BACTERIOPHAGE GENETICS 295 



SO that they can be examined individually. In phage there is no way of exam- 

 ining the products of an individual mating without at the same time exam- 

 ining all the other phage particles which arose in the same bacterium. Since 

 in any bacterium there are hundreds of independent mating acts, it is difficult 

 to set up an experiment in which one can be sure of examining all the 

 products of a single mating act and avoid the confusion due to different 

 recombinants being produced in different single events. 



The problem was investigated in two ways, both of which were first used 

 by Hershey. The experiments involve a modification of the standard pro- 

 cedure of carrying out phage crosses in such a way that the observation is 

 made of the particles Hberated by individual, mixedly infected bacteria. The 

 cross itself is done as indicated in Section III, A; but before the infected cells 

 have time to break open and liberate their progeny phage, they are diluted 

 into many individual test tubes to such an extent that each tube has on the 

 average much less than one infected cell. In this way the probability that 

 any tube has more than one cell can be made as small as is necessary. After 

 the cells in this diluted state have been allowed time to produce their pro- 

 geny, the contents of each tube is examined. Most of the tubes will, of course, 

 contain no phage, while those which do contain any will, for the most part, 

 contain only particles liberated by a single cell. This single-burst technique 

 was originally introduced by Delbriick (1945), and Hershey and Rotman 

 used it to determine whether the reciprocal recombinants produced by single 

 cells were correlated with each other. The correlation coefficient calculated 

 between the per cent recombination of each of the two reciprocal types was 

 not found to be significantly different from that of uncorrelated events. 

 However, the meaning of this lack of correlation was not clear. Since many 

 mating events take place in the cell and since the various recombinants 

 could grow at unequal rates after they were formed, one cannot easily tell 

 whether the lack of correlation is due to the way in which the recombinants 

 are formed or to the events which take place subsequent to the process which 

 formed them. Another method of analyzing the single-burst data gives con- 

 siderably more convincing results. By using two very closely linked genetic 

 markers, one can obtain a situation in which many of the bursts produce 

 none of a particular recombinant; for these bursts one can ask whether the 

 other recombinant type was produced in its usual amoimt. In this experi- 

 ment, also, Hershey and Rotman found that the recombinants seemed to be 

 formed in independent events. It was suggested by Visconti and Delbriick 

 (1953) that this result could also be explained by subsequent growth of one 

 of the recombinants, coupled vnth. a loss of the genetic structures between 

 the time that they are formed in a pool where mating is possible and the time 

 they are ^vrapped in their protein coats and ready to infect another cell. 

 However, this explanation was made unlikely by Hershey's finding that the 



