354 BACTERIOPHAGES 



tioning in the same cell. This is the genetic explanation for the 

 phenomenon of phenotypic mixing (Chapter XVI). If the 

 two mutations under study are in different phage particles, the 

 mixed infection simulates the trans heterozygote, and if the 

 mutant loci are in the same phage and the second phage is wild 

 type, the mixed infection simulates the cis heterozygote. 



The rll mutants of phage T4 have the phenotypic properties 

 of producing r plaques on host B and failure to lyse host K12, 

 although infection of this host does occur. The wild phenotype 

 is characterized by r+ plaques on host B and efficient lysis of host 

 K12. All the rll mutants are similar in these two properties 

 although they may differ in other characteristics such as map 

 location, transmission coefficient, and frequency of reverse 

 mutation. If host K12 is mixedly infected with two rll mutants 

 and efficient lysis occurs, one concludes that they are in different 

 functional groups. If the mixedly infected bacteria fail to lyse 

 it is concluded that the mutants are in the same functional 

 group. On the basis of this test Benzer (1955) concluded that 

 the rll region contained two independent functional units as 

 shown in Figure 12, Benzer (1957) proposed the name cistron 

 to describe the functional unit, defined on the basis of the cis- 

 trans test as that genetic region in which any two mutants in the 

 trans configuration fail to give the wild phenotype, but in the cis 

 configuration do give the wild phenotype. 



b. The Mutational Unit 



Most rll mutants can be located unequivocally with refer- 

 ence to other loci in the linkage group by recombination fre- 

 quency with neighboring mutants. However, Benzer (1955) 

 found certain anomalous rll mutants that failed to give wild 

 type recombinants with a number of closely linked mutants 

 extending over a definite span of the rll region, but gave normal 

 recombination with rll mutants outside that span. Such mutants 

 are indicated in Figure 12 by horizontal bars extending over the 

 region of no recombination. This observation suggests that a 



