BACTERIOPHAGE GENETICS 341 



will prevent an equilibrium mixture of recombinants being 

 obtained even if the vegetative pool can reach genetic equi- 

 libriuin. 



The equilibrium value of 0.5 for unlinked markers applies 

 only to bacteria infected with equal numbers of the two parental 

 types. The proportion of the two types in the vegetative pool 

 and in the phage progeny is presumably the same as in the 

 parental phage particles infecting a given bacterium. Any 

 disproportion in the infecting types will result in a decreased 

 opportunity for mating between unlike types, as pointed out by 

 Hershey and Rotman (1949) who developed a correction factor 

 applicable to the yields from single bursts. If a bacterium is 

 infected with m particles of one type and n particles of the other 

 type, the recombinant frequency would be proportional to the 

 factor Amn'{m + nY. When the average multiplicities of the 

 two infecting types are equal, the random adsorption of the 

 phage particles results in unequal multiplicities in individual 

 bacteria and this decreases the average recombinant frequency. 

 A correction for this effect is given by Lennox, Levinthal, and 

 Smith in an appendix to the paper by Levinthal and Visconti 

 (1953). The observed recombinant frequency should reach 

 about 90 per cent of the theoretical value at an average multi- 

 plicity of 5 for each infecting type. 



It is evident that some progeny phage particles from mixedly 

 infected bacteria descend from particles that had no opportunity 

 to mate with unlike particles, whereas other progeny phage 

 particles descend from vegetative phage lines in which more than 

 one mating with unlike particles occurred. The observed 

 recombinant frequencies are consistent with the assumption 

 that the act of mating is random with respect to type of partner. 

 As one consequence of repeated matings in the vegetative pool, 

 crosses made with unequal multiplicitbs of the two parental 

 types may yield a higher frequency of recombinants than of 

 minority parents when the markers are unlinked (Doermann, 

 1953). 



