THE MANNER OF OCCURRENCE OF MUTATION 469 
longer life-cycle or a greater number of cell divisions in the 
germ-cycle, will in general produce a greater proportion of 
mutants per generation than younger animals, or animals of 
species having a shorter life-cycle or a smaller number of cell 
divisions in the germ-cycle. 
VII. THE DEGREE OF LOCALIZATION OF THE EVENT WHICH 
PRODUCES THE MUTATION 
It is noteworthy that the mosaic white reported above was a 
male. Similarly, a mosaic yellow fly which has arisen in a more 
recent experiment of the author’s, and which was tested and 
found to be an allelomorph of the original yellow, was also a 
male. These two are the only somatically mosaic mutants 
which have been genetically tested and verified as containing a 
mutant factor, but a considerable number of other mosaics 
have been observed by other workers which are almost undoubt- 
edly mutants, although they were not tested. In all these 
cases, where a recessive sex-linked mutant gene was involved, 
the mosaic fly was a male. This result does not mean that 
mutation does not occur in females; it is obvious that ivory, 
for example, arose in the female parent of the brood, and Sturte- 
vant has recently found an apparent female somatic mutant 
involving the dominant sex-linked character notch. The more 
reasonable explanation is that the mutations occur similarly in 
males and females, but that they occur in only one X chromo- 
some at a time, and therefore recessive mutant genes cannot 
manifest themselves in the female sex, on account of the pres- 
ence there of the dominant unmutated allelomorph in the homol- 
ogous X chromosome. In the male, since there is only one X 
chromosome and since the Y does not dominate, any ee 
occurring in the X could be immediately visible. 
The conclusion that a mutation happens in only one X chro- 
mosome at a time implies that the agent which ordinarily pro- 
10 The new yellow is apparently identical with the old, and not ‘achete,’ like 
the allelomorph of yellow which Weinstein found. Weinstein’s yellow appeared 
in just three brother males among a family of ordinary size and was therefore 
due to a mutation in the oégonial stage (Weinstein, 718). 
