OF MUTANT CHARACTERS. 273 



The double dominant form, star dich^te (dichn'tc being the most 

 important third-chromosome mutant), is probal)Iy tlie most useful 

 single stock we employ. For example, it has almost entirely suj)- 

 planted the former methods of testing the chromosome group of a 

 new mutant, and likewise furnishes the first test aj)pliod in determin- 

 ing loci within the chromosome. 



NICK M. 



(Text-figure 84.) 



ORIGIN OF NICK. 



In an experiment to determine the cause of the linkage disturl)ance 

 found in lethal 2 (Morgan and Bridges, 1916, p. 51), Bridges found a 

 single male which showed a very slight nick or notch at the tip of the 

 wing (culture 2012, August 7, 1915). This nick cliaracter would not 

 have been noticed had not the fly been very exceptional in another 

 regard, for he was otherwise wild-type (though noted as very dark), 

 which is a condition so rare in the particular experiment that only one 

 other wild-type male occurred in some thousands of offspring. It is 

 our custom to test flies whose occurrence is rare in order to be sure that 

 they are actually as they appear to be, and are not the result of error 

 in classification or parentage. For this reason the male was out- 

 crossed to an eosin tan vermilion female. In Fj all the daughters were 

 wild-type, which showed that no error had been made in cla.ssifying 

 the fly as not-tan, and tan was the only character in the parent experi- 

 ment in which there was any such difficulty in classification. The 

 sons were eosin tan vermiliom, as expected. An Fi pair gave in Fj 

 45 not-nick and 12 nick offspring (culture 2210, August 28, 1914). 

 The nicks were equally distributed among females and males, where- 

 fore it was know^n that the character was not sex-linked. The signif- 

 icant feature of this F2 was that most of the nick flies showed a 

 dark body-color like black, and there were a few other blacks thiit 

 were not-nick. To test whether this body-color were really black, a 

 "black" nick male was out-crossed to a bkxck female from stock. .\I1 

 of the Fi flies were black. The presence of black in the F-j effectively 

 disposed of the question of the classification of the rare fly in the lethal 

 experiment — he was due to contamination by some method and Imd no 

 place in the experiment. 



DESCRIPTION OF NICK. 



Some further tests were undertaken with the character nick, since 

 it seemed to be a hitherto unknown mutant. The niiiin charact<;ristic 

 of nick is the excision of a piece of the wing-blade from the region in 

 which the fourth longitudinal vein meets the margin— tliiit is, at the 

 tip and inner margin. This section may be very tiny (fig. 84) or more 

 extensive than in an extreme "notch." The more extreme nicks 



