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KAISER AND JACOB 



TABLE 3 

 Crosses X Clear X 434 Turbid 



Note: Progeny of crosses of the type X clear by 434 turbid, plated on C600, 

 C600(X), or C600(434) were scored for the number of clear and turbid plaques. 

 The clear mutants of X employed in these crosses were selected to mark different 

 regions of the c segment: C47, C30, Ci, and C50 to mark region I, C42 and coi to mark 

 region II, and C44 to mark region III. 



limits of recombination between X and 434 lie, on one side, between 

 C44 and C47 and, on the other side, between C50 and C42. The precise location 

 of the limits between these pairs of markers is suggested by the fre- 

 quencies of recombination with C44 and C42, i.e., 3.5% recombination to 

 the right of 44 and 0.54 % recombination to the left of C42. 



The recombination pattern in X X 434 crosses can be understood if it 

 is postulated that the Ci region controls immunity specificity, that there 

 is a Ci region specific for the immunity of X and a Ci region specific for 

 the immunity of 434. 



2. 434-^ Hybrid. Hybrid phages which have only the immunity de- 

 termining region of 434 and the rest of their chromosome from X were 

 isolated from 5 successive backcrosses of 434 to X. The first step was to 

 cross 434, which forms tiny turbid plaques, with Xci, which forms large 

 clear plaques. A few large turbid plaques were found among the progeny. 

 They were tested and all found to have the immunity of 434. The second 

 step was to cross one of the large turbid 434 to XC44 and to isolate a 434 C44 



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