GENETICS OF BACTERIOPHAGE 



ing rise to the heterozygotes must differ by at least two genetic factors. Our ex- 

 periments have been hmited to differently linked pairs of h and r markers. The 

 experimental method is simple. One samples mottled plaques containmg the 

 segregants from heterozygotes and replates the phage progeny on mixed indi- 

 cator. The types of virus recognized in this way form the basis for classifying the 

 original heterozygote. 



By selecting mottled placjues, we limit the exammation to heterozygotes segre- 

 gating for r and r+. The analysis yields information only about the pattern of 

 segregation of the additional marker within this class. For any cross hr X wild- 

 type, only three results are found. 



TABLE 2. PER CENT DISTRIBUTION OF r-r+ HETEROZYGOTES WITH RESPECT 

 TO SEGREGATION PATTERN: CROSSES WITH EQUAL MULTIPLICITY OF 



INFECTION 



Per cent yields of recombinants in each cross are shown in parentheses. 



(1) Segregants h and hr, corresponding to one parent and one recombinant of 

 the original cross. Heterozygotes of this class lack the h^ marker, or lose it during 

 segregation. 



(2) Segregants r and wild-type, corresponding to the second parent and second 

 recombinant of the original cross. Heterozygotes of this class lack the h marker. 



(3) Segregants hr and wild-type, corresponding to the two parents of the origi- 

 nal cross, coming from doubly heterozygous phage particles. These segregants 

 are necessarily accompanied by their recombinants. 



Two qualitative results are evident. First, the heterozygotes segregate into 

 pairs. This shows that segregation precedes multiphcation, or that daughter 

 heterozygotes segregate in only one way. Second, the double heterozygotes do 

 not segregate into the two recombinants, h and r, but only into the two parents, 

 hr and wild-type. It should be noted, however, that these two alternatives could 

 not be distinguished in the cross hrl X wild-type, because of the large yields of re- 

 combinants in either case. 



The quantitative results are summarized in Table 2. The doubly heterozygous 

 class is a small minority in the crosses involving unlinked or distant markers, and 

 forms a surprisingly small majority even for the closely linked factors h and rl8. 



The inferences that can be drawn from Table 2 are supported also by tests on a 

 smaller scale of heterozygotes from the crosses h X rl and h X rl3. 



In view of the small proportion of doubly heterozygous particles produced in 

 the crosses hrl X wild-type, and hr7 X wild-type, there was some question 



185 



