SYLLABUS 129 



are those numbered ri, r/, and ri3. When each of these is crossed with 

 T2h, a different frequency of recombinants is obtained, as follows: 



Frequency of each recombinant 

 Tah X T2ri 15 % 



Tah X T2Y7 7 % 



T2h X T2ri3 0.8% 



These different frequencies with which recombinants turn up are 

 an expression of different degrees of linkage of the h locus with each 

 of the r loci, as will be explained more fully in the section on crosses 

 between different r mutants. 



Doennann (personal communication) has carried the analysis of 

 the h X ri3 cross one step further, by breaking up the mixedly infected 

 bacteria at a stage when the bacterium contains only relatively few 

 infective particles. The question that Doermann wanted to decide was 

 this: are the recombinants made by interactions stemming directly 

 from the infecting particles, or are they made principally by interactions 

 between their offspring? In the latter case, it should be expected that 

 the frequency of recombinants is exceedingly low when the bacterium 

 is opened at a time when few offspring have as yet been formed. Doer- 

 mann's results show very definitely that the recombinants are repre- 

 sented in the early offspring with the same frequency with which they 

 are represented in the final crop. This is perhaps the clearest evidence 

 we have for saying that the complete phage particles formed inside the 

 bacterium at an early stage are not themselves the parents of those 

 which appear later. 



We next turn to crosses between different r mutants of T2, say, the 

 cross between ri and ri3 (Hershey and Rotman, 1948). Here, too, four 

 types of particles result from the cross, namely, the two parental types 

 T2ri and T2ri3, and the two recombinant types, T2ri3 and T2 + +. 

 Here, however, the three types carrying one or more r mutations form 

 the same type of plaque. They are phenotypically alike. They can be 

 distinguished only by genetic tests, as follows: T2ri will give a certain 

 proportion of wild-type progeny in a cross with T2ri3, but not in a 

 cross v^th T2riri3, and of course also not in a cross with itself, which 

 amounts to a simple multiple infection with one type. Similarly, T2ri3 

 will give wild tj^pe progeny in a cross with T2ri, but not in crosses with 

 T2riri3, or with T2ri3. Finally, T2riri3 will not give wild type in any 

 of the three possible crosses. 



Such genetic tests are much more cumbersome than the simple 

 inspection of the plaque type which was sufficient in the h x r crosses 

 described in the previous section. In so far as they have been carried 



