SYLLABUS 127 



If, among the infecting particles, the proportion of one type is 

 considerably reduced, some of the bursting bacteria, although adsorbing 

 both types, yield phage belonging to the majority type only. Quantita- 

 tive analysis of this phenomenon shows that a hmited number of phage 

 particles can take part in growth in one bacterium, this number being 

 of the order of 8 to 10 for phage T2 {limited participation, Dulbecco, 

 1949 a). 



30. Mixed infections with mutants carrying two genetic markers. 

 Recombination. Linkage. — We now turn to those mixed infections 

 which require for their interpretation the idea of genetic recombina- 

 tion. We will begin with the case investigated most recently because 

 it gives the clearest and the most detailed picture of the situation. This 

 is the case of a mixed infection of a bacterium with a host range mu- 

 tant (h) and a plaque mutant (r) of T2 (Hershey and Rotman, 1949). 



From such a mixed infection there results four types of particles, 

 two like the parental types and two recombinant types, designated as 

 rh and ++ (wild type). These four types can be distinguished when 

 platings are made on a mixture of B and B/2. (See photograph.) 



If yields of individual bacteria are examined, it is found that almost 

 every burst contains particles of each of the four types, which can be 

 separately enmnerated. Hershey and Rotman have done these experi- 

 ments on a large scale, with the following principal results: 



1 ) The average yields of the two parental types are alike. 



2) The average yields of the two recombinant types are alike. 



3) In individual bacteria the yields of the four types vary widely, 

 just as the total burst sizes do. 



4) The proportional yields of the four types show little or no cor- 

 relation with each other. That is, if the recombinant rh appears with a 

 higher than average frequency in an individual burst, this is not cor- 

 related with a higher or lower than average frequency of the other 

 recombinant type, ++, or with a higher or lower frequency of either of 

 the parental types. 



One qualification has to be made to the preceding point. If, in the 

 yield obtained from any given bacterium, the two parental types appear 

 with very unequal frequency, then the frequencies of both the recom- 

 binant types are lower than average. One should probably take the 

 disparity in the yields of the two parental tjrpes as indicating that the 

 parental type which appears with a low yield has had a late or other- 

 wise disadvantageous start in determining what is to go on inside the 



