212 ERNST FREESE 



thu^ to dctciininc in two- and tlircc-factor crosses the relative distance 

 and order of the mutations in tlic /•// i'c<j;i()n of the phage genome 

 (cliromosome). Benzer (1961) lias been able to niaj) a much larger num- 

 ber of mutants by using a set of rll mutants which contain a large 

 alteration within the rll region. Such a set is shown in Fig. 2. Each rll 



ht cM hr in rll Iv rl h m M M la In M 



" Ml ■! I — I ■ —Mnr-i I I I I 



■ Slondord 



I^ZZZ r 1272 

 BZZZH r 1241 

 r 4 3 

 r PTI 

 r PB242 

 r A 105 

 r 638 



S««in«n( Al A2 A3 A4 A3 A6 B 



Fig. 2. At the top, the rll region is shown compared with the entire genetic 

 map of the phage. This map is a composite of markers mapped in T4 and the 

 related i:)hage T2. Seven segments of the rll region are defined by a set of "dele- 

 tions" beginning at different points and extending to the right band (and possiblj' 

 beyond, as indicated by shading). (From Benzer, 1961, p. 404.) 



mutation can thus be rapidly located in one or more of the segments 

 demarcated by the alteration, if the mutants are crossed, by a simple 

 spot test, against each mutant in the set. This ingenious trick reduces 

 the amount of labor tremendously. A genetic map of spontaneous mutants 

 is shown in Fig. 3. The same method has been used to determine rapidly 

 the relative location of a large number of mutants induced by different 

 means and to decide which of them could recombine with each other and 

 which could not. Results of such examinations will be described in 

 this paper. 



D. FURTHER SUBDIVISION OF MUTATIONS 



In the attempt to subdivide mutations further, "chromosomal muta- 

 tions" and "gene mutations" have been defined. But the word "gene" 

 referred here to the older concept of a gene being at the same time the 

 functional, recombinational, and mutational unit. Recent examinations 

 of the genetic fine structure have proven that a functional unit contains 

 many mutational units and can l^o finely subdivided by recombination; 



