130 BENZER ET AL. 



out by Hershey and Rotman for crosses between r mutants, the genetic 

 tests have given resuhs which are in full agreement with the rules 

 listed in the previous paragraph for h x r crosses. The crosses between 

 r mutants have been useful, moreover, in throwing a little more light on 

 linkage. Hershey and Rotman have isolated about 15 different r mu- 

 tants, and have determined the frequency with which the wild type 

 recombinant turns up in crosses between many of the possible pairs. 

 The principal point which seems clear from the analysis is this: if a 

 given pair yields a low frequency of wild type recombinants, this is not 

 due to a generahzed inability of either of these mutants to give any re- 

 combinations, because if either of them is crossed with a suitable third 

 mutant, a high frequency of recombinants can be obtained from each 

 of them. 



31. Mixed infections with unrelated strains. Interference and 

 mutual exclusion. — In the preceding sections we have discussed mixed 

 infection with exceedingly closely related phages. The two parental 

 types were derived from each other by one or two known mutational 

 steps. We now turn to the opposite extreme, mixed infections in which 

 the two parental types are as unrelated as possible, while yet permitting 

 infection of the same host bacterium. The two cases best analyzed are 

 those of mixed infection with Ti and T2 on the one hand (Delbriick 

 and Luria, 1942), and with Ti and T7 on the other hand (Delbriick, 

 1945). In both cases the two parental types are morphologically dis- 

 tinct and show no serological cross reaction whatsoever. The principal 

 result obtained in these cases is the phenomenon of mutual exclusion, 

 which means that any one bacterium will yield upon bursting either 

 one or the other of the two parental types, but never both. This is 

 shown most simply and convincingly by the use of a special trick, 

 namely plating of the mixedly infected bacteria on a mixture of two 

 indicator strains, one of which is sensitive to one of the parental types 

 and resistant to the other, and vice versa. On such plates, a clear plaque 

 can be formed only from an infected bacterium which, upon bursting, 

 yields at least one particle capable of lysing one of the indicator strains 

 and at least one particle capable of lysing the other indicator strain. 

 The frequency with which such clear plaques turn up is below 1%, 

 the limit of reliable observation. 



The principle of mutual exclusion has been verified for several 

 other cases of mixed infection with unrelated strains, and no exception 

 to this rule has yet been found. 



The state of exclusion of the other virus is sometimes, but not al- 

 ways, very quickly established after infection. For instance, we have 



