MUTATION AND PHENOTYPIC VARIATION IN PHAGES 303 



phage. As a result, wild type phages of this group tend to be of 

 the lysis-inhibiting variety (r+), but mutate with rather high 

 frequency to the r (rapidly-lysing) form, which does not produce 

 lysis-inhibition. 



Figure 9. PlaquesofT4 (Doermann strain) and some of its mutants. 1. Wild 

 type, showing one overlapping r mutant plaque. 2. Minute W241. 3. Turbid 

 halo tu44. 4. Wild type (turbid centers) and A41 (clear), from a plating of 

 the mixture on mixed indicator (B -\- K12/4). 5. The mutant r48. 6. The 

 double mutant r48?n41. 7. The double mutant /48tu44. Unpublished 

 photographs by A. H. Doermann. ^ 



The r mutations in T2 and T4 occur at many different genetic 

 loci (Hershey and Rotman, 1948; Doermann and Hill, 1953), 

 which accounts for the high over-all frequency of the mutations. 

 The expression of the r character depends on the bacterium; 



