82 



fV^/^'^'t'^SP'^*?*!^?^'^?'?^-**'*'^^^'^^'*^^ 



CELL HEREDITY 



E. coli 

 B 



FIGURE 3.8. The appearance of plaques 

 of r and r bacteriophage multiplying 

 on a film of the bacterium, E. coli, strain 

 6. A plaque is the clear area formed by 

 the lysis (disintegration) of the bacterial 

 host. 



form small plaques with a fuzzy, slightly turbid margin (Figure 3.8). 

 Types of r mutants exist which are differentiated by their behavior on 

 strains other than B (Table 3.3). Mutants of class rl form large clear 

 plaques on all three tester strains; those of class rll are mutant on strain 

 B, wild type on strain S, and form no plaques on strain K; mutants of 

 class rlll form wild-type plaques on strains S and K. At the bottom of 

 Table 3.3 are shown the maps that would be found if studies were 

 restricted by the use of only one tester strain of bacterium. On strain 

 B all three mutants can be recognized as such and would be located on 

 the map; on strain S only the rl mutants could be located; on strain K, 

 rl mutants would be mapped as such but since a few rll mutants can 

 multiply on K to form minute plaques, these would be located at a site 

 probably called the minute locus. 



Beyond all this, the arbitrary choice of the wild type determines the 

 appearance of the map. If there were only one allelic state that gave 

 the rll"*" character and many rll alleles that could arise from it by muta- 

 tion, then there would be one rll^ locus on the genetic map if r"*" were 

 chosen as wild type. If, on the other hand, a particular rll allele were 

 chosen as wild type, then each new rll mutant found which could be 

 separated from the others by recombination would be given a separate, 

 albeit neighboring, locus. We will see in Chapter 6 that alleles can be 

 fragmented by recombination so this is a real possibility. But despite 

 the relativity of genetic maps, their construction in great detail is in 

 itself one of the most powerful methods for the analysis of the behavior 

 and fine structure of the chromosome and of the gene. 



