BIOLOGICAL ORDER 



(c) The inductive bacteria may give rise to mutants that pro- 

 duce /?-galactosidase and permease in the absence of an inducer. 

 They are called constitutive; i+ being the symbol for the char- 

 acter controlling inductivity versus constitutivity, the mutation is 



i + 



i~, where i+ stands for inductivity, i~ for constitutivity 

 (Figure 15). 



\'^ = inductive 



i 



i+ 



i+ z+ y+ 



I I K I 



O = permease 



• = y5-galactosidase 



1 =constitutive 



No inducer present 



Figure 15. A4utation from Inductivity toward Constitutivity. 



The bacterium carrying the gene i+ is inductive: in the absence of inducer, it 

 manufactures neither permease nor /3-galactosidase, despite the presence of both 

 genes z+ and y+. If the gene i+ mutates toward constitutivity (i~), both 

 enzymes are synthesized in the absence of an inducer. 



These variations are hereditary. And as the substrate of heredity 

 in bacteria is the nucleic acid of the chromosome, it seems prob- 

 able that the three features, or characters — inductivity, synthesis of 

 /?-galactosidase, synthesis of permease — are each controlled by a 

 unit of genetic material, by a specific gene. That the substrate of 

 hereditary information is really located in the chromosome and is a 

 unit of function is easy to show, with the help of recombination. A 

 digression is necessary here. 



[42] 



