344 BACTERIOPHAGES 



matured in these experiments at the rate of about 1 particles 

 per minute. If one assumes that the vegetative phage pool 

 remains constant at a size of about 30 particles, then each par- 

 ticle can mate and duplicate every 2 minutes to give the observed 

 rates of genetic recombination and phage maturation. This 

 estimate of the size of the vegetative phage pool is in reasonable 

 agreement with the size of the pool (50 to 100 phage equivalents) 

 estimated by Hershey (1953a) from purely chemical evidence. 

 The experiments of Levinthal and Visconti (1953) are thus con- 

 sistent with the possibility that mating is a prelude to phage 

 duplication but of course they do not prove the essentiality of 

 such a relationship. 



4. Heterozygosis 



Although the Visconti-Delbriick theory is an eminently satis- 

 factory description of the kinetics of genetic recombination in 

 bacteriophages it does not attempt to offer a mechanism for 

 genetic recombination, and in particular it does not explain the 

 occurrence of heterozygous phage particles. A detailed de- 

 scription of heterozygous phage particles was given by Hershey 

 and Chase (1951) who discovered them. If bacteria are mixedly 

 infected with r and r+ phage particles and plated before lysis 

 each mixed yielding bacterium will produce a mottled plaque. 

 The mottled appearance is the result of lysis inhibition in some 

 regions of the plaque where r^ phage particles are multiplying 

 and complete lysis in other regions where r particles are growing. 

 A mottled plaque is produced whenever both r and r+ phages 

 grow together, a fact that can be checked by sampling the 

 plaque and demonstrating that both kinds of phage are present. 



Hershey and Chase noted that lysates of bacteria mixedly 

 infected with T2r and T2r+ phage particles contained about 2 

 per cent of aberrant phage particles that produced mottled 

 plaques. These plaques contained r and /+ phage particles 

 indicating that the aberrant phage particles segregated to pro- 

 duce both r and r+ progeny, hence they were heterozygous for 

 the r locus. 



