152 



belief has several times been expressed 

 in the Drosophila literature that this 

 holds true in the case of "natural" mu- 

 tations in this organism, but it has been 

 founded only on "general impres- 

 sions"; Baur, however, has demon- 

 strated the truth of it in Antirrhminn. 

 On the whole, the visible mutations 

 caused by raying were found to be 

 similar, in their general characteristics, 

 to those previously detected in non- 

 rayed material in the extensive ob- 

 servations on visible mutations in Dro- 

 sophila carried out by Bridges and 

 others. A considerable proportion of 

 the induced visible mutations were, it 

 is true, in loci in which mutation ap- 

 parently had never been observed be- 

 fore, and some of these involved 

 morphological effects of a sort not ex- 

 actly like any seen previously {e.g., 

 "splotched wing," "sex-combless," 

 etc.), but, on the other hand, there 

 were also numerous repetitions of mu- 

 tations previously known. In fact, the 

 majority of the well-known mutations 

 in the X-chromosome of Drosophila 

 melanogaster, such as "white-eye," 

 "miniature wing," "forked bristles," 

 etc., were reobtained, some of them 

 several times. Among the visible muta- 

 tions found, the great majority were 

 recessive, yet there was a considerable 

 "sprinkling" of dominants, just as in 

 other work. All in all, then, there can 

 be no doubt that many, at least, of the 

 changes produced by X-rays are of 

 just the same kind as the "gene muta- 

 tions" which are obtained, with so 

 much greater rarity, without such 

 treatment, and which we believe fur- 

 nish the building blocks of evolution. 

 In addition to the gene mutations, it 

 was found that there is also caused by 

 X-ray treatment a high proportion of 

 rearrangements in the linear order of 

 the genes. This was evidenced in gen- 

 eral by the frequent inherited disturb- 

 ances in crossover frequency (at least 



MULLER 



3 per cent, were detected in the X- 

 chromosome alone, many accompanied 

 but some unaccompanied by lethal 

 effects), and evidenced specifically by 

 various cases that were proved in other 

 ways to involve inversions, "deficien- 

 cies," fragmentations, translocations, 

 etc., of portions of a chromosome. 

 These cases are making possible at- 

 tacks on a number of genetic problems 

 otherwise difficult of approach. 



The transmitting action of X-rays 

 on the genes is not confined to the 

 sperm cells, for treatment of the un- 

 fertilized females causes mutations 

 about as readily as treatment of the 

 males. The effect is produced both on 

 oocytes and early oogonia. It should 

 be noted especially that, as in mam- 

 mals. X-rays (in the doses used) cause 

 a period of extreme infertility, which 

 commences soon after treatment and 

 later is partially recovered from. It 

 can be stated positively that the re- 

 turn of fertility does not mean that the 

 new crop of eggs is unaffected, for 

 these, like those mature eggs that man- 

 aged to survive, were found in the 

 present experiments to contain a high 

 proportion of mutant genes (chiefly 

 lethals, as usual). The practice, com- 

 mon in current X-ray therapy, of giv- 

 ing treatments that do not certainly 

 result in permanent sterilization, has 

 been defended chiefly on the ground 

 of a purely theoretical conception that 

 eggs produced after the return of fer- 

 tility must necessarily represent "un- 

 injured" tissue. As this presumption is 

 hereby demonstrated to be faulty it 

 would seem incumbent for medical 

 practice to be modified accordingly, 

 at least until genetically sound experi- 

 mentation upon mammals can be 

 shown to yield results of a decisively 

 negative character. Such work upon 

 mammals would involve a highly elab- 

 orate undertaking, as compared with 

 the above experiments on flies. 



