BENZER 



tances 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-Del- 

 briick correction 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 Pseiido-allelisif?.— The 

 functional relatedness of two closely 

 linked mutations causing similar de- 

 fects may be tested by constructing 

 diploid heterozv^gotes containing the 

 two mutations in different configura- 

 tions.^' - The cis form, with both mu- 

 tations in one chromosome, usually be- 

 haves as wild type, since the second 

 chromosome supplies an intact func- 

 tional unit (or units). However, the 

 trajis form, containing one of the mu- 

 tations in each chromosome, may or 

 may not produce the wild phenorvpe. 

 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 mu- 

 tants, the diploid heterozygote can be 

 simulated by a mixed infection with 

 two kinds of phage. The rll pheno- 

 type is a failure to lyse K, whereas the 

 wild phenotype is to cause lysis. If K 

 is mixed ly infected with wild type and 

 rll mutant, the cells lyse, liberating 

 both types of phage. Thus the pres- 

 ence of wild type in the cell supplies 

 the function which is defective in rll 

 type, and the rll mutation can be con- 

 sidered "recessive." Although it has 

 not yet been tested, the cis configura- 



277 



tion of double rll mutant plus wild 

 type is also presumed to produce lysis 

 in all cases. The tra?is configuration is 

 obtained by infecting K with the pair 

 of rll mutants in question. This is 

 found to give lysis or not, 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 

 segments. If both mutants belong to 

 the same segment, mixed infection of 

 K gives the mutant phenotype (very 

 few cells lyse). If the two mutants be- 

 long to different segments, extensive 

 lysis occurs with liberation of both in- 

 fecting types (and recombinants). 

 These results are summarized in Fig- 

 ure 3. Thus, on the basis of this test, 

 the two segments of the rll region 

 correspond to independent functional 

 units. 



octive 



active 



presumed 

 octive 



inoctive 



inactive 



active 



14 Visconti, N., and Delbriick, M., Ge- 

 netics }8:5-33, 1953. 



Fig. 3. Summary of tests for "position- 

 effect pseudo-allelism" of rll mutants. 

 Each diagram represents a diploid hetero- 

 zygote as simulated by mixed infection 

 of a bacterium (K) with two types of 

 phage containing the indicated mutations. 

 Active means extensive lysis of the 

 mixedly infected cells; inactive means 

 very little lysis. The dotted line repre- 

 sents a dividing point in the rll region, 

 the position of which is defined by these 

 results. 



Actuall\% for mixed infection of K 

 with two (non leaky) mutants of the 

 same segment, a very small proportion 

 of the cells do lyse and liberate wild 

 recombinants, that proportion increas- 

 ing with the linkage distance between 



