1240 BIOLOGICAL EFFECTS OF RADIATION 



genes differ in their stabilities. Demerec has shown (23) that for the 

 mutable miniature gene of Drosophila virilis, the effect of X-rays is very- 

 slight. Similar data have been obtained in Zea Mays by Stadler (166). 

 It will be remembered, moreover, that the rough information given 

 by the recurrence values of Table 1 indicates differences for some genes, 

 differences which point in the same direction. It would appear that 

 differences exist between the slopes of the dosage-effect carves of different 

 genes. Now the curves of the lethal-mutation rate under discussion 

 represent the sums of the curves of the genes composing the population. 

 Their shape must obviously depend on the distribution of the slopes of 

 the curves for individual genes. Both Stadler's and Demerec's data 

 agree in indicating a low value of the slope for frequently mutating genes, 

 which would tend toward a sigmoid curve for the whole population. 

 It may be remarked that the gap between the control value — which 

 should be the first point on the curve, but has not been so indicated in 

 Fig. 6 — and the first experimental value is considerable. Precisely this 

 region is, however, most important for defining the nature of the 

 curve. 



With the lethal mutations, Oliver (125) has attempted to show that 

 the distribution of lethals at different dosages is identical (Fig. 5). His 

 data are not sufficient to settle the question. Moreover, without tests 

 of the allelomorphism of the lethals, all that such tests can show is that the 

 total number of genes available for mutation in a given section of chromo- 

 some is constant at the different dosages. The individual genes within 

 the section, which still must constitute a large number, may still vary. 



Timofeeff-Ressovsky (185) has tabulated the ratio of lethal to viable 

 mutations at different dosages to distinguish "qualitatively" different 

 effects. The ratio is sensibly the same at all dosages, which he takes 

 to indicate that the type of mutation produced (visible or lethal) is 

 independent of dosage. This does not, of course, bear on the question 

 raised above, namely, the slope of the dosage-effect curve for individual 

 loci. A quantitative determination of the relation between lethals asso- 

 ciated with chromosome abnormalities and those which are not has been 

 attempted by Oliver (125). Both types appear to vary linearly with the 

 dosage, although whether the slopes are the same cannot be stated with 

 assurance. This comparison is vitiated to a great extent by the large 

 number of deficiencies undoubtedly present among the lethals not showing 

 any major reduction in crossing over (cf. Patterson, 138). Since these 

 are essentially small-scale chromosome aberrations, the comparison may 

 well involve an identity. Except for the two cases mentioned no data 

 are available for viable mutations where the question could be settled, 

 albeit with great labor. Neither has there been made a comparison 

 of the dosage-effect curves for lethals in the autosomes with those in 

 the X-chromosome, which is more feasible. 



