342 BACTERIOPHAGES 



g. A Problem in Population Genetics 



On the basis of the observations described above, Visconti 

 and Delbriick (1953) developed a theory to explain quantita- 

 tively the frequencies of recoinbinants observed in various mixed 

 infection experiments involving phages T2 and T4. They made 

 the following specific assumptions that were consistent with the 

 available data. Phage infection involves the introduction of 

 phage DNA into the host cell and as a result mature phage is 

 converted into vegetative phage. Replication and recombina- 

 tion proceed in the pool of vegetative phage until interrupted by 

 lysis of the host cell. Phage is withdrawn from the vegetative 

 pool at the same rate that new phage is produced, and is con- 

 verted by an irreversible process into mature phage. Mature 

 phage particles do not replicate and do not mix genetically with 

 the vegetative phage pool. Within this pool the vegetative 

 phage particles mate pairwise and at random with respect to 

 partner, and each mating involves a genetically complete phage 

 particle. The authors considered two possibilities with respect 

 to order of mating, synchronous and random, and concluded that 

 random-in-time matings agreed better with data from experi- 

 ments designed to test this point. Genetic recombination in 

 phage is then a problem in population genetics in which the 

 vegetative phage pool is the population under consideration. 



On the basis of these assumptions the authors derived equa- 

 tions to describe the results of mixed infection experiments in- 

 volving parental phage particles differing by two or three genetic 

 factors. For instance, in the two-factor cross Tlhr'^ X Tlh'^r 

 the frequency among the progeny of the T2hr recombinant is 

 given by the equation 



a = (1 _/2)(i _ e~"''>)/A 



In this equation the parameter / is related to the genotype fre- 

 quencies of the infecting parents by the following expression : 



Frequency of majority parent = (1 + /)/2 



