BIOLOGICAL ORDER 



repressor is present. In the zygote type II, the female cytoplasm 

 is i~. This corresponds to constitutivity. No repressor is present, 

 and the enzyme is synthesized, thanks to the gene z+ introduced 

 by the male. But the male has introduced at the same time the 



phase I 



• :/?-galactosidase 

 X = repressor 



• X • 



HOURS 



Figure 19. Synthesis of /3-Galactosidase by a Heterozygote. 



The heterozygote of type II shown in the preceding figure has been formed 

 by the transfer of the male genes i+ z+ into the i~ z~ female. The female, being 

 constitutive (i~), is devoid of repressor, and ^-galactosidase is synthesized, in 

 the absence of an inducer, as soon as the gene z+ enters the female. This is 

 the first phase. But the male has introduced also the inductive gene i+: a 

 repressor is synthesized, and the synthesis of galactosidase stopped. This is the 

 second phase. If, however, an inducer is added, the synthesis of the enzyme 

 continues. 



gene i+, which, according to the hypothesis, should be responsible 

 for the manufacture of a repressor. Something seems to be wrong. 

 If, however, the synthesis of galactosidase is followed long enough, 

 it is then found that the rate of enzyme synthesis decreases and 

 reaches a zero value (Figure 19). A repressor has really been formed. 



[48] 



