A SOMATIC MLTAIION IN DHOSOI'HILA MKLANÜÜAS IKH \4'.i 



There is, as is well known, good evidence in favor ot the view 

 that mutation only occurs in one member ot a cliromosome pair. 

 If a mutation occurs in a somatic cell, either in one member of an 

 autosome pair, or in one of the two A'-clironiosomcs of a female, then 

 tlie gene will have no visil)le effect, unless it is dominant. In the 

 latter cases that })orti()n of the individual which receives the nuitated 

 chromosome will manifest the corresponding dominant character. A 

 recessive gene will not have any somatic effect, since the normal 

 allelomorph present in the other member of the pair conceals it. 



But if a recessive somatic mutation takes place in the single A'- 

 chromosome of a mcile, then the corresponding character will show 

 at once in those parts whose cells have received the mutated A', be- 

 cause the y-chromosome lacks the normal allelomorph. 



Recessive mutations are in Drosophild far more frequent than 

 dominant ones and we would from what has been said above accor- 

 dingly expect that the majority of somatic mutations observed should 

 be of the latter type, /. c, sex-linked récessives that manifest them- 

 selves in male mosaics. This was also found to be the case. 



If a somatic mutation of this type occurs late in the life cycle, 

 then only a small part of the individual will show the corresponding 

 mutant character, while the rest of the male w411 be normal. If, on 

 the other hand, the mutation takes place early, in one of the seg- 

 mentation nuclei, then a large part will manifest the character. And 

 if the mutation occurs before the germ tract has separated off from 

 the common germinal-somatic blastomeres, the following possibilities 

 present themselves. The testes may either be derived from cells 

 which have received the mutated A^, or they may descend from cells 

 which carry a normal A'-chromosome. The A'-sperms will therefore 

 in the first case contain the mutant gene and transmit the mutation 

 to the offspring, while in the latter the mutation will not be inhe- 

 rited. Theoretically there is also a third possibility. One testis (or 

 part of one testis) may be derived from cells which carry the mutant 

 gene, while the other testis genetically is like the normal portion of 

 the mosaic. Under these conditions only some of the A'-sperms will 

 transmit the mutant gene to the offspring, others will not. 



Thus, the external examination of such mosaics combined with 

 the genetic test may not only give data as to the stage in the life 

 cycle at which the mutation took place, but eventually also offer an 

 opportunity for deductions as to the stage at which the germ tract 



