BACTERIOPHAGE GENETICS 335 



infecting phage particle in a mixedly infected bacterium formed 

 its own vegetative pool. The efficiency of the recombination 

 process requires thorough mixing of genetic material from the 

 two kinds of parents. 



3. The Visconti-Delbruck Theory 



By 1953 a considerable body of quantitative data had accumu- 

 lated as a result of genetic studies with phages T2 and T4, and 

 the time was appropriate for the description of a theoretical 

 mechanism for genetic recombination in these phages. The 

 genetic studies had produced a number of facts with which any 

 proposed mechanism must be consistent. These facts as recog- 

 nized in 1953 will be briefly reviewed. 



a. Many Genetic Loci Are Available 



Of 15 independent T2r mutants tested by recombination, all 

 were identical phenotypically by plaque morphology, yet each 

 was distinguishable genetically. Mixed multiple infection with 

 any two mutant r stocks gave a proportion of wild type recom- 

 binants that was characteristic of the pair, whereas multiple 

 infection with any single r stock gave only r progeny (Hershey 

 and Rotman, 1 948, 1 949) . Each r mutant of independent origin 

 was given a serial number such as T2rl, T2/2, etc., as a distinc- 

 tive label. Two distinct host range loci were available for phage 

 T2 (Hershey and Davidson, 1951) and a plaque type mutant m 

 (minute) which is distinct from the r plaque mutations. An- 

 other kind of plaque type mutation called tu for turbid was re- 

 ported in phage T4 by Doermann and Hill (1953) as well as the 

 usual r mutations. A number of distinct tu and m loci are 

 known. 



h. Complementary Recombinants Are Formed 



In a cross such as T2rh^ X T2r^h one finds both kinds of re- 

 combinants among the progeny: T2rh and Tlr'^h^. In crosses 

 involving two distinct r mutants such as T2rl and T2r7, one 



