1214 



BIOLOGICAL EFFECTS OF RADIATION 



measured; or a particular locus is chosen, and frequency of its change 

 noted. In the first case, many different genes, with possibly different 

 rates of mutation, contribute to the total percentage. On the other 

 hand, if a particular locus is studied, the rate of mutation is so low even 

 at high X-ray dosages, that experiments are exceedingly laborious. 

 Moreover, there is no possible estimate of the frequency of undetected 

 mutations. In spite of this, it is clear that in order to interpret the data 

 from experiments where whole populations of genes are involved, the 

 characteristics of these populations must be known analytically. 



An exploratory survey can be provided by the enumeration of the 

 recurrences of mutants at given loci. The frequency with which any 

 gene appears in a group of mutations will be proportional to its own 

 mutation frequency. It follows then that a comparison of the numbers 

 of recurrences at different loci provides a measure of the relative muta- 

 bilities of these loci. This procedure is admittedly rough; it takes no 

 regard of differences in viability, or detectability of different pheno types; 

 moreover the data are themselves collected in a haphazard manner, 

 not for the specific purpose. But they do give a rough indication of 

 what is to be expected. 



Such a tabulation was provided for spontaneous mutations by 

 Morgan, Bridges, and Sturtevant (93). For the radiation experiments, 

 a comparable table can be constructed by using only the sex-linked 

 mutants arising in experiments where all these would have been observed 

 and recorded. The two groups are recorded in Table 1. For the 

 autosomes, no X-ray data are as yet published. 



It is clear that the recurrences of mutants at the different loci vary 

 in the same manner in both groups, generally speaking. Two conclu- 



Tablb 1. — Recurrences in Spontaneous and Induced Mutations in the 

 A'-Chromosome of Drosophila melanogaster 



Looiis 



Yellow 



Scute 



White 



Facet 



Echinus 



Ruby 



Crossveinless . 



Cut 



Singed 



Tan 



Locus 



Lozenge 



Ascutex 



Vermilion. . . . 

 Miniature. . . . 

 Furrowed . . . . 



Garnet 



Rudimentary. 



Forked 



Fused 



Spontane- 

 ous 



10 

 1 



15 

 7 

 2 

 5 



15 



12 

 2 



Induced 



2 

 2 

 4 



13 

 2 

 1 

 8 



17 

 3 



The data for the spontaneoiLS mutations are from .Morgan, Bridge-s, Sturtevant (93); the induced 

 mutations are taken from the papers of Gowen and Gay (52), Hanson and Winkelman (71), Oruneberg 

 (56), Dubinin (38), Serebrovsky and Dubinin (162), and unpublished data of the writer. These are 

 cases in which all viable sex -linked mutations could be detected, and were reported. 



