258 



action of the activator type. In the 

 presence of the aberrant chromosome- 

 9 and in the absence of Dt, the stand- 

 ard a allele has given occasional en- 

 dosperm dots apparently due to 

 mutation to A. This strongly indicates 

 that the standard ^ is a repressed A, 

 and, if so, its reversion under the influ- 

 ence of Dt must also be due to some 

 modification of conditions affecting 

 gene expression. 



Whether or not there is acceptance 

 of my hypothesis that these manifesta- 

 tions of unstable gene behavior are 

 brought about by the transposition of 

 invisible bits of heterochromatin to the 

 locus of the gene affected, this brilliant 

 investigation clearly shows that expres- 

 sion effects may be the actual cause of 

 apparent gene mutations, even when 

 the mutation observed shows no indi- 

 cation of a change of position or of 

 any associated chromosomal alteration. 



The resulting difficulty in the analy- 

 sis of observed mutations further em- 

 phasizes the necessity for carrying on 

 the analysis with the advantages of the 

 detailed study of mutation at specific 

 loci. If we think of these results in 

 terms of the generalizing assumptions 

 characteristic of the study of mutation 

 en masse, we may be inclined to apply 

 the findings to the nature of gene in- 

 stability in general, or even to the na- 

 ture of mutant alleles in general. If we 

 think of them against the background 

 of diverse mutations of some inten- 

 sively studied gene, we are inclined to 

 make detailed comparisons of the mu- 

 tants of this category with those of 

 other types and other modes of origin 

 in the hope of developing criterions 

 that distinguish mutants of different 

 kinds. 



Meanwhile, in the study of gene mu- 

 tation, we are for the present in an 

 anomalous position. A mutant may 

 meet every test of gene mutation, and 

 yet, if it is not capable of reverse 



STABLER 



mutation there is ground for the sus- 

 picion that it may be due to gene loss, 

 while, if it is capable of reverse muta- 

 tion, there is ground for the suspicion 

 that it may be due to an expression-ef- 

 fect. The only escape from this di- 

 lemma is through the more intensive 

 study of the mutations of specific 

 genes selected as best suited to detailed 

 genetic analysis, in the hope of de- 

 veloping more sensitive criterions for 

 the identification of gene mutations. 



REFERENCES AND NOTES 



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1938. 



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17. Given as the presidential address, Amer- 



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 It is a report of the cooperative investi- 

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