A SOMATIC MUTATION IN DUOSOFHILA MELANOGASTER 149 



as examination proved that the mosaic had no female portions at all, 

 the singed as well as the non-singed parts showing, without exception, 

 typical male characteristics. 



The possil)ility that we were dealing with a somatic mosaic could 

 also safely be excluded. If the individual were a somatic mosaic 

 due to autosomal elimination in the same way that gynandromorphs 

 are accounted for by X-chromosomal elimination, this interpretation 

 would demand first a mutation in the black purple curved X Streak 

 curved stock and secondly an autosomal elimination. Such a coin- 

 cidence would be so rare as to be considered entirely out of question. 



As a fully satisfactory explanation remained that of a somatic 

 mutation (see Introduction). The mosaic was a male. Two recessive 

 sex-linked mutations which produce entirely analogous bristle and 

 hair alterations were known beforehand, viz., singed and forked*, (/*, 

 allelomorph of forked, at 56,5). There was every reason to believe 

 that the singed part of the individual in all cells contained one of 

 these genes, or an allelomorph thereof. Since about half of the in- 

 dividual had normal bristles and hairs, this mutant gene could not 

 have been present in the Z-chromosome which this male received 

 from his mother. The mutation must have occurred later, in one 

 of the daughter X's of the dividing egg nucleus, or in another of the 

 earliest segmentation nuclei. The male was on this assumption ex- 

 pected to be of normal fertility, and if the testes (or one of them) 

 were derived from the cell which received the mutated X, then we 

 would have an opportunity of proving conclusively the correctness 

 of the above supposition. 



If both testes were derived from the cells which contained the 

 A' carrying the unmutated gene for non-singed, then, of course, all 

 the daughters of the mosaic will give only wild-type sons. If, on the 

 other hand, both testes have developed from the cell which received 

 the mutated A', then all the daughters of the exceptional male will give 

 sons half of which are singed. Finally, if one testis belonged to the 

 singed, and the other to the non-singed part of the mosaic, then half 

 of his daughters would be expected to give sons, half of which were 

 singed. The other half of his daughters would give only non-singed sons. 



IV. TESTS OF THE EXCEPTIONAL MALE. 



Since the Streak character in the mosaic was somewhat difficult 

 to detect, it was regarded as necessary to certify genetically that the 



