140 THE SECOND-CHROMOSOME GROUP 



In most of these cases the character is also "specifically" truncate, 

 and usually due to the original truncate gene rather than to a fresh 

 mutation. In the absence of its usual intensifies truncate may lurk 

 unsuspected in a stock or an experiment for many generations and is 

 difficult to eliminate. For this reason it is practically impossible to be 

 certain in any unexpected case of truncate appearance that there has 

 been a fresh mutation. It is not ordinarily profitable to pay any 

 attention to these appearances of truncate, but in two instances in 

 which, because of the pedigree, there was less than the usual likelihood 

 that the truncate was due to the original gene, tests were made, and in 

 both cases the character was found to be either truncate or else a very 

 similar allelomorph, though these tests could throw no new light on the 

 question of whether the gene were the original or a fresh truncate 

 mutation. 



SNUB. 



The first of these tests was made by Muller (unpublished). The 

 second case appeared in the ninth generation of some experiments on 

 "duplication." A cross had been made between a female with the 

 new sex-linked recessive wing-character "cut" and a not-cut male 

 (February 17, 1916, culture 3338, Bridges). All the daughters were 

 expected to be wild-type and all the sons cut; but 9 of the 77 daughters 

 were seen to be slightly truncated, the character being called "snub," 

 while some 18 of the 67 sons were cut with shortened and blunted wing 

 ends (cut snub). 



One of these cut-snub males outcrossed to a wild female gave about 

 a quarter of the FI flies with the snub or truncate character. This 

 result showed that the character was a dominant, though a "poor" 

 one; that is, not all the flies genetically alike (heterozygotes) showed 

 the character somatically. The snub appeared among the FI males as 

 well as among the females, and this fact showed that the character was 

 non-sex-linked, for had it been sex-linked it could have appeared only 

 among the daughters of the above cross. 



When snub flies were bred together, the result was usually an approxi- 

 mation to a 2 snub : 1 not-snub ratio, often with the snubs below expec- 

 tation because of the above-noted occasional failure of heterozygous 

 snubs to show the character somatically. This ratio and the fact that 

 it proved impossible to obtain a pure-breeding snub stock suggested 

 that the mutant was lethal when homozygous, as are most of our 

 dominants. In cut-snub stock the approximation to the 2 : 1 ratios 

 was much closer, and it seems certain that the character cut favors 

 the differentiation of snub (see cultures 1 to 10, table 9). We are well 

 acquainted with such intensifiers or modifiers in other cases, and 

 truncate itself was known to be very susceptible to intensification. 

 A few of the cut-snub pairs gave nearly all of the flies snub (see espe- 

 cially 10 and 12, table 9, Morgan), and it seems probable that in these 



