MUTATION AND PHENOTYPIC VARIATION IN PHAGES 299 



species (Girard, 1943; Flu and Flu, 1946; Lazarus and Gunni- 

 son, 1947). 



In T2, h mutations can occur at a number of widely separated 

 genetic loci (Hershey and Davidson, 1951 ; Baylor, Hurst, Allen, 

 and Bertani, 1957). Presumably these correspond to physi- 

 ologically different processes concerned with the organization of 

 tail structure in the phage. Streisinger and Franklin (1956) 

 studied the genetic structure of one of the regions controlling 

 specificity of adsorption in T2. They find that different muta- 

 tions tend to occur at a number of different sites, which prove to 

 be closely linked but separable by genetic recombination. Re- 

 versions to the wild phenotype, however, tend to occur at the 

 site of the original mutation. One can nevertheless recognize 

 several different phenotypes, differing in sensitivity to heat and 

 in other ways, among such reverse mutants. 



Remarkably enough, as already mentioned, the two-fold 

 difference in host range between T2 and T4 seems to be due to 

 genetic changes at a single locus. The same locus accounts 

 for the difference in serological specificity (Streisinger, 1956b). 



In addition to the examples cited host range mutations have 

 been described by Shwartzman (1927), Sertic (1929b), Sertic 

 and Gough (1930), Luria (1945a, b), Hershey (1946b), Wahl and 

 Blum-Emerique (1950, 1952a), Bresch (1953), Fraser and Dul- 

 becco (1953), and Appleyard, McGregor, and Baird (1956). 

 Probably all of these are mutations affecting specificity of ad- 

 sorption. 



b. Mutations Affecting Growth Potential 



We discuss here all those mutations affecting host range that 

 do not influence specificity of adsorption. In principle these 

 could belong to many different classes, but only a few are recog- 

 nized. For instance, no mutations are known to affect specifi- 

 cally the injection mechanism. The h mutation in Tl is a some- 

 what borderline case (Chapter X). The mutant can infect B/1 . 

 The wild type can adsorb to B/1 but the adsorption is reversible 



