MUTATION AND PHENOTYPIC VARIATION IN PHAGES 317 



6. Summary 



Variation in phages can be divided into two broad classes, 

 hereditary or not, and hereditary variation can be further sub- 

 divided into two categories, mutation and genetic recombina- 

 tion. Genetic recombination between phages is a well-defined 

 phenomenon described in Chapter XVIII. Genetic recombina- 

 tion is micntioned here only when it threatens to be confused 

 with mutation, that is, when the initial genetic purity of a phage 

 clone cannot be guaranteed because of possible "contamination" 

 from the host. 



Nonheritable variation is most informative in two examples. 

 The first, called phenotypic mixing, calls for interactions be- 

 tween two related phages multiplying in the same host and, 

 specifically, for an interaction between two alleles of an h (host- 

 range) gene in such phages. The effect is to produce phage 

 particles differing in h phenotype and genotype, and phage par- 

 ticles of mixed h phenotype. Particles of mixed genotype 

 (heterozygotes. Chapter XVIII) are also produced, but this 

 probably has little to do with phenotypic mixing. 



The second example of nonheritable variation, called host- 

 induced modification, results from unknown causes by which 

 certain characteristics of the phenotype of an entire phage popu- 

 lation are determined by the bacteria in which it was produced. 

 The host range character is the principal one affected but, in 

 contrast to phenotypic mixing, not at the level of attachment 

 of phage to bacterium. Both physiological and genetic factors 

 in the bacterium can be recognized to play a role in this phe- 

 nomenon. The nature of the change in the phage is not known. 

 One instance of spontaneous phenotypic variation in phage has 

 also been described. 



Mutations in phages can be classified according to character 

 affected, as is done in this chapter, but this classification is of 

 little theoretical interest. More important is the separation into 

 expected and unexpected categories. The expected categories 

 can be dismissed as follows. They arise spontaneously during 

 phage growth and their frequency can be increased by a few 



