70 A. D. HERSHEY AND RAQUEL ROTMAN 



mixing of the cell contents is relatively incomplete, and the distance between 

 unlike clones relatively great, for small total populations. It has to be stipu- 

 lated further that the terminal mixing is independent of the final concentra- 

 tion of virus in the cell, to account for the lack of dependence of proportion 

 of recombinants on burst size. Some hypothesis of this sort may prove useful 

 if further experiments fail to strengthen the present evidence for reciprocal 

 exchange. 



It is notable that two very different lines of evidence, ours and that of 

 LuRiA (1947), have led to the idea of independently multiplying subunits of 

 the virus. Our results differ from Luria's only in calling for a system of linkage 

 superimposed on the set of independent units. It remains to be seen whether 

 a combination of genetic and radiological techniques bears out the present 

 conclusions, and perhaps leads to an identification of the radiation-sensitive 

 units with the linkage structures. 



SUMMARY 



Genetic recombination between two viruses differing by two mutational 

 steps has been studied by infecting bacteria with the pair, and counting the 

 numbers of the four types of virus found in yields from single bacteria. The 

 crosses so examined include hy.rl (unlinked), AX^Z (linked), and hXrl3 

 (closely linked), where h refers to a mutant of altered host range, and rl, r7, 

 and rl3 are different mutations producing the same alteration in type ol 

 plaque. The reverse crosses, ^rXwild type, were also studied. The results may 

 be summarized as follows. 



Nearly all mixedly infected bacteria yield both parental types of virus and 

 two recombinants, according to the scheme h-^r = h r-{--wi\d type. The ten 

 percent or so of bacteria yielding only one of the parental types seldom or 

 never yield any recombinants. The rest of the bacteria always yield two recom- 

 binants, except for the occasional absence of one or both in the crosses between 

 closely linked factors. 



The average yields of the two recombinants in any one cross are the same, 

 and are independent of the direction of exchange, so that reverse crosses in- 

 volving the same pair of mutant factors yield the same number of recombi- 

 nants. The proportionate yields of recombinants from individual bacteria are 

 independent of burst size, and of the total multiplicity of infection, but depend 

 on the relative yields of the two parental types. The effect of the latter is not 

 marked, however, and the variations from bacterium to bacterium must be 

 chiefly the result of variations in the number of genetic exchanges and in the 

 growth of recombinants subsequent to exchange. These variations may be de- 

 scribed by saying that one finds a moderately skewed distribution, with mode 

 less than the mean, and with mean and standard deviation dependent on the 

 linkage relations as follows: for hXrl, 15 + 6, for hXr7, 7 + 4, for hXrlJ, 1 ± 1 , 

 expressed in round numbers as percent of either recombinant in the total yield 

 of virus. 



A weak but moderately convincing correlation between the proportionate 



177 



