THE MUTATED GENE 175 



of gynandromorphs — Bee Goldschmidt, 193 Id — but they do not 

 furnish much information for the present problem.) If mutant 

 genes are present in the X-chromosome, the mosaic male spots 

 will also show the mutant character, and the facts showed that 

 the development of these is autonomous. Even very small 

 mosaic parts behave as if they were independent of the rest 

 of the animal. In these cases, then, the mutant genes must have 

 acted within the cells in which they were located, and their action 

 may have begun rather late in development. But it ought to be 

 emphasized that the characters involved here are such as are 

 actually formed only at the end of differentiation within the cells, 

 e.g., pigment formation or development of bristles. Develop- 

 mental processes that have to do with growth and differentiation 

 of a whole Anlage do not seem to react within the individual 

 cells; in mosaics, they appear only in a whole organ. This 

 applies, for example, to wing form. 



These facts, derived from sex mosaics, reappear in cases where 

 mosaics have been produced in different ways. There are genes 

 like the Claret gene in Drosophila (Sturtevant 1929), or the 

 Minute-w gene (Bridges 1925), which produce elimination of 

 chromosomes and therefore mosaics. There is the possibility of 

 producing a similar result by inducing somatic mutations by X-ray 

 action upon individuals that have been marked by definite genes, 

 preferably by heterozygous sex-linked genes. A special study 

 has been made by Patterson (1929a) using eye colors in Dro- 

 sophila. The results coincide with those found in other cases of 

 mosaicism, viz., that mosaic areas may be formed from a whole 

 eye down to a single aberrant ommatidium. The genie effect 

 upon pigment formation then appears rather localized (see page 

 176 for more facts). Here it w r as also possible to perform the 

 important experiment of producing the somatic mutation at 

 different times of development. The result was, as expected, 

 that the earlier the treatment the larger the mosaic area. 



A third method of attack upon the same problem is furnished 

 by the study of what has been called unstable genes. In this 

 chapter, we are not concerned with the explanation of this 

 phenomenon but with the results. It is assumed that in these 

 cases mutant genes have the tendency to revert to the Wild-type 

 (or, more rarely, if ever, to another) allelomorph during develop- 

 ment. The result is a mosaic spot of different tissue, the size 



