BACTERIOPHAGE GENETICS 297 



and the r+ phage in the plaque, can be detected by direct observation. How- 

 ever, in order to detect particles which produce two different kinds of off- 

 spring for other genetic markers, it is necessary to allow the plaque to 

 develop, pick it, and retest the phage it contains. Since this process is ex- 

 tremely time-consuming and only a small number of mixed plaques are found, 

 most of the quantitative analyses have been done by selecting plaques which 

 have the observable motthng and testing them for other genetic markers 

 which were introduced in the original cross. 



Using the host-range marker h and various rapidly lysing r mutants, 

 Hershey and Chase established the following facts about the heterozygous 

 particles: If the r and the h which are used in the original mixed infection 

 were separated by at least twenty recombination units (i.e., the recombina- 

 tion frequency is at least 20 %), then most of the mottled plaques, picked 

 and tested for the phage they produced, were found to be pure with respect 

 to the h marker. In the cross hr 1 X h'^r'^ the mottled plaques would yield 

 either hr and /«r+ or A+r and /i"'"r+. These particles are designated hrlhr^ and 

 h'^rlh'^r'^, respectively. However, 6 % of the mottled plaques did yield both 

 h and /i+, i.e., they were produced by phage of the type hrjh^r'^ or hr'^lh^r. 

 If an r was used which was closely linked to the h, then the situation changed 

 considerably. With h and rl3, which are two recombination units apart, 

 75 % of the mottled plaques yielded the two parental types, one of which 

 contained h and the other /i+; the remaining 25 % yielded one parental type 

 and one recombinant, and so were pure with respect to the h marker. 



Using three-factor crosses, Levinthal (1954) showed that if a particle was 

 selected which was heterozygous for the middle of the three markers, that 

 is, it produced mixed offspring with respect to this marker, it was generally 

 recombinant with respect to the outer pair of markers, i.e., its genetic 

 information for the outer markers came from the two different parents. In 

 the cross abc X a'^b'^c^ particles selected as being 6/6+ were usually of the 

 form abc'^lab'^c'*' or «+ bcja^ 6+ c. Since for the markers used in this case there 

 was a considerable amount of recombination between a and c even if the 

 heterozygotes were not selected, it was only possible to show that at least 

 75 % of the particles heterozygous for the middle marker arose in structures 

 which were recombinant for the outer markers. Trautner (1957), making use 

 of the color mutants in Tl, was able to extend this conclusion considerably 

 and demonstrated that over 90 % of the particles in Tl which were heterozy- 

 gous for a central genetic marker were recombinant for the pair on either 

 side. 



These experiments, as well as others performed by Hershey using two r 

 markers in the original mixed infection, can be summarized pictorially in the 

 following way. We can assume that the phage genetic structure is ordinarily 

 composed of two parallel subunits, each of which contains the same genetic 



