CHAPTER XVIII 



BACTERIOPHAGE GENETICS 



The mixed growth of two closely related phages in the same 

 host cell with genetic recombination was announced simul- 

 taneously by Hershey (1946a, b) and by Delbriick and Bailey 

 (1946). Hershey studied mixed infections with mutant strains 

 of phage T2, such as T2hr^ and T2h^r and obtained recombinant 

 strains such as Tlh'^r'^ and T2hr. Delbriick and Bailey used 

 related phage strains such as T2r and T4r"^, obtaining recom- 

 binant types such as T2r'^ and T4r. Because they were the first 

 phages to be used for genetic studies and because certain proper- 

 ties (highly efficient adsorption to the host cell and useful plaque 

 type and host range mutations) were suitable, phages T2 and 

 T4 have been more thoroughly studied and are better known 

 genetically than any other phages. Most of the important 

 phenomena of phage genetics have been discovered with this 

 pair of phages and so most of this chapter will be devoted to a 

 survey of papers on phage T2 and T4. 



1. Mixed Infection and Recombination 



Given two related phage strains that differ from each other 

 by at least two distinguishable hereditary characteristics, it is 

 possible to test the pair for genetic '-ecombination, providing 

 only "that a common host is available. Genetic recombination 

 has been found in every phage examined with the single excep- 

 tion of coliphage S13 (Zahler, Lennox, and Vatter, 1954). As 

 the list includes temperate and virulent phages, phages for both 

 gram-positive and gram-negative organisms, large phages and 

 small, it seems to be a fairly safe conclusion that genetic recom- 

 bination between closely related phages under conditions of 



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