LETHAL FACTOES IN DROSOPHILA 555 



somes. If it is a crossover it should be found to have the same 

 locus as the factor with the red. 



The white eyed females mated to eosiii males gave white- 

 eosin^ long winged females (with the factor for lethal) : white- 

 eosin miniature females; white eyed miniature males; and a few 

 white eyed long winged males as crossovers. 



The white-eosin long winged females were again mated to 

 eosin miniature males. This mating gave white-eosin long winged 

 females with the lethal factor, eosin miniature females without 

 the lethal factor, eosin long winged females and, white-eosin 

 miniature females, eosin miniature males, white eyed miniature 

 males, eosin long winged males and white long winged males. 

 The single crossovers are the white miniature and the eosin long 

 winged males, while the white long winged males are double 

 crossovers. In table 18 some of the counts are given. 



The crossover value of white with the lethal involved is 15.6 

 and that of the lethal with miniature is 19.9. Therefore the 

 locus of this lethal is at 16.7, that is, 1.1 plus 15.6. The locus 

 of the original lethal was shown to be at 23.7, so that this lethal 

 with a locus at 16.7 must be the new lethal whose advent led to 

 the production of the high sex ratio. The mother of the high 

 sex ratio carried in one of her X chromosomes the original lethal 

 at 23.7 and in the other X chromosome, the one derived from 

 the father, the new lethal at 16.7. 



If this female contained two lethals some of her descendants, 

 should have one and some the other ; and these two kinds should 

 be expected to give slightly different linkage values with white. 

 If, then, the values obtained from all of her descendants be 

 plotted they should give a bimodal curve. Diagram 1 was made 

 from such data except that the diagram does not include the data 

 in table 18. 



In diagram 1 there is a strong mode at 15 which corresponds 

 to the new lethal, but the mode which corresponds to the original 

 lethal gives only a weak mode at 22; indeed the curve is not 

 obviously bimodal, small number of determinations not being 

 sufficient to distinguish clearly between the two lethals. The 



