144 OT'IO I.. M Ol IK 



separates oti troni tlie common germinal-somatie stork (Mohgan 

 and Bridges, 1919; Mlllku, 1920). 



Morgan and Bridges describe ten mosaics, all males, which are 

 explained as due to somatic mutations of the type last mentioned. 

 Some of these are more or less doubtful, involving only a very small 

 part of the individual or open to alternative interpretations. But it 

 is of importance that the atypical portion in the majority of the cases 

 exhibited somatic characteristics similar to those produced by known 

 sex-linked récessives. Only two of the mosaics were tested and in 

 none of these did the testes carry the mutant gene. The same was 

 true of the two cases in Dr. simulans recorded by Sturtevant(1921) ^. 



The most conclusive case is the one recorded by Müller, (1920). 

 A male with a normal red left eye and a white right eye occurred 

 in a line which for many generations had bred true for the ordinär 

 red eye color. When the male was tested, half of his grandsons had 

 white eyes. Thus, it was clear that the white eye color of the right 

 eye was due to a sex-linked recessive mutant gene. This gene proved 

 to be identical with the old white gene {w; at 1,5). The fertilized egg 

 from which the exceptional male arose must accordingly have con- 

 tained the original unmutated gene for red eyes (since the left eye 

 was red). A somatic mutation to white must have occurred in one 

 of the very early cleavage nuclei and the right eye and some, at least, 

 of the germ cells — all of them so far as the evidence went — must 

 have been derived from this cell. Since the gene white only affects 

 the eye color, there was no means of controlling the distribution of 

 the mutant cells in other parts of the individual. And the test was not 

 arranged in such a way as to give a conclusive answer to the question 

 whether both testes or only one of them contained the mutant factor. 



The somatic mutation here to be described is perhaps the most stri- 

 king case so far recorded, since it involves a mutation which is more 

 favorable than any other just for the study of mosaics and gynandro- 

 morphs, viz., the sex-linked recessive singed {sir, at 20,9; Mohr, 1922). 



The singed mutation produces a very characteristic curling of all 

 the bristles and small hairs all over the individual. The hairs and 



^ Recently Breitünukcheu (1922) in his breeding experiments with the beetle 

 Bruchus qiiadrimaculatus found no less than 31 females in which the elytra were of 

 different color, and he interprets these exceptions as due to autosomal dominant 

 sex-limited somatic mutations. However, the genetic demonstration of the correct- 

 ness of this assumption is as yet lacking, since in none of these numerous cases 

 (later also 17 additional mosaics were observed) did the gonads carry the mutant gene. 



