GENETIC EFFECTS IN MAMMALS 851 



rate. It is, however, interesting to compare the results obtained for 

 dominants with the data on mutations at the specific loci. Two of the 

 dominants affected the tail, one affected the ears and possibly the coat 

 color, one produced white spotting and one mottling. Thus, only three 

 of the dominants affected the characters associated with the specific loci. 

 In the population in which these now established dominants were found, 

 thirty-two mutations were observed at the specific loci. It can be con- 

 cluded that, at least for the coat color and ears, the mutation rate to 

 dominant visibles at all loci is lower than the mutation rate to recessive 

 visibles at a total of only seven selected loci. 



It was mentioned by Russell (1951) that about one-half of the twenty- 

 eight induced mutations at the S locus showed, in addition to the recessive 

 spotting effect, a dominant reduction in body size. With the accumula- 

 tion of more data on body weights in descendant generations it now 

 appears that most, or perhaps all, of the *S-locus mutations will show this 

 effect to some extent. There is also increasing evidence for an associated 

 reduction in viability which would place these mutations under the 

 classification of dominant subvitals. In any case, it is apparent that the 

 mutation rate to dominants detected by a change at the S locus is much 

 higher than would have been anticipated from the over-all rate to 

 dominants affecting coat color, ears, and tail. It is possible that dominant 

 mutations affecting size are far more frequent than those causing coat 

 color changes or gross morphological abnormalities. It is also possible 

 that the >S-locus mutations fall in a special class. 



RECESSIVE LETHALS, SEMILETHALS, AND VIABLES 



The only data on the possible occurrence of radiation-induced sex- 

 linked recessives in mammals is that presented by Charles (1950) who 

 briefly reports two sex-linked lethals in 3072 offspring of male mice 

 exposed to a mean dose of 60 r of X rays and one in 2755 control animals. 

 The rest of this section is concerned with information obtained on auto- 

 somal recessives. 



Hertwig (1939, 1941) reports two recessive visibles, both causing 

 retarded growth, and four recessive lethals in the descendants of thirty-six 

 male offspring of presterile-period matings of male mice exposed to from 

 600 to 1200 r of X rays. Three visibles, one causing anemia, one oligo- 

 dactyly (both described in detail in Hertwig, 1942a, b), and one causing 

 dwarfism, were found in the descendants of eighty-two male offspring of 

 poststerile-period matings of irradiated males. The dose range, 800- 

 1600 r, is given only for the first fifty-eight (sixty ?) male offspring tested. 

 Two other possible lethals are reported as incompletely tested, but it is 

 not stated whether these came from presterile- or poststerile-period 

 matings. Two lethals were found in the descendants of seventy-two 



