850 RADIATION BIOLOGY 



ingly useful as tools for genetic and cytological research. Snell (1946) 

 has attempted a genetic analysis of the frequencies of occurrence of the 

 nondisjunction classes of gametes found in two translocation lines and 

 has also obtained some information on the positions of the centromeres on 

 the two chromosomes involved in one of these lines. Slizynski (1952) has 

 begun the task of determining, in the mouse, which genetically deter- 

 mined linkage group corresponds to which cytologically identifiable 

 chromosome. 



DOMINANT VISIBLES 



No dominant mutations with externally visible effects were observed by 

 Snell (1935) in 178 offspring of male mice given a mean dose of 681 r of 

 X rays (numbers obtained from Snell, 1933b). However, autopsies in 

 the third generation revealed what proved to be a dominant mutation 

 affecting spleen shape. Penetrance was incomplete. Affected individ- 

 uals had markedly reduced vigor and somewhat subnormal fertility. 

 The autopsied F 3 mice were descended from ninety-one offspring of 

 treated males, but, as the descendant lines were not always large enough 

 to give near certainty of recovering even dominant mutations, the 

 mutation rate must be taken as one mutation in something less than 

 ninety-one sperm for a dose of approximately 700 r. 



Charles (1950) found seven dominant visible mutations, including 

 those detected only by autopsy, in 3072 offspring of male mice exposed to 

 a mean dose of 60 r. The difference from the controls, in which there 

 were no mutations in 2755 animals, is significant. Two of the mutant 

 phenotypes were characterized by abnormal connections of minor tribu- 

 taries of the vena cava, one had extra nodules of adrenal cortical tissue, 

 one showed incomplete suture of the parietal bones, one exhibited "con- 

 fused" behavior occasionally associated with cataract and deafness and 

 two had altered hair color, one of these also having reduced eye size and 

 lowered fertility. 



Five dominant visible mutations in approximately 30,000 offspring of 

 poststerile-period matings of male mice exposed to 600 r of X rays are 

 recorded by Russell (1951) in a preliminary account of an investigation 

 designed primarily for the detection of recessive mutations at seven 

 specific loci. The loci were chosen so that recessives could be detected 

 by rapid examination of the coat color and ears. Dominant mutations, 

 anywhere in the genome, that had effects on these characters were, there- 

 fore, automatically detected. In order not to interfere appreciably with 

 the rapidity of observation necessitated by the large number of animals 

 required for the determination of mutation rates at the specific loci, the 

 only additional character routinely observed for dominants was the tail. 

 For this reason, the mutation rate must be far below the over-all rate to 

 dominant visibles and is, therefore, of only limited value as an absolute 



