510 BACTERIOPHAGES 



[More recent investigations have shown that for a maximum re- 

 combination frequency the multiplicity of infection should be 

 equal and high for each phage type (around eight of each par- 

 ental type is recommended for T2). Dulbecco (1949b) has 

 shown that at least 10 phage particles can participate in growth 

 in a single cell. Adsorption should be performed under condi- 

 tions which permit initiation of phage development to be de- 

 layed (see p. 483) long enough to allow most of the late-reacting 

 phage to be adsorbed before exclusion occurs. If the total num- 

 ber of adsorbed phage is low or unequal, the frequency of ob- 

 served recombinants should be corrected as described by Lennox 

 et al. (1953).] 



Hershey and Rotman (1948), pursuing this line of research, 

 discovered that not all r mutations are genetically identical al- 

 though the plaques may be indistinguishable morphologically. 

 They isolated a number of independent r mutants of T2r+ and 

 numbered them in the order of isolation as T2r7, T2r2, etc. A 

 culture of B multiply infected with T2/-7 gave only r plaques, as 

 did one multiply infected with T2r2. However, if B were 

 mixedly infected with T2r7 and T2r2, about 15 per cent of the 

 progeny were T2r^. Further investigation of the r progeny 

 demonstrated that they were of 3 types, T2r7, T2r2, and T2r/r2. 

 These 3 forms are identical morphologically but can be distin- 

 guished by making mixed infections with the parental types. The 

 T2/-7 type would give 15 per cent r+ when tested by mixed 

 infection with a known T2r2 stock and no r+ when tested with a 

 known T2r7 stock. However, the T2r7r2 type would give 

 no r+ plaques on mixed infection with either T2r7 or T2r2. 

 On the basis of his analysis of the r mutants of T2, Hershey con- 

 cluded that there are at least 2 independent linkage groups of dis- 

 tinct r loci. 



[Benzer (1955) has developed a method for measuring very 

 small recombination frequencies between members of one of 

 these groups of r mutants (rll, the one containing r2). The 

 method is based on the fact that although the wild type phage can 

 grow on the lysogenic bacteria, Escherichia coli K-12 (X), the r 



