BACTEKIOPHAGE GENETICS 309 



ordered by this type of analysis, one can again conclude that the genetic map 

 must be a one-dimensional structure. The special and important character- 

 istic of this demonstration is that it does not in any way depend on a deter- 

 mination of recombination frequency, but only on the presence or absence of 

 recombinants in crosses between pairs of mutants. 



Fig. 6. Possible arrangements of deletions in a genetic map. If the map were two- 

 climensional, one should occasionally observe deletions which overlap as in (a). All of 

 the observed data are consistent with a model of the form (b). Any overlapping deletions 

 in either model mean that no recombinants are observed between the two mutants. 



C. Function Units 



During the course of these investigations, Benzer also found that the ill 

 mutants could be divided into two classes by a test which did not involve 

 genetic recombination but that seems to be based on the function of various 

 parts of the genetic material. If two different mutant phages are both 

 allowed to adsorb to a single K12 (A) cell, one sometimes finds the cell yielding 

 few or no progeny phage; with other pairs of mutants the cell yields an essen- 

 tially normal burst. It is found that all point mutants of the rll class can be 

 divided into two classes, A and B, such that any two members of the same 

 class, if infecting a single cell, will not yield a normal burst, whereas any two 

 mutants in different classes, infecting the samie cell, will yield a normal 

 burst. This method of dividing the mutants into the two classes obviously 

 does not involve the production of recombinants, since some pairs of mutants 

 which are in the same class are less closely linked than other pairs in which 

 the two mutants are in different classes. However, it is found that this 

 division into the two classes also corresponds to a separation on the genetic 

 map. All of those mutants which are in class A lie at one end of the genetic 

 map while all those in class B he at the other end of the map. Benzer called 

 these two regions "cistrons" by analogy with a cis-trans genetic test which is 

 used m higher organisms. 



The most direct interpretation of these findings appears to be that sug- 

 gested by Benzer. In this picture a cistron is a region of the genetic map 

 defined in terms of the test used to separate A mutants from B mutants. It 



