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Traditional microbial genetics is based largely upon
observation of growth. Phenotypic variation has been
observed on the basis of a gene’s capacity to permit
growth under conditions of selection; eg, a bacterium
containing a gene that confers resistance to ampicillin can
be distinguished from a bacterium lacking the gene by its
growth in the presence of the antibiotic, which serves as
the agent of selection. Note that selection of the gene
requires its expression, which under appropriate conditions can be observed at the level of phenotype.
