WHAT IS A GENE? 159 



the basis of transduction frequencies and these subgroups have been 

 found to coincide with the biochemical properties of the mutants. For 

 example, the 77 tryptophane mutants fall into four groups: A, members 

 of which grow on either tryptophane, indole glycerol phosphate, indole, 

 or anthranilic acid; B, which grow on either tryptophane, indole glycerol 

 phosphate or indole and accumulate anthranilic acid; C, which grow on 

 either tryptophane or indole, but accumulate indole glycerol phosphate; 

 and D, which will grow only on tryptophane. All subgroups are linked 

 with a cystine B locus and seem to be located in the order cys B-.try A: 

 try B:try D:try C. 



This order can be confirmed by making three-point crosses via trans- 

 duction, in which a closely linked cystineless locus is utilized. Since the 

 nutritional deficiencies of members of the various groups differ, it is 

 possible to detect the various recombination classes. As an illustration 

 of the method, results obtained by employing try B-2 try D-11, and 

 cys B-18 are presented in Table 6.1. The size of the cys~ ind~ class 

 eliminates the order try D-.cys B-.try B, which would require four ex- 

 changes. In reciprocal experiments where phages from try D-11 

 cys B-18 bacteria were used to transduce try B-2 bacteria, the results 

 shown in Table 6.2 were obtained. The order try B:try D.cys B is 

 eliminated by the size of the cys~ class which requires a quadruple ex- 

 change. Thus we can conclude the order is try D-.try B-.cys B. By 

 means of similar experiments the order was found to be that shown in 

 Figure 6.3. 



The linkage order is here the same as the order of reaction sequences; 

 yet at least the functional units B, D and C are further subdivisible. 

 A more extensive subdivision of functional units is to be found among 

 the alleles for histidine synthesis, analyzed in a similar fashion and 

 arranged as shown in Figure 6.4. Once again, the functional units are 



TABLE 6.2 



Results of Transducing try B-2 Bacteria ^ith 



Phages Grown on Uy DA 1 cys B-18 Bacteria 



(From Demerec, 1956, in Mutation, Brookhaven 



National Laboratory Symposium, p. 75) 



