OF MUTANT CHARACTERS. 273 



The double dominant form, star dichaete (dichsete being the most 

 important third-chromosome mutant), is probably the most useful 

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

 planted the former methods of testing the chromosome group of a 

 new mutant, and likewise furnishes the first test applied in determin- 

 ing loci within the chromosome. 



NICK (CX 



(Text-figure 84.) 



ORIGIN OF NICK. 



In an experiment to determine the cause of the linkage disturbance 

 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 character 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 FI all the daughters were 

 wild-type, which showed that no error had been made in classifying 

 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 F 2 

 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 known that the character was not sex-linked. The signif- 

 icant feature of this F 2 was that most of the nick flies showed a 

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

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

 " black" nick male was out-crossed to a black female from stock. All 

 of the FI flies were black. The presence of black in the F 2 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 had 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 main characteristic 

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

 which the fourth longitudinal vein meets the margin that 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 



