Vol. 41, 1955 GENETICS: S. BENZER 351 



located within the group. This method works best for mutants which are stable 

 (i.e., low reversion rate) and nonleaky, so that large numbers of phage particles can 

 be plated. Reversions or pronounced leaking effects obviously cause an obscuring 

 background. 



This test has been applied to a large group of stable, nonleaky rll mutants. 

 Their approximate locations as deduced from these tests are shown in Figure 4. 

 Some of the mutants showed anomalies which made it impossible to locate them 

 as members of a series. They gave very little recombination with any of the mu- 

 tants located within a certain span, while behaving normally with respect to mu- 

 tants located outside that span. They are indicated in Figure 4 by horizontal 

 lines extending over the span. 



segment A 



I segment B v 



r r r r r r I r r 



47 104 101 103 105 106 I 51 102 



H |-HHtH lll|l I m il l I I I I I I IjiM iii ii i liii I 



I 



a b c d 



Fig. 4.— Preliminary locations of various rll mutants, based upon spot tests 



Spot tests on numerous other mutants have shown that mutants of varied re- 

 version rates, transmission coefficients, and rates of "partial reversion" occur at 

 scattered positions in both segments. 



Mapping of "MicroclustersJ'— The spot test enables us to pick out "mieroclus- 

 ters," i.e., groups of very closely neighboring mutations. Four such groups selected 

 for further study are indicated in Figure 4, and the results of mapping them are 

 given in Figure 5. While some intervals show reasonably good additivity proper- 

 ties, there are some mutants which give violently anomalous results. Thus in 

 mic'rocluster a, r47 gives no wild recombinants (i.e., less than 1 in 10«) with any of 

 the other three mutants, but two pairs of the three do show recombination. These 

 results can be understood if it is assumed that each mutation extends over a certain 

 length of the chromosome, and production of wild type requires recombination 

 within the space between those lengths. According to this interpretation, the 

 mutations would cover the lengths indicated by the bars in Figure 5. These anom- 

 alies resemble those observed in the spot tests, only they are more limited in 



span. . J J 



This observation raises the question of whether there exist true "pomt muta- 

 tions (i.e., involving an alteration of only one nucleotide pair) or whether all muta- 

 tions involve more or less long pieces of the chromosome. It must be remembered 

 that the mutants used in these experiments were selected for extreme stability 

 against reversion. This procedure would be expected to enrich the proportion of 

 mutants containing gross chromosomal alterations. So far as is known, the anom- 

 alous cases observed could equally well be imagined to be due to double (i.e., 

 two near-by "point") mutations, inversions, or deletions of the wild-type chromo- 

 some. In continuing these experiments, it would seem well advised to employ 

 only mutants for which some reversion is observed. 



216 



