OF MUTANT CHARACTERS. 297 



probability is that the gene is in the second chromosome, especially in 

 view of the evidence that it is not in the X or third chromosome. 



As is suggested in its name, dachsoid has certain points of resem- 

 blance to the second-chromosome character dachs. It was therefore 

 surmised that the gene might be allelomorphic to the dachs gene, and 

 the following tests were made: 



Six offspring of the four pairs in which dachsoid first appeared 

 were selected at random. Two-thirds of these would be expected to 

 be heterozygous for dachsoid; and the chance that none was hetero- 



/1\ 6 1 

 zygous is only (-J = . These 6 individuals were then mated, 



\o/ i 2io 



some to homozygous dachs, some to flies heterozygous for dachs. All 

 produced a considerable number of offspring (105 to 199 per pair) and 

 another similar mating (to homozygous dachs) produced 9 offspring. 

 None of the FI flies showed any trace of the characteristics of dachs or 

 dachsoid, or any other unusual characters. It therefore follows that 

 the flies heterozygous both for dachs and for dachsoid are normal in 

 appearance. In all probability, therefore, the genes of the two char- 

 acters are not allelomorphic. 



THE CONSTRUCTION OF THE MAP OF THE SECOND 

 CHROMOSOME. 



The map given on page 127 is constructed on the basis of the total 

 data available on each cross-over value. The first step taken was to 

 collect and summarize this data in the form in which it appears in 

 table 140. 



In constructing the map of the second chromosome on the basis of 

 all the available data, the procedure was roughly as follows: The 

 first locus to be considered was that of black, since the "second" chro- 

 mosome had originally been defined quite arbitrarily as that chromo- 

 some which carries the gene for black and such other genes as may be 

 found to be linked to black, while correspondingly the third chromosome 

 was that chromosome which carries the gene for pink and such other 

 genes as may be found to be linked to pink. Furthermore, it so 

 happened that of the early mutations which were stably mapped 

 (black, purple, vestigial, and curved) black was the one located 

 farthest to the left, and was therefore chosen as the zero-point of this 

 early map. Even after black had been displaced from the position 

 at zero, it still remained the base of reference of the entire second 

 chromosome, in relation to which all other loci are mapped, either 

 directly, in the case of those close by, or indirectly, through refer- 

 ence to intermediate bases in the case of those farther away. Black 

 was therefore accepted as the constructional zero-point of the map, 

 and all other loci were to be mapped as lying to the right or to the left 

 of black by a specific number of units. The loci to the right, or in a 



