THE SECOND-CHROMOSOME GROUP OF MUTANT 



CHARACTERS. 



BY C. B. BRIDGES AND T. H. MORGAN. 



INTRODUCTION. 



This paper deals with 39 mutant races whose genes lie in the "second 

 chromosome.' ' This number includes all of those previously described, 

 and gives a complete account of all the second-chromosome mutants 

 found before 1916. The latter have been for the most part used or 

 mentioned in previous papers by ourselves and others. It has been 

 our rule never to hold back any useful mutant type until it had been 

 recorded by its discoverers, and in consequence a number of the char- 

 acters here described for the first time have already been used widely. 

 This applies equally to the third-chromosome mutants, an account 

 of which we hope to publish soon. 



. In addition to the 39 mutant types here described there remain 

 about 35 others discovered since 1915 which are still to be described. 

 In making our selection for the present publication we have included 

 those which are most essential for future work both in localization of 

 genes and in special experiments. For example, star eye, because of 

 the location of the gene at the extreme 'left' end of the chromosome, of 

 its dominance, and of its other excellent characteristics, is now the most 

 generally useful second- chromosome mutant. Again, purple eye- 

 color is the only workable eye-color in this chromosome. It has been 

 involved in the localization of many of the genes in the chromosome. 

 Its position is at the center of the chromosome, which center shows 

 certain important peculiarities. Furthermore, its location near black 

 gives a working distance suitable for analyzing linkage and coincidence 

 characteristics; the distance between purple and black is short enough 

 to exclude double crossing-over and long enough to exclude the large 

 and uncertain probable error incident to small percentages of crossing- 

 over. A third important mutant type, speck, whose gene is located 

 near the right end of the chromosome, is used as the basis of reference 

 of the genes of that end of the chromosome. Finally, curved is, 

 in addition to its excellent viability, ease of identification, and other 

 useful features, very valuable from its position at the right of the cen- 

 tral group of the most useful and best located genes. 



The mutants have been discussed in order of their discovery, since 

 this method involves least use of material requiring special explana- 



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