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Gene Mutations Caused by Radiation 



H. J. MULLER 



Indiana University 

 Bloomington, Indiana 



The Relation of Radiation Mutations to Those of 

 Spontaneous Occurrence 



Perhaps the most striking thing about the gene mutations induced 

 by radiation, in organisms in which they have been intensively studied, 

 is the fact that they have thus far been in no wise distinguishable from 

 the so-called spontaneous mutations. Pick out any locus in Drosophila 

 in which a spontaneous mutation is known, and the production of a 

 similar-appearing mutation in this locus by means of ionizing radiation 

 may be guaranteed. Moreover, if multiple alleles of the locus are known 

 to have occurred spontaneously, a similar series of multiple alleles can 

 in time be obtained by radiation also. We do not mean to imply here 

 that mutant genes whose effects look alike are necessarily the same in 

 their inner genie structure but only that no consistent differences have 

 been found in the alleles, or series of alleles, arising with and without 

 radiation. It is further to be observed that, if mutations in the reverse 

 direction have been obtained spontaneously, they too can be produced 

 by exposure to radiation. And it is probably true, conversely, that any 

 gene mutation arising as a result of irradiation could be found in un- 

 treated material, if a prolonged and intensive search were made. 



It must be admitted that it has not yet proved possible to put this 

 comparison into quantitative form. That is, although spontaneous gene 

 mutations in Drosophila have been known for over 40 years and those 

 induced by radiation for 24 years, the per-locus rate of mutation, es- 

 pecially that occurring spontaneously, is so low as to have prevented 

 the making of a comparison of the per-locus distribution of mutation 

 rates, or what has been termed the mutational spectrum, in spontaneous 

 and radiation samples. That would require a long-term project, the 

 funds of which were not subject to year-by-year uncertainties, because 

 the so-called "personal equation" is so strong a factor in the detection 

 of visible mutations. Thus in a given series of radiation experiments 



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