyed females and white- 

 eyed males, none of which show sterility. 



If by chance the flies chosen have affected genes, we get : 



w X - R X ^ 



^ Gametes 

 w X — O 



WWXX — wRXx— wxO — RXO Flies of the F2 generation ; only white- 

 eyed females are sterile 



But this does not agree with the facts of the experiment. 



b. If the defect was brought in by the red-eyed grandfather 

 and the female is free from it, then it follows: 



w X — w X ^ , 

 _, „ Gametes 



R X — O 



wRXx — wXO Fi Red-eyed females, white-eyed males; all fertile. 



