66 SEX-LINKED INHERITANCE IN DROSOPHILA. 



The locus of this lethal is at 16.7; the locus of lethal sa was found 

 to be at 23.7, so that the lethal at 16.7 is evidently the second lethal or 

 lethal sh whose advent gave rise to the high sex-ratio. This interpre- 

 tation is in accord with the curve which Miss Stark published, for 

 although the mode which corresponds to lethal sa is weak, the mode 

 at 15-16 is well marked. 



The two other lethals, lethals sc and sd, which came up in the course 

 of these experiments by Miss Stark, are treated in other sections of this 

 paper. 



BAR. 



(Plate II, figures 12 and 13.) 



The dominant sex-linked mutant called bar-eye (formerly called 

 barred) appeared in February 191 3 in an experiment involving rudi- 

 mentary and long-winged flies (Tice, 1914). A female that is hetero- 

 zygous for bar has an eye that is intermediate between the rounded eye 

 of the wild fly and the narrow band of the bar stock. This hetero- 

 zygous bar female is always readily distinguishable from the normal, 

 but can not always be separated from the pure bar. Bar is therefore 

 nearly always used as a dominant and back-crosses are made with 

 normal males. 



Bar is the most useful sex-linked character so far discovered, on 

 account of its dominance, the certainty of its classification, and its 

 position near the right end of the X chromosome. The locus of bar at 

 57 was determined on the basis of the data of table 65. 



NOTCH. 



A sex-linked dominant factor that brings about a notch at the ends 

 of the w^ngs appeared in March 1913, and has been described and 

 figured by Dexter (1914, p. 753, and fig. 13, p. 730). The factor acts 

 as a lethal for the male. Consequently a female heterozygous for 

 notch bred to a wild male gives a 2 : i sex-ratio; half of her daughters 

 are notch and half normal; the sons are only normal. The actual figures 

 obtained by Dexter were 235 notch females, 270 normal females, and 

 235 normal males. 



The location of notch in the X chromosome was not determined by 

 Dexter, but the mutant has appeared anew three or four times and the 

 position has been found by Bridges to be approximately at 2.6. 



