308 BACTERIOPHAGES 



range of T2, now called T2 bar, is backcrossed with T2r (non- 

 bar), the yield (containing some r particles) is again entirely bar 

 in character. 



These results are remarkable in several respects. First, the 

 bar character, unlike the familiar genetic markers in these phages, 

 shows "spreading inheritance"; it does not segregate among the 

 offspring of bar by nonbar crosses. Second, it is associated with 

 a gross chemical difference in the DNA, which does not seem to 

 be referable to any antecedent mutational change (tests of this 

 possibility are still rudimentary, however). Third, the ability of 

 T4 to grow in K12 is associated with this gross chemical differ- 

 ence, which offers an opportunity to explore possible causes for 

 the restricted host range of T2. Fourth, the results suggest the 

 possibility of isolating bar mutants of T2, but this possibility has 

 not been adequately tested. Such a mutation would be of an 

 entirely new kind. Fifth, an obvious clue is offered to the nature 

 of one kind of exclusion between dissimilar phages (Chapter 

 XVII). 



Additional indications that anomalous types of mutation exist 

 are discussed in section 5 below. 



4. Mutation Frequency 



Phage mutations are believed to occur exclusively during 

 growth of phage (Luria, 1945a; Hershey, 1946b, 1953d; 

 Weigle, 1953; Jacob, 1954c; Latarjet, 1949, 1954). The rates 

 of natural mutation are not known with great exactness, but 

 probably vary at least over the range from 10~^ to 10~^° per locus 

 per generation. 



Luria (1945a) and Hershey (1946a) measured rates of h 

 mutation in Tl and T2. In T2 the measured rate lies between 

 10~^ and 10~^, which may be too high or ambiguous, depending 

 on how the locus is defined (Hershey, 1953d; Streisinger and 

 Franklin, 1956), and is almost certainly too low since phenotypic 

 mixing is neglected. 



Hershey (1946a, b) and Luria (1951) estimated rates of r 

 mutation in T2 at about 10""* per phage particle per generation. 



