MULLER 



fifteen thousand per cent, in the muta- 

 tion rate over that in the untreated 

 germ cells. 



Regarding the t\'pes of mutations 

 produced, it was found that, as was to 

 have been expected both on theoretical 

 grounds and on the basis of the pre- 

 vious mutation studies of Altenburg 

 and the writer, the lethals (recessive 

 for the lethal effect, though some were 

 dominant for visible effects) greatly 

 outnumbered the non-lethals produc- 

 ing a visible morphological abnormal- 

 ity. There were some "semi-lethals" 

 also (defining these as mutants having 

 a viability ordinarily between about 

 0.5 per cent, and 10 per cent, of the 

 normal), but, fortunately for the use 

 of lethals as an index of mutation rate, 

 these were not nearly so numerous as 

 the lethals. The elusive class of "in- 

 visible" mutations that caused an even 

 lesser reduction of viability, not 

 readily confusable with lethals, ap- 

 peared larger than that of the semi- 

 lethals, but they were not subjected to 

 study. In addition, it was also possible 

 to obtain evidence in these experiments 

 for the first time, of the occurrence of 

 dominant lethal genetic changes, both 

 in the X and in the other chromo- 

 somes. Since the zygotes receiving 

 these never developed to maturity, 

 such lethals could not be detected in- 

 dividually, but their number was so 

 great that through egg counts and 

 effects on the sex ratio evidence could 

 be obtained of them e7i masse. It was 

 found that their numbers are of the 

 same order of magnitude as those of 

 the recessive lethals. The "partial ste- 

 rility" of treated males is, to an ap- 

 preciable extent at least, caused by 

 these dominant lethals. Another abun- 

 dant class of mutations not previously 

 recognized was found to be those 

 which, when heterozygous, cause ste- 

 rility but produce no detectable 



151 



change in appearance; these too occur 

 in numbers rather similar to those of 

 the recessive lethals, and they may 

 hereafter afford one of the readiest 

 indices of the general mutation rate, 

 when this is high. The sterility thus 

 caused, occurring as it does in the off- 

 spring of the treated individuals, is of 

 course a separate phenomenon from 

 the "partial sterility" of the treated in- 

 dividuals themselves, caused by the 

 dominant lethals. 



In the statement that the proportion 

 of "individually detectable mutations" 

 was about one seventh for the X, and 

 therefore nearly one half for all the 

 chromatin, only the recessive lethals 

 and semi-lethals and the "visible" mu- 

 tants were referred to. If the dominant 

 lethals, the dominant and recessive ste- 

 rility genes and the "invisible" genes 

 that merely reduce (or otherwise 

 affect) viability or fertility had been 

 taken into account, the percentage of 

 mutants given would have been far 

 higher, and it is accordingly evident 

 that in reality the great majority of 

 the treated sperm cells contained mu- 

 tations of some kind or other. It ap- 

 pears that the rate of gene mutation 

 after X-ray treatment is high enough, 

 in proportion to the total number of 

 genes, so that it will be practicable to 

 study it even in the case of individual 

 loci, in an attack on problems of allelo- 

 morphism, etc. 



Returning to a consideration of the 

 induced mutations that produced visi- 

 ble effects, it is to be noted that the 

 conditions of the present experiment 

 allowed the detection of many which 

 approached or overlapped the normal 

 type to such an extent that ordinarily 

 they would have escaped observation, 

 and definite evidence was thus ob- 

 tained of the relatively high frequency 

 of such changes here, as compared 

 with the more conspicuous ones. The 



