144 



can be shown to have occurred in the 

 X chromosomes of males, since it may 

 be taken as established that crossing 

 over does not occur between the X 

 and the Y of a male. 



We have seen earlier in this paper 

 that infrabar arose from bar in a male, 

 and that its later behavior was not in 

 agreement with the view that it repre- 

 sented a quantitative change in the bar 

 gene, as it should if due to unequal 

 crossing over. I have also obtained 

 yellow, a fused allelomorph, and a 

 lozenge allelomorph under the same 

 circumstances, namely, from mothers 

 with attached X's and in experiments 

 where known sex-linked genes were 

 present, so that breaking apart of the 

 attached X's was known not to have 

 occurred. 



Unpublished data are available for 

 5 other cases of the same sort, either 

 from attached-X or from "high-non- 

 disjunction" mothers, as follows: rudi- 

 mentary (C. B. Bridges), a dusky alle- 

 lomorph (C. B. Bridges), a sable allelo- 

 morph (E. M. Wallace), white (L. V. 

 Morgan), and a new lozenge allelo- 

 morph from lozenge (C. B. Bridges). 

 In all of these cases, as in that of in- 

 frabar, the mutant type first appeared 

 as a single male. 



Muller (1920) reported the occur- 

 rence of white as a "somatic" mutation 

 in a male. From a stock in which white 

 was not present he obtained a male 

 with one wild-tvpe eye and one white 

 eye. This male also transmitted white 

 to some (all that were tested) of his 

 daughters. In the same paper Muller 

 described briefly a mosaic male that 

 was partly yellow, and transmitted the 

 new character to his offspring. Doctor 

 Bridges informs me that he has a sim- 

 ilar (unpublished) record for yellow. 

 Mohr (1923 a) reports a similar case 

 for a singed allelomorph, though here 

 some of the X-bearing spemis carried 

 singed, while others did not. I have 



STURTEVANT 



observed two other such cases,— both 

 in D. s'nmdans. The mutant types 

 dusky and fused (both corresponding 

 to the types of the same names in D. 

 7}7elaiiogaster) each appeared first in 

 an individual that showed the new 

 character in only one wing; and in 

 each case tests showed that some of 

 the X-bearing sperms carried the new 

 gene, while others did not. In all cases 

 discussed in the last two paragraphs, 

 genetic tests have established the alle- 

 lomorphism of the new mutant genes 

 to the old ones whose names they bear. 



There is thus clear evidence that 

 mutations have arisen in the follow- 

 ing sex-linked loci in the germ-cells 

 of males: yellow (3 times), white 

 (twice), lozenge (twice), dusky 

 (twice), fused (twice), singed (once), 

 bar (once). It should be noted that all 

 these loci are among the more mutable 

 ones of Drosophila. 



Another class of cases to which the 

 unequal-crossing-over hypothesis is 

 probably not applicable is that in which 

 mutation can be shown to have oc- 

 curred at some stage other than mat- 

 uration. A number of such instances 

 are on record for Drosophila. The 

 mosaic males described above are ex- 

 amples, and a longer list of cases for 

 autosomal mutant types and for sex- 

 linked mutations in females could 

 easily be compiled. But since there is 

 evidence that crossing over does very 

 rarely occur at somatic divisions, this 

 evidence can hardly be considered de- 

 cisive. In the case of certain t\'pes of 

 frequently recurring somatic muta- 

 tions in plants, however, the mutation 

 occurs far too often to make an appeal 

 to somatic crossing over seem plausi- 

 ble. The clearest example of this sort 

 is the variegated pericarp of maize 

 studied by Emerson (1917) and others, 

 in which a given gene mutates many 

 separate times in a single individual 

 plant. 



