Vol. 41, 1955 GENETICS: S. BENZER 347 



rates, S can be used as a nondiscriminating host. The failure of rll mutants to 

 plate on K enables one to detect very small proportions of wild-type particles due 

 to reversion or due to recombination between different rll mutants. 



m r r tu 



42 47 51 41 



1 \mU 1— 



p HD% ^^ r^^\OT\ 



Fig. 1. — Partial linkage map of T4 (Doermann), indicating the location of the rll region, 

 m and tu designate "minute plaque" and "turbid plaque" mutations. The circular inset shows, 

 diagrammatically, the corresponding dimensions of the DNA chain magnified 1,000 diameters. 



Fate of rll Mutants in K. — Wild-type and rll mutants adsorb equally well to 

 strains S and K. Whereas the wild type provokes lysis and liberation of a burst of 

 progeny on both strains, the rll mutant grows normally only on S. Infection of K 

 with an rll mutant provokes very little (and/or very late) lysis, although all in- 

 fected cells are killed. The block in growth of rll mutant is associated with the 

 presence of the carried phage X. The reason for this association is unknown. 



Quantitative Differences in Phenotype. — While all rll mutants show the same 

 phenotypic effect of poor multiplication on K, they differ in the degree of this 

 effect. A certain proportion of K infected with rll actually liberates some prog- 

 eny, which can be detected by plating the infected cells on B. The fraction of 

 infected cells yielding progeny defines a "transmission coefficient" characteristic 

 of the mutant. The transmission coefficient is insensitive to the multiplicity of in- 

 fection but depends strongly upon the physiological state of the bacteria (K) and 

 upon temperature. Under given conditions, however, the coefficient can be 

 used as a com.parative index of degree of phenotypic effect, a "leaky" mutant having 

 a high coefficient. As can be seen in Table 2, a wide range of values is found. 



* Three parameters are given for each mutant. The map position is computed from the sum of the nearest 

 intervals shown in Figure 2 and is given in percentage recombination units, taking the position of r47 as zero. The 

 "transmission coefficient" is a measure of phenotypic effect determined by infecting bacteria K with the mutant 

 in question and is given as the fraction of such infected cells yielding plaques on strain B. The "reversion index" 

 is the average fraction of wild-type particles arising in lysates of the mutant grown from a small inoculum on a non- 

 selective host. 



Plaques on K. — Some rll mutants produce no plaques on K, even when as many 

 as 10* particles (as measured by plaque count on B) of a stock are plated. Other 

 rll mutants, however, produce various proportions of plaques on K. When the 



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