GENE MUTATIONS 431 



visible mutations as a group require at least three generations and then 

 recover only a portion of the total. The mutants can be recognized at a 

 glance, in contrast to the detailed examination, by highly trained ob- 

 servers, necessary when searching for mutations at all loci. 



Another advantage of the method lies in the fact that the type of 

 information obtained by it should be suitable for a meaningful compari- 

 son with results from Drosophila. Data on the rate of mutation of 

 visibles at all loci would, for example, be less favorable because the num- 

 ber of mutants detected is dependent on the minuteness of examination 

 of the anatomy and physiology of the organism, and it would be hard to 

 determine comparable levels for this in species as widely different as 

 fruit flies and mice. It is important to have data that can be compared 

 because the total information on mutation in mice is bound to lag be- 

 hind that for Drosophila. The estimating of human hazards will, for a 

 long time, be dependent on some extrapolation from fruit flies to men, 

 and this will be less conjectural if at least one meaningful cross-reference 

 point is established between Drosophila and a mammal. 



It is encouraging to know that, as work on specific loci in the mouse 

 proceeds, Muller and his collaborators are making large contributions 

 to data of this kind in Drosophila. In addition to increasing the reli- 

 ability of the Drosophila side of the comparison, these data will be of 

 help in evaluating the results obtained in the mouse. Thus, the extent 

 of variation found in the induced rates of the many loci being tested in 

 Drosophila will have a bearing on the confidence with which general 

 conclusions can be drawn from the average rate obtained for the seven 

 loci under examination in the mouse. 



Muller's data are also likely to prove of great aid in the planning of 

 further experiments on mice. The Drosophila results will include induced 

 mutation rates at the specific loci in eggs as well as in sperm, and in 

 immature as well as mature gametes. Observations are also being made 

 to determine what proportion of the mutations is associated wdth de- 

 tectable chromosomal aberrations. Information on some of these factors 

 may be obtained in the mouse, but, as it is a major task to collect 

 enough data for a reliable over-all rate, there can be only limited sub- 

 division of the data or repetition of the experiment under other condi- 

 tions. The Drosophila results on these and other factors should show 

 which are likely to be the important ones to test. 



It is too early yet to draw conclusions from the results being obtained 

 at Oak Ridge.* A pilot experiment was undertaken while the stocks of 

 mice were being built up to the size necessary for the large-scale program. 

 The findings from this experiment have settled many of the problems, 



* Russell, W. L., X-ray-induced mutations in mice, Cold Spring Harbor Symposia 

 Quant. Biol., XVI, 1951 (in press). 



