34 SEX-LINKED INHERITANCE IN DROSOPHILA. 



are allelomorphic. Unless due to crossing-over it must have been a 

 mutation from spot back to yellow. Improbable as this may seem to 

 those who look upon mutations as due to losses from the germ-plasm, 

 yet we have records of several other cases where similar mutations 

 "backwards" have taken place, notably in the case of eosin to white, 

 under conditions where the alternative interpretation of crossing-over 

 is excluded. 



SABLE. 



(Plate I, figure 2.) 



In an experiment involving black body-color^ a fly appeared (July 

 19, 191 2) whose body-color differed slightly from ordinary black in 

 that the trident mark on the thorax was sharper and the color itself 

 was brighter and clearer. This fly, a male, was mated to black females 

 and gave some black males and females, but also some gray (wild 

 body-color) males and females, showing not only that he was heterozy- 

 gous for ordinary recessive black, but at the same time that his dark 

 color must be due to another kind of black. The gray Fi flies when 

 mated together gave a series of gray and dark flies in F2 about as follows : 

 In the females 3 grays to i dark; in the males 3 grays to 5 dark in color. 

 The result indicated that the new black color, which we call sable, was 

 due to a sex-Hnked factor. It was difficult to discover which of the 

 heterogeneous F2 males were the new blacks. Suspected males were 

 bred (singly) to wild females, and the F2 dark males, from those cultures 

 that gave the closest approach to a 2 gray 9 : i gray cf : i darkcT, were 

 bred to their sisters in pairs in order to obtain sable females and males. 

 Thus stock homozygous for sable but still containing black as an 

 impurity was obtained. It became necessary to free it from black by 

 successive individual out-crossings to wild flies and extractions. 



This account of how sable was purified shows how difficult it is to 

 separate two recessive factors that give closely similar somatic effects. 

 If a character like sable should be present in any other black stock, or 

 if a character like black should be present in sable, very erratic results 

 would be obtained if such stocks were used in experiments, before such 

 a population had been separated into its component races. 



Sable males of the purified stock were mated to wild females and gave 

 wild-type (gray) males and females. These inbred gave the results 

 shown in table 6. 



No sable females appeared in F2, as seen in table 6. The reciprocal 

 cross gave the results shown in table 7. 



*The first dark body-color mutation "black" (see plate II, figs. 7, 8) had appeared much earlier 

 (Morgan 191 16, 1912c). It is an autosomal character, a member of the second group of linked 

 gens. Still another dark mutant, "ebony, " had also appeared, which was found to be a member 

 of the third group of gens. 



