BACTERIOPHAGE GENETICS 287 



transferred into a growth medium and left until the progeny are produced. 

 Except for the controls necessary to determine the multipUcity of infection, 

 these steps are the only ones necessary to carry out a cross. 



r"*" h"*" = wild 



. . single step 

 '^^ '^^ mutants 



double step 

 mutants 



Fig. 2. The mutational pattern of a T2 or T4 phage. K one starts with the wild or 

 standard, type, designated r+ h+, single-step mutants can be obtained either to h or 

 to any of the many genetically different r's. A second mutation can then produce the 

 rh type. However, the rh's produced by the two different pathways or on different 

 attempts will in general not be genetically identical. Although all of the steps are revers- 

 ible, the frequencies are extremely variable and the frequencies for the reversions are 

 very different from one mutation to the next. 



C. Two-factor Crosses 



If either a multiple or a single infection is carried out using only the phage 

 in one stock for the parental particles, no new types are found in the progeny 

 except for the occasional mutants. The same is true if two different phage 

 types mixedly infect a single ceU, as long as the two types differ with respect 

 to only a single mutation. If, however, two different types of mutant phage, 

 for example, an r and an h are used, then some particles appear in the progeny 

 which are like wild type in having neither the r nor the h property, and other 

 particles are formed which have both the r and the h (Fig. 2). These rh par- 

 ticles behave in the same way as those obtained by successive selection of the 

 double mutants. Likewise, if two stocks of r mutants which arose indepen- 

 dently are crossed in the same way, wild-type particles are also observed in 

 the progeny. However, the phage particles containing both the r mutations 

 can be identified only by the fact that if they are further crossed to either of 

 the parenteral r^s individually, no wild-type plaques are observed. On the 



