A SOMATIC MUTATION IN THE 

 SINGED LOCUS OF THE X-CHROMOSOME 

 IN DROSOPHILA MELANOGASTER 



BY OTTO L. M OH H 



ANATOMICAL INSTITUTE, CHRISTIANIA UNIVKHSITY. NORWAY 



I. INTRODUCTION, 



THE doctrine that mutations occur in the germ cells shortly 

 before or during maturation has until recently been widely 

 accepted (see Muller's quotations from different text books, 1920). 

 We now know that mutation is not a phenomenon restricted to ma- 

 turation or peculiar to germ cells but may occur in any cell and at 

 any time during the life cycle. 



Thus, botanists have shown that several cases of vegetative varia- 

 tion in plants are due to mutations which occurred in ordinary so- 

 malic cells. It may in this connection be sufficient to recall the re- 

 sults of ?2merson and his co-workers. Emerson gives (1922) a dis- 

 cussion of the botanical literature on somatic mutations in plants, to 

 which discussion may be referred. 



So far only a few cases are known in zoological material, 

 where the only conclusive ones are recorded in Drosophila, (Bridges, 

 1919; Morgan and Bridges, 1919; Müller, 1920; Sturtevant, 1921). 



The visible somatic result of a mutation outside the germ tract 

 is the formation of a mosaic. 



The overwhelming number of mosaics in Drosophila are sex- 

 mosaics, i. e., gynandromorphs. By far the large majority of the 

 gynandromorphs are, as Morgan and Bridges have shown, accounted 

 for by A'-chromosomal elimination, while a few cases seem to have 

 arisen from binucleated eggs. (Here, as well as for the following, 

 see Morgan and Bridges, 1919). 



Somatic mosaics might theoretically be expected to arise in the 

 same way by autosomal elimination or from binucleated eggs. Only 

 one case which was interpreted in this way has so far been recorded. 



The rest of the mosaics described in Drosophila are explained as 

 due to somatic mutation. These cases are rare. 



