The Code is almost universal. Mutations can occur at different sites within a gene. The test to determine whether two mutations reside in the same gene is the trans complementation test. The complementation test provides the operational definition of a gene. Straight genetic experiments can reveal a lot about the molecular nature of the gene.
Bacteria have several ways of exchanging genetic information with one another.
Episomes can mobilize genetic markers so that they can travel from one bacterium to another. Fine structure mapping can be done with bacteria. Sexduction produces partial diploids that can be used in complementation and dominance studies. It is possible to prepare large, chemically pure preparations of single genes or parts of genes. These can be analyzed down to the level of the nucleotide sequence.