SEX-RATIO IN DROSOPHILA AMPELOPHILA. 369 



Another indication of the presence of a lethal factor is the fact 

 that in generation 4, when he used female 5 which gave a normal 

 ratio, as mother of the next generation, all of her daughters also 

 threw normal ratios. This female did not carry the lethal 

 factor and none of her offspring showed abnormal ratios. 



A third indication of a lethal factor is the fact that Moenkhaus 

 found in crossing his male and female strains that the female is 

 almost wholly responsible for the transmission of the sex-ratio. 

 For he found if females from a strain possessing a high female 

 ratio be mated with males from a strain possessing a low female 

 ratio or vice versa, the offspring will show a sex-ratio which is 

 wholly or very near that of the strain from which the females 

 were taken. If his female strain carried a lethal factor and if 

 his male strain was a normal strain, as it appears to be, the above 

 result would be the expected one. For, if a lethal bearing female 

 is mated to a normal male, she will transmit the lethal factor to 

 half her daughters but a male from the lethal strain cannot 

 transmit the lethal factor because all of the lethal bearing males 

 die. Therefore the end result would be as Moenkhaus found, 

 that the offspring will show sex-ratios like that of the strains 

 from which the females came. 



Of course, the only sure way of testing for a sex-linked lethal 

 factor is to cross the suspected female to a male carrying another 

 sex-linked factor and the resulting Fo will be characterized by a 

 deficiency in the class of normal males. So it is not possible to 

 determine beyond doubt whether the unusual ratios in Moenk- 

 haus's female strain were due to a lethal factor. But since 

 several lethals have already been found and all of his data sub- 

 stantiate this assumption, it seems that this explanation is prob- 

 ably the correct one. 



DISCUSSION. 



The writer's three attempts to modify by selection, the sex- 

 ratio in the fruit fly have resulted in no clear-cut evidence that 

 it was modifiable. In each experiment, the work covered more 

 generations and many more individuals than the work of Moenk- 

 haus. Since Moenkhaus's results could not be obtained in any 

 one of the three experiments, it leads one to question Moenk- 

 haus's conclusions or, at least, to question the general application 



