294 BACTERIOPHAGES 



b. Phenocopies 



The term phenocopy means, literally, a nonheritable variation 

 that mimics in phenotype a heritable variation. We use it here 

 to include all nonheritable variations that seem, like mutations, 

 to arise "spontaneously," that is, under conditions not subject to 

 experimental control. Since only one example has been studied 

 the name is not very important. 



Stocks of T5 and related phages contain about one particle 

 per thousand that is unusually resistant to thermal inactivation 

 (Lark and Adams, 1953, Chapter V). In this respect the ex- 

 ceptional particles resemble a much rarer mutant present in the 

 same stocks, but in the majority of the resistant particles the 

 resistance is not heritable. Remarkably enough, the number of 

 phenocopies per single cell yield of T5 is clonally distributed, 

 that is, most yields contain no resistant particles, some contain 

 only one or two, but a few contain 10 or more. Luria (1953b) 

 suggested a kind of pseudo-mutation to account for this result. 

 Probably an equally satisfactory interpretation would ascribe 

 the variation to differences in the conditions of growth in 

 different host cells (Adams, 1953a). In either case some sort of 

 host control seems to be called for, but it is clearly different from 

 that seen in typical host-controlled variations, in which every 

 particle in single bursts is affected. Different phages related to 

 T5 grown in different hosts yield,s similar proportions of heat 

 resistant particles (Adams, 1953a). 



c. Phenotypic Mixing 



The typical examples of host-controlled variation reflect an 

 interaction, probably indirect, between the genome of the modi- 

 fying host and that of the phage to produce phage particles of 

 modified phenotype. In mixed infection with related phages 

 one sometimes sees an interaction between the two phage gen- 

 omes to produce phage particles of modified phenotype. This 

 phenomenon is called phenotypic mixing and its resemblance to 

 host-controlled variation ends with the somewhat cryptic 

 analogy suggested by the above statements. 



