264 



THE AMEBIC AX NATURALIST 



[Vol. LII 



THREE MUTATIONS IN PREVIOUSLY KNOWN LOCI 

 Three mutations is the sex-chromosome have occurred in my 

 cultures of Drosophila melanogaster (ampelophila) . Two were 

 reappearances of genes already known, namely, white and rudi- 

 mentary ; the third was the appearance of a new gene at the white 

 locus and has been named coral, symbol w f0 . In each case it is 

 clear that the changes occurred in the wild type gene of a mater- 

 nal chromosome. The evidence also indicates that the new gene 

 arose relatively late in the history of the egg in each case, whereas 

 if the mutation had occurred in the early oogonial stages several 

 individuals with the new gene should have appeared. In eases 

 where such information is known, it seems worth recording since 

 it will make possible a later consideration of the relative stabil- 

 ity of genes by a summing up of the frequency of mutations in 

 the different loci. 



As has been pointed out by Muller 1 recessive genes might ex- 

 ist for a long time before making an appearance, in case they 

 were closely linked to a lethal. The character produced by the 

 gene would ultimately be allowed to appear as the result of a 

 cross over which would separate the gene and the lethal from 

 the same chromosome. Previous to the time of crossing-over the 

 character produced by the gene would never be seen, since all 

 individuals pure for it would also be pure for the lethal and not 

 survive. The gene could be indefinitely transmitted along with 

 the lethal through heterozygous individuals. I mention this 

 point because it is necessary in establishing the time of origin 

 of a mutation to consider whether its appearance may be due 

 merely to its recent separation from a lethal, which had obscured 

 it. The three mutations dealt with here could not have been 

 masked by a lethal because they were in the X-chromosome, and 

 the presence of a lethal would have been apparent, as it would 

 have produced a lethal sex ratio. No such lethal ratio has been 

 found in connection with any of the three mutants either before 

 or since their appearance. In these cases, then, it is safe to as- 

 sume that the appearance of the first mutant marks the time of 

 the mutation. If the mutation had occurred in earlier genera- 

 tions, several individuals bearing the character would have ap- 

 peared instead of one. 



In the case of the reappearance of a character, careful consid- 

 eration must be given to the possibility of contamination, as has 



