RECOMBINATION IN BACTERIOPHAGE 63 



small proportion of recombinants, probably due to the appreciable number of 

 superimposed unmixed bursts. 



Table 8 



Single Bursts from the Cross hXr7 with Low Multiplicity of Infection 



See legend table 4. The proportion of recombinants have not been corrected for dispropor- 

 tionate yields of parental types of virus. Adsorption time one minute. Multiplicity 1.0 h and 0.5 r 

 per bacterium. 



The principal point to be made here is that the variation in yields of recom- 

 binants from tube to tube is not exceptional. In this connection it must be 

 mentioned that 5 of the 49 tubes contained h and r virus without any recom- 

 binants. Of these, one was exceptional in containing only 24 viral particles, and 

 another for the extreme disproportion of parental types (88 percent h). The 

 remaining three, each containing from 450 to 570 particles, are probably 

 superimposed unmixed h and r bursts. 



It should be noted also that the correlation between the yields of the two 

 recombinants in this set is not significant, since the observed correlation is 

 exaggerated by the tubes containing unmixed h and r bursts without recom- 

 binants. Whether the poor correlation is accidental, or an effect of the low 

 multiplicity of infection, remains to be determined. The negative correlation 

 between burst size and proportion of recombinants can, however, be ascribed 

 to the superimposed mixed and unmixed bursts, as well as to the unmixed 

 h and r bursts, in some of the tubes. 



IDENTITY OF RECOMBINANTS WITH THE CORRESPONDING ANCESTRAL TYPES 



According to any simple hypothesis of factorial recombination, one expects 

 the recombinant virus arising in crosses not to differ genetically from the cor- 

 responding ancestral type. Two kinds of test indicate that this is so. In the 

 first kind of test (Hershey and Rotman 1948) stocks of the phenotypic wild 

 type arising from the cross between two different r mutants were backcrossed 

 to authentic wild type. No r mutants appeared in such crosses, and it was 

 concluded that the stocks were genetically identical. 



The second kind of test is the following. The double mutant h r7, itself ob- 

 tained by crossing the two single mutants, was crossed with wild type and the 

 recombinants h and r were re-isolated. These were then tested by making the 

 homologous (parental h by recombinant h and parental r by recombinant r) 

 and heterologous (parental h by recombinant r and parental r by recombinant 

 h) back crosses. In both cases, the homologous cross yielded only one type of 



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