CONTROL BY REPRESSION 35 



system whose functioning has been profitably explored through 

 studies of dominance and position in mating experiments. In this 

 species, conjugation involves" the injection of a chromosome from a 

 male (Hfr) into a female (F~) cell and generally results in the for- 

 mation of an incomplete zvgote ( merozvgote ) ; and segregation of 

 recombinants from heteromerozygotes is sufficiently slow to allow 

 time for experimentation (Wollman et al., 1956). 



A revealing anahsis of the functioning of the i gene and the ex- 

 pression of inducibilitv in the /3-galactosidase system has been pre- 

 sented bv Pardee et al. ( 1958, 1959 ) . In very thoughtfully designed 

 conjugation experiments, they crossed a male of the genetic consti- 

 tution ;3+i+ to a female of the genetic constitution z~i~. Thus, the 

 male, but not the female, carried an intact "structural" allele for 

 enzvme svnthesis, and the male had an "inducible" allele, whereas 

 the female had a "constitutive" one. Additionally, the male had 

 markers for streptomvcin sensitivity and T6 phage sensitivity, and 

 the female had the corresponding "resistant" markers. It was there- 

 fore possible to use streptomycin and T6 phage to prevent induc- 

 tion of the male cells by any added inducer and to block remating. 

 The results of such a cross are shown in Fig. 2-5, which illustrates 

 the synthesis of the enzvme, in the absence or in the presence of 

 inducer, over a period of several hours. It will be seen that, in the 

 absence of inducer, enzyme svnthesis occurs over an interval of time 

 and then ceases. Addition of inducer at the point indicated brings 

 about continued enzvme synthesis. Accordingly, the initially con- 

 stitutive zygotes have become inducible. When z~i~ males are 

 crossed with r.+i+ females in the absence of inducer, no enzyme 

 synthesis is observed, even several hours after the mating. In other 

 words, the allele from the male is never expressed. The "inducible" 

 (i+) allele is, therefore, indicated to be dominant with respect to 

 the "constitutive" {i") allele. The authors concluded from the 

 kinetics of expression of the i+ character that the \ locus controls 

 the svnthesis of a specific substance that represses the formation of 

 /3-galactosidase, and that the constitutive state results from loss of 

 tlie capacity to make active repressor. The results of these experi- 

 ments also show that there is no appreciable mixing of the male and 

 female cvtoplasms during mating; the cytoplasm of the female deter- 

 mines initial inducibility or constitutivity. Cohn et al. (1960) have 

 studied tlie svnthesis of ^-galactosidase in Shigella dysenteriae fol- 



