GENETIC EFFECTS IN MAMMALS 



853 



the seven loci is (25.0 + 3.7) X 10 -8 per roentgen, per locus. The 

 observed variation in induced rates at the different loci is significantly 

 greater than chance variation. 



The characteristics of the mutations are of interest from several points 

 of view. It has already been mentioned that most, or all, of the $-locus 

 mutations have, in addition to the recessive spotting effect, a dominant 

 effect causing reduction in body size and possibly lowered viability. No 

 marked dominant effects have been observed for the other mutations, but 



Table 12-10. X-ray-induced Mutations at Specific Loci in the Mouse 



(Russell, 1951) 



a See text for explanation. 



6 Two of the four mutations may be a cluster from a single mutation. 

 c In the pilot experiment, mutations at the S locus were scored on a smaller sample 

 of animals than that on which mutations at the other loci were determined. 



Table 12-11. Viability of homozygotes of X-ray-induced Mutations at Specific 



Loci in the Mouse 



a Other mutations at this locus that have been partially tested indicate a consider- 

 able addition to the lethal category. 



the possibility of a slight reduction in viability has not yet been excluded. 

 Three of the induced mutations, one at the B locus, one at the C locus and 

 one at the P locus, were to intermediate alleles. The effect of the muta- 

 tions in homozygous condition is still being tested. The available 

 information, which includes some not published before, is given in 

 Table 12-11. The mutations listed do not constitute a random sample 

 because their inclusion depended on the speed with which the test could 

 be completed and this, in turn, was affected by the nature of the mutation 

 and the locus involved. When all the mutations have been tested, the 

 proportion of viables in the total may turn out to be lower than that in 



