276 



is very little chance of introducing a 

 wild-type particle present in the 

 stock). If S is used as the host, both rll 

 mutant and any reversions which arise 

 can multiply with little selection, as 

 shown by control mixtures. The aver- 

 age fraction of wild-type particles 

 present in several lysates is an index 

 which can be shown to be roughly 

 proportional to the probability of re- 

 version per duphcation of the rll mu- 

 tant. Under the conditions of measure- 

 ment the index is of the order of 10- 

 20 times the probability of reversion 

 per duplication. The plaques appearing 

 on K must be tested by picking and 

 replating on B. This eliminates the 

 "spurious" plaques produced by par- 

 tial reversions and by leaky mutants, 

 which show up as r type on B. As may 

 be seen in Table 2, the reversion in- 

 dices for rll mutants vary over a very 

 wide range. One mutant has been 

 found which reverts 10 times more 

 frequently than r51, so that the re- 

 version rates cover a known range of 

 over 10-^-fold. 



It has not been proved that these 

 apparent reversions constitute a genu- 

 ine return to the original wild type. 

 However, the possibility of suppres- 

 sor mutations distant from the site of 

 the rll mutation has been ruled out by 

 backcrosses to the original wild type, 

 Krieg ^^ found very few, if any, r- 

 type recombinants in backcrosses of 

 several reversions, localizing the re- 

 verse changes to within a few tenths 

 of a per cent linkage distance from 

 the original rll mutations. One case of 

 "partial reversion" has also been tested 

 by backcrossing, and failure to observe 

 rll-type recombinants localized the 

 "partial reverse mutation" to within 

 the rll region. 



Mapping of the rll Region— A cross 

 between two rll mutants is made by 



BENZER 



infecting a culture of B with equal 

 multiplicities (three per bacterium) of 

 each type. The yield after lysis con- 

 tains the two parental types and, if the 

 parents are genetically distinct, two 

 recombinant types, the double mutant 

 and wild type. In the average yield 

 from many cells, the recombinant 

 types occur in equal numbers. ^^ In all 

 cases thus far tested, double rll mu- 

 tants, like single mutants, do not pro- 

 duce plaques on K. On the assumption 

 that this is generally true, the propor- 

 tion of recombinants in the yield can 

 be measured simply by doubling the 

 ratio of the plaque count on K (which 

 registers only the wild recombinant) 

 to the count on B (which registers all 

 types). The percentage of wild type 

 thus measured agrees well with a di- 

 rect count of plaque types on B. 



In this way, a series of six rll mu- 

 tants of T4 (the first six isolated— not 

 selected in any way) have been crossed 

 with each other and with r47 and r51 

 (kindly supplied by A. H. Doermann) 

 in 23 of the 28 possible pairs. The re- 

 sults of these crosses are given in Fig- 

 ure 2 and are compatible with the indi- 

 cated seriation of the mutants. The dis- 



^^^ 



BA 



321 



13 Krieg, D., personal communication. 



Fig. 2. Larger-scale map of eight rll mu- 

 tants, including Doermann's r47 and r51. 

 Newly isolated mutants are numbered 

 starting with 101. The recombination 

 value (in per cent) for each cross is ob- 

 tained bv plating the progeny on K and 

 on B and doubling the ratio of plaque 

 count on K to count on B. 



