No. 57G] 



VAMATIOX IX DEOSonilLA 



723 



expected ratios, or classes, or in some other peculiarity of 

 genetic behavior. In the case before us, we found that 

 the F 1 generation consisted of at least two types; viz., 

 Beaded and not-Beaded flies. These were shown to differ 

 genetically. To obtain such a result must mean that at 

 least one of the parents was heterozygous in at least one 

 gene. This result is however a fairly constant one; and 

 by virtue of the long-continued inbreeding of the Beaded 

 stock this heterozygosity must surely have been weeded 

 out before now if there were no serious hindrance to 

 homozygosity. The classic example of this sort of effect 

 is that of the yellow mice. 



But the development of Beaded wings can not be 

 brought about by the action of a single lethal gene, for if 

 this were true it would be impossible to obtain a stock 

 of Beaded flies that would breed true, and yet such a 

 stock, as has already been said, is the one from which 

 these very crosses derive their Headed ancestors. There 

 must therefore be at least one pair of allelomorphs of 

 which one member is effective in producing Beaded wings, 

 and can exist in the homozygous condition and possibly 

 also another pair of allelomorphs of which one member is 

 a recessive lethal gene. We can explain many of the 

 facts so far obtained on the supposition, that there are 

 these two independently Mendelizing pairs of allelo- 

 morphs concerned in the production of Beaded wings. 

 The pair containing the lethal gene we will call L (nor- 

 mal) and 1 (lethal) ; and the other pair B' (Beaded) and 

 b' (normal). The occurrence of the two genes B' and 1 

 in one individual usually causes such an individual to 

 have Beaded wings, though Beaded-winged flies also 

 occur which do not cany the lethal gene, but are homo- 

 zygous for B'. 



It should be possible then to isolate a stock of Beaded- 

 winged Hies not carrying this lethal factor, I. Such flies 

 should give a much smaller percentage of Headed-winged 

 offspring in the F, generation of a cross with Wild stock 

 (or perhaps none at all, if B' were recessive), than would 

 those flies carrying 1L. Such a stock has not yet been ob- 



