Figure 7.3f. Steps in RFLP genotyping. The "light bulb" carried by the probe is often a radioactive isotope. To make the DNA visible in this case, a sheet of photographic film is placed over the gel for a certain amount of time. (Again, some steps have been omitted here.) The radioactivity in the probe will expose the film while areas without radioactivity will not expose the film. When the negative is developed and viewed, there will be a characteristic set of dark bands that correspond to the areas of the gel on which the probe bound to its complementary sequence. The following figure illustrates what would be seen in this example.

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