BIOLOGICAL ORDER 



structure a+b+ corresponds to the "normal" gene z+, that is, to 

 the information for the synthesis of the enzyme. A gene is a 

 sequence of nucleic bases, and any nucleic base, whether in position 

 a or b, can be replaced by another (Table II). As a result, one 



Table II. The Activity of Normal and Mutated Genes. 

 "Normal" gene > active protein — normal enzyme 



Mutation I > active protein — modified enzyme (less potent) 



Mutation II > inactive protein — no enzymatic activity 



sequence of bases is replaced by another, and in the protein one amino 

 acid is replaced by another. The normal gene z+ produces the en- 

 zyme. The mutated gene z~ does not produce the original enzyme, 

 but instead a new, altered protein. It may have a slight enzymatic 

 activity, or be totally devoid of activity. An important fraction of 

 the negative mutants z~ nevertheless contains a protein able to 

 cross-react with the anti-/3-galactosidase antibody and to displace 

 ^-galactosidase when combined with the antibody. Some z~ 

 mutants give a complete cross-reaction; that is, they are able to 

 displace entirely the enzyme from the enzyme-antibody complex. 

 Some others give incomplete cross-reactions; that is, they can saturate 

 only a fraction (20 to 60 per 100) of the antibody. 



All this shows that the mutations affecting the gene z+ are mani- 

 fold. The proteins produced by the various z~ mutants differ from 

 the enzyme /3-galactosidase and differ also in each of the mutants. 

 So when the gene z+(a+b+), the normal structure, is present, the 

 enzyme is synthesized (Figure 17). When the mutated gene z~, 

 whether a~ or b~, is present, no enzyme is synthesized. Now the 

 mutants a~ and b~ are different. Let us then mix the various z~ mu- 

 tants. The male carries a gene a+b~ and the female a gene a~b + . 

 The zygote has two different sets of genes and is therefore called 

 heterozygote for gene z. Such a heterozygote is unable to manufac- 

 ture /?-galactosidase, despite the presence of all the information. In 



[44] 



