150 



expected mutational windfall. To these 

 circumstances are due the wide-spread 

 desire on the part of biologists to gain 

 some measure of control over the 

 hereditary changes within the genes. 



It has been repeatedly reported that 

 germinal changes, presumably muta- 

 tional, could be induced by X or ra- 

 dium rays, but, as in the case of the 

 similarly published claims involving 

 other agents (alcohol, lead, antibodies, 

 etc.), the work has been done in such 

 a way that the meaning of the data, 

 as analyzed from a modern genetic 

 standpoint, has been highly dispu- 

 tatious at best; moreover, what were 

 apparently the clearest cases have 

 given negative or contrary results on 

 repetition. Nevertheless, on theoretical 

 grounds, it has appeared to the present 

 writer that radiations of short wave 

 length should be especially promising 

 for the production of mutational 

 changes, and for this and other rea- 

 sons a series of experiments concerned 

 with this problem has been undertaken 

 during the past year on the fruit fly, 

 Drosophila mela?iogaster, in an attempt 

 to provide critical data. The well- 

 known favorableness of this species for 

 genetic study, and the special methods 

 evolved during the writer's eight years' 

 intensive work on its mutation rate 

 (including the work on temperature, 

 to be referred to later), have finally 

 made possible the finding of some de- 

 cisive efi^ects, consequent upon the ap- 

 plication of X-rays. The eflfects here 

 referred to are truly mutational, and 

 not to be confused with the well- 

 known eflfects of X-rays upon the dis- 

 tribution of the chromatin, expressed 

 by non-disjunction, non-inherited 

 crossover modifications, etc. In the 

 present condensed digest of the work, 

 only the broad facts and conclusions 

 therefrom, and some of the problems 

 raised, can be presented, without any 



MULLER 



details of the genetic methods em- 

 ployed, or of the individual results ob- 

 tained. 



It has been found quite conclusively 

 that treatment of the sperm with rela- 

 tively heavy doses of X-rays induces 

 the occurrence of true "gene muta- 

 tions" in a high proportion of the 

 treated germ cells. Several hundred 

 mutants have been obtained in this 

 way in a short time and considerably 

 more than a hundred of the mutant 

 genes have been followed through 

 three, four or more generations. They 

 are (nearly all of them, at any rate) 

 stable in their inheritance, and most 

 of them behave in the manner typical 

 of the A4endelian chromosomal mutant 

 genes found in organisms generally. 

 The nature of the crosses was such as 

 to be much more favorable for the 

 detection of mutations in the X-chro- 

 mosomes than in the other chromo- 

 somes, so that most of the mutant 

 genes dealt with were sex-linked; there 

 was, however, ample proof that mu- 

 tations were occurring similarly 

 throughout the chromatin. When the 

 heaviest treatment was given to the 

 sperm, about a seventh of the oflFspring 

 that hatched from them and bred con- 

 tained individually detectable muta- 

 tions in their treated X-chromosome. 

 Since the X forms about one fourth 

 of the haploid chromatin, then, if we 

 assume an equal rate of mutation in all 

 the chromosomes (per unit of their 

 length), it follows that almost "every 

 other one" of the sperm cells capable 

 of producing a fertile adult contained 

 an "individually detectable" mutation 

 in some chromosome or other. Thou- 

 sands of untreated parent flies were 

 bred as controls in the same way as 

 the treated ones. Comparison of the 

 mutation rates under the two sets of 

 conditions showed that the heavy 

 treatment had caused a rise of about 



