I have questions regarding "selection using insertional inactivation".
In a typical DNA recombination experiment using plasmid pBR322 where a new gene in inserted in the place of tet gene, it is said that entities with the recombinant plasmid can be selected by growing all the bacteria in two different medium: one with tetracycline and one with ampicillin.
I know that pBR322 contains two genes that make it resistant to both tetracycline and ampicillin, and also that the newly created plasmid will only be resistant to ampicillin, since its tet gene is no longer functional.
So in the plate with tetracycline, only entities with the original plasmid will grow. In the plate with ampicillin, however, entities with the original plasmid and the modified plasmid will grow.
How can this be a selection when two different types of entities are growing in one plate?
Am I misunderstanding the meaning of "selection" in this case?
My explanation above seems very immature, so it would be great if someone can provide me with deeper understanding.