|MadSci Network: Molecular Biology|
The transformation efficiency, even with a nice plasmid and E. coli in good shape, is very small. Actually, most of the plasmid stays in solution and most of the bacteria remain non- transformed.
Let's use numbers, even if they might be wrong. Your have say, 109 input bacteria, and you end up the next day with about 100 colonies. So a single bug has about one chance in 107 to catch a plasmid molecule. So that's one chance in 1014 to catch two at the same time.
In other words, each of your resistant colonies has a chance in 107 to have a second plasmid. On every plate you have one chance in 10,000 to have a double-transformed bug.
[Not all of those double-transformants will be transformed with both an inserted-vector and a religand, so the chance of observing the particular double-transformant in question will be even lower, depending on the ratio of inserted to religanded vectors. -SM, Moderator]
Hope that helps!
Good luck with ongoing experiments.
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