350 



GENETICS: S. BENZER 



Proc. N. a. S. 



test, the two segments of the rll region correspond to independent functional 

 units. 



Actually, for mixed infection of K with two (nonleaky) mutants of the same seg- 

 ment, a very small proportion of the cells do lyse and liberate wild recombinants, 

 that proportion increasing with the linkage distance between the mutations. For 

 two rll mutants separated by 1 per cent linkage distance (measured by a standard 

 cross on B) the proportion of mixedly infected K yielding any wild particles is 

 about 0.2 per cent. 



This value has bearing upon the effect upon K/B values of the heterozygous phage par- 

 ticles which arise in a cross between two rll mutants on B. In such a cross between closely 

 linked rll mutants, the progeny should include about 2 per cent of particles containing a 

 trans configuration heterozygous piece. ^^ When one of these is plated on K, there is a cer- 

 tain chance that a wild recombinant may form in the first cycle of infection, leading to pro- 

 duction of a plaque. If it is assumed that these are no more likely to do so than a mixed 

 infection of K with two complete mutant particles, it can be concluded that the effect of 

 these heterozygous particles upon the count on K is negligible, provided that both rll mu- 

 tants belong to the same segment. For mutants in different segments, however, the "effi- 

 ciency" of the heterozygous particles should 

 be much greater, and recombination values 

 measured by the K/B method should run 

 considerably higher than the true values. 

 The recombination values in Figure 2 for 

 crosses which transgress the segmental 

 divide are probably subject to some correc- 

 tion for this reason. 



active 



-i— f- 



active 



» i I 



presumed 

 octive 



4+ 



-N- 



-4— t- 



-f-H 



inactive 



inactive 



active 



Fig. 3. — Summarj^ of tests for "position-effect 

 pseudo-allelism" of rll mutants. Each diagram 

 represents a diploid heterozygote as simulated by 

 mixed infection of a bacterium (K) with two 

 types of phage containing the indicated muta- 

 tions. Active means extensive lysis of the mix- 

 edly infected cells,- inactive means very little 

 lysis. The dotted line represents a dividing 

 point in the rll region, the position of which is 

 defined by these results. 



Rough Mapping hy Spot Test. — If a 

 stock of either of two rll mutants is 

 plated on K, no plaques arise; but if 

 both are plated together, some bacteria 

 become infected by both mutants and, 

 if this leads to the occurrence of wild- 

 type recombinants, plaques are pro- 

 duced. If the two mutants are such 

 that wild recombinants cannot arise between them (e.g., if they contain identical 

 mutations), no plaques appear. A given rll mutant may thus be tested against 

 several others on a single plate by first seeding the plate with K plus the mutant 

 in question (in the usual soft agar top layer) and then spotting with drops contain- 

 ing the other rll mutants. 



Inspection of such a plate immediately places the unknown mutant in the proper 

 segment, since spotting any mutant of segment A against any mutant of segment 

 B gives a very clear spot, due to the extensive lysis of mixedly infected bacteria. 

 However, for a pair of mutations belonging to the same segment, plaques are pro- 

 duced only by the relatively few mixedly infected bacteria which give rise to wild 

 recombinants. The greater the linkage distance between the mutations, the larger 

 the number of plaques that appear in the spot. A group of mutants of the same 

 segment may thus be seriated by seeding one plate with each and spotting with all 

 the others. Given a previously seriated group, a new mutant can thus be quickly 



215 



