The commonness of a given size of DNA fragment is called a population frequency. This figure illustrates the RFLP process. First, a restriction enzyme cuts the DNA strand into thousands of fragments of nearly all possible sizes. The fragments are then separated by size on a gel through the process known as electrophoresis. The DNA at this point is invisible in the gel unless it is stained with a dye.
Excess probe is washed off the blot, then the blot is laid onto X-ray film.
The radiation from the probes exposes the film, producing dark bands. For RFLP analysis to be reliable, all complex steps of the analysis must be carefully controlled. Because of the complexities of populations, databases must be interpreted with extreme care. For example, DNA fragment sizes rare in one population may be more common in other populations. Further, sub-populations or populations within populations must be considered.