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THE AMEBIC AN NATURALIST [Vol. XLVIII 



TABLE VIII 



Normal Females from Family 2, Generation 2, Back-crossed to Wild 

 Males, Showing Percentages of Beaded-winged Offspring 



times has as large a percentage of Beaded-winged off- 

 spring when mated to Wild, as does a fly direct from pure 

 Beaded stock when mated to Wild, though a comparison 

 of Tables I and V shows that this is not the usual occur- 

 rence. This suggests at once the action of a lethal gene 

 (Morgan, 19126). Morgan has shown that in a certain 

 stock of Drosophila there are twice as many females as 

 males in the offspring of one half the females. No matter 

 to what male such a female be mated, her daughters are 

 twice as numerous as her sons, and one half of her 

 daughters also repeat this phenomenon, and one half of 

 the daughters of these again. This fact finds its explana- 

 tion in the assumption that there is in one of the sex- 

 chromosomes of such females a gene which prevents the 

 development of any male which gets it. 



Now if such a gene had the power of expressing itself 

 as a dominant in those flies that carried it in the hetero- 

 zygous condition, if, for example, it caused the wings to be 

 Beaded, it would be possible to select such flies at sight, 

 and these flies could then be depended upon to repeat the 

 phenomenon. (Morgan accomplishes the same end by 

 mating such flies to mutants carrying a gene with which 

 the lethal gene shows close linkage, such as that for white- 

 eyes. He then finds that the red-eyed females carry the 

 lethal gene, unless, as rarely happens, a 4 'cross-over" has 

 occurred.) 



Such a sex-linked lethal gene producing a dominant 

 wing character has actually been found to occur in the 

 case of a mutant which arose in the Beaded stock, and 

 which will be discussed later. For the present we must 

 note that if the lethal gene were not associated with sex, 

 its presence could be detected by the absence of certain 



