BENZER 



Quantitative Differences in Phefio- 

 type.— While all rll mutants show the 

 same phenotvpic effect of poor multi- 

 plication on K, thev' differ in the de- 

 gree of this effect. A certain propor- 

 tion of K infected with rll actually 

 liberates some progeny, which can be 

 detected by plating the infected cells 

 on R. The fraction of infected cells 

 \'ieldincT progeny defines a "transmis- 

 sion 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 comparative 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. 



Table 2 



Properties of T4 Mutants of the 

 rll Group * 



Reversion 

 Index 

 Mutant Map Transmission (units 



Number Position Coefficient of IQ-**) 

 r47 0.03 <0.01 



* 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 recom- 

 bination 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 ques- 

 tion 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. 



275 



Plaques on /<".— Some rll mutants 

 produce no plaques on K, even when 

 as many as 10*^ particles (as measured 

 b\' plaque count on B) of a stock are 

 plated. Other rll mutants, however, 

 produce various proportions of 

 plaques on K. When the plaques ap- 

 pearing on K are picked and retested, 

 they fall into three categories: (1) a 

 type which, like the original mutant, 

 produces very few plaques on K and 

 r-type plaques on B; (2) a type which 

 produces plaques (often smaller than 

 wild type) on K with good efficiency 

 but r-type plaques on B; and (3) a 

 type indistinguishable from the orig- 

 inal wild. These three types are un- 

 derstood to be due to the following: 



D 



(1) "leaking" effects, i.e., ability of 

 the mutant to grow slightly on K, so 

 that there is a chance for a few visible 

 plaques to form; (2) a mutation which 

 partially undoes the effect of the rll 

 mutation, so that multiplication in K 

 is possible, but the full wild phenotype 

 is not achieved; and (3) apparent re- 

 verse mutation, which may or may not 

 be genuine, to the original wild type. 



The proportion of each type occur- 

 ring in a stock is characteristic and re- 

 producible for a particular rll mutant 

 but differs enormously from one rll 

 mutant to another. There is no evident 

 correlation in the rates of occurrence 

 of the three tv^pes. 



Reversion Rates of rll Ahitants — 

 Reversion of r mutants to a form indis- 

 tinguishable from wild type was dem- 

 onstrated by Hershev,^ who made use 

 of the selective advantage of wild type 

 on B to enrich its proportion in serial 

 transfers. Given the inability of rll 

 mutants to produce plaques on K, such 

 reversions are easily detected, even in 

 very small proportion. An index to the 

 frequency of reversion of a particular 

 rll mutant can be obtained by prepar- 

 ing a Ivsate from a small inoculum 

 (about 100 particles, say, so that there 



