SEX-LINKED LETHALS IN DROSOPHILA 91 



of the class whose occurrence would necessitate a double cross- 

 over. A double cross-over class is always much less likely to be 

 represented than any single cross-over class. Inspection of 

 table 7 shows that the class red miniature is the only one not 

 represented. The order of loci which would make the red minia- 

 ture the double cross-over class is lethal, white, miniature (L W M). 

 If lethal were not outside of the white to miniature section of the 

 chromosome (that is, if the order of factors were white, lethal, 

 miniature, W L M) the double cross-over male class would be the 

 red, long class. The fact that this class is represented argues 

 against the probability of white, lethal, miniature as the order 

 of factors and points to lethal, white, miniature as the correct 

 order. ( 



A confirmation of lethal, white, miniature as the order of 

 factors was obtained by using yellow and miniature as the fixed 

 points instead of white and miniature. A large amount of data 

 has shown that yellow is about 1.1 units from white on the side 

 of white away from miniature. The data just given show that 

 lethal lies at a distance of 0.4 units from white jj|ird probably 

 on the same side from white as does yellow. Lethal should lie, 

 therefore, between yellow and white at a distance of 0.7 from 

 yellow and 0.4 from white. 



Sturtevant made the following mating (not published). A 

 gray, lethal long-winged female was mated to a yellow minia- 

 ture male. They produced 169 gray red-eyed females and 64: 

 gray white-eyed males. The female used was, as shown by the 

 sex-ratio, heterozygous for lethal and for white. Of her daugh- 

 ters, half should be heterozygous for lethal and should not bear 

 white, while the other half should be heterozygous for white, 

 but should not bear lethal. 



Six females of the latter type crossed individually to their 

 white brothers gave non-lethal F2 ratios in which white males 

 and females occurred. Six other females gave the following 

 lethal ratios in which no white appeared (table 8). 



