THE PHENOTYPIG EXPRESSION OF GENES 

 DETERMINING VARIOUS TYPES OF DRUG 

 RESISTANCE FOLLOWING THEIR INHERIT- 

 ANCE BY SENSITIVE BACTERIA 



W. Hayes 



Department of Bacteriology, Postgraduate Medical School, Loudon 



When a gene in a bacterial cell is changed to an allelic state 

 by mutation, or is replaced by an allele during recombination 

 in parasexual systems, the altered genotype of the cell will 

 not immediately be expressed in a corresponding change of 

 phenotype. This delay of expression, known as phenotypic 

 lag, can theoretically be accounted for by the operation of 

 one or more of several circumstances. Firstly, genes can only 

 manifest their effects through the enzymic potentialities and 

 organization of the cytoplasm. The new gene finds itself 

 confronted with a cytoplasm adapted to the expression of the 

 allele which it has replaced. Thus, if the new gene determines 

 the synthesis of an enzyme of which the cell is devoid, at least 

 one molecule of enzyme must be created, and its dependent 

 synthesis initiated, before expression can occur. Alterna- 

 tively, if the new gene differs from its allele in being unable 

 to synthesize a particular enzyme, then its expression will be 

 delayed until the enzyme molecules already present in the 

 cytoplasm have been diluted out by successive divisions of 

 the cell. Secondly, bacteria are peculiar in that each cell 

 possesses, at least during the logarithmic phase of growth, 

 two or more nuclear analogues. The occurrence of a mutation 

 involves a gene in only one of these nuclei so that the mutant 

 cell initially contains one or more wild-type genes as well as 

 the mutant gene and therefore resembles a heterokaryon. If, 

 as appears usually to be the case, the mutant character is 

 recessive to the wild-type character, then the mutant gene 



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