282 



are widely scattered. An unambiguous 

 seriation of the mutants, with roughly 

 additive distances, can be accom- 

 plished, except for certain anomalous 

 cases. 



The simultaneous presence of a 

 wild-type phage particle in K enables 

 the multiplication of rll mutants to 

 proceed, apparently by supplying a 

 function in which the mutant is defi- 

 cient. A heterozygous diploid in the 

 trajis configuration is simulated bv a 

 mixed infection of K with two mu- 

 tant types. The application of the 

 phenotype test to pairs of rll mutants 

 leads to the division of the region into 

 two functionally separable segments. 



Spontaneous reversion to wild-type 

 had been observed for most of these 

 mutants. It remains to be seen whether 

 these are genuine reversions. Each mu- 

 tant reverts at a characteristic rate, but 

 the rates for different mutants differ 

 enormously. Partial reversions to in- 

 termediate types are also observed. 



BENZER 



The mutants differ greatly in de- 

 gree of residual ability to grow on K. 

 There is no evident correlation be- 

 tween map position, reversion rate, 

 and degree of residual activity of the 

 various mutants. 



The selective feature of K for wild- 

 type recombinants offers the possibil- 

 ity of extending the recombination 

 studies to an analysis of the fine details 

 of the region. 



Preliminary studies of this type in- 

 dicate that the units of recombination 

 are not larger than the order of one 

 dozen nucleotide pairs and that muta- 

 tions may involve various lengths of 

 "chromosome." 



I am much indebted to A. D. Her- 

 shey and A. H. Doermann for stocks 

 of their genetically mapped mutants, 

 to Sydney Brenner and David Krieg 

 for stimulating discussion, and to Max 

 Delbriick for his invaluable moderat- 

 ing influence. 



