Vol. 41, 1955 



GENETICS: S. BENZER 



349 



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-J21 



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 direct count of plaque types on B. 



In this way, a series of six rll mutants of T4 (the first six isolated — not selected 

 in any way) have been crossed with each other and with r47 and rol (kindly supplied 

 by A. H. Doermann) in 23 of the 28 possible pairs. The results of these crosses are 

 given in Figure 2 and are compatible with the indicated seriation of the mutants. 

 The distances are only roughly additive; there is some systematic deviation in the 

 sense that a long distance tends to be smaller than the sum of its component shorter 

 ones. Part of this discrepancy is accounted for by the Visconti-Delbruck correc- 

 tion for multiple rounds of mating.'^ Reversion rates were small enough to be 

 negligible in these crosses. Thus, while all rll mutants in this set fall into a small 

 portion of the phage linkage map, it is possible to seriate them unambiguously, 

 and their positions within the region are 

 well scattered. 



Tests for Pseudo-allelism. — The func- 

 tional relatedness of two closely linked 

 mutations causing similar defects may 

 be tested by constructing diploid heter- 

 ozygotes containing the two mutations 

 in different configurations.'- ^ The as 

 form, with both mutations in one 

 chromosome, usually behaves as wild 

 type, since the second chromosome sup- 

 plies an intact functional unit (or units). 

 However, the trans form, containhig 

 one of the mutations in each chromo- 

 some, may or may not produce the 

 wild phenotype. If it does, it is 

 concluded that the two mutations in 

 question are located in separate functional units. 



In applying this test to the rll mutants, the diploid heterozygote can be simulated 

 l)y a mixed infection with two kinds of phage. The rll phenotype is a failure to 

 lyse K, whereas the wild phenotype is to cause lysis. If K is mixedly infected with 

 wild type and rll mutant, the cells lyse, liberating both types of phage. Thus 

 the presence of wild type in the cell supplies the function which is defective in rll 

 type, and the rll mutation can be considered "recessive." Although it has not yet 

 been tested, the cis configuration of double rll mutant plus wild type is also pre- 

 sumed to produce lysis in all cases. The trans configuration is obtained by infecting 

 K with the pair of rll mutants in question. This is found to give lysis or not, de- 

 depending upon which rll mutants compose the pair. The results are summarized 

 by the dotted line in Figure 2, indicating a division of the rll region into two seg- 

 ments. If both mutants belong to the same segment, mixed infection of K gives 

 the mutant phenotype (very few cells lyse). If the two mutants belong to different 

 segments, extensive lysis occurs with liberation of both infecting types (and recom- 

 binants). These results are summarized in Figure 3. Thus, on the basis of this 



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

 tants, including Doermann's r47 and r51. 

 Newly isolated mutants are numbered starting 

 with /W. The recombination value (in per 

 cent) for each cross is obtained by plating the 

 progeny on K and on B and doubling the ratio 

 of plaque count on K to count on B. 



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