In the last fifteen years, DNA has played an increasingly important role in our legal system. Tissue evidence is now routinely collected during criminal investigations in hopes that it will provide genetic clues linking perpetrators to crimes. Similarly, lawyers commonly use DNA fingerprints to link suspected fathers to their biological children in paternity cases.

DNA fingerprints allow geneticists to determine whether two tissue samples came from the same individual or not. This genetic comparison is done not by looking at all of the DNA contained in a given set of tissue samples, but instead by marking a small number of segments of DNA and then checking for the presence or absence of those segments in each sample.

One of the most common techniques used for creating DNA fingerprints is called variable number tandem repeat (VNTR) analysis. This technique isolates only those DNA segments in a person's genome that have the same sequence of repeating letters (ATCATCATC, for example).

To do this, lab technicians isolate the segments from the rest of the DNA strand, using chemicals called restriction enzymes that cut each segment where the repeated sequence begins and ends. They then separate the repeating segments according to length in a process called gel electrophoresis. Next, they introduce a small set of radioactive "markers" to the sample. These markers are segments of DNA of known length that mirror the code of and bind to sample segments of the same length. The sample segment above (ATCATCATC), for example, would be tagged by a marker with the corresponding code, TAGTAGTAG. Markers that do not bind to sample segments are then rinsed away, leaving in place only those markers that bound to corresponding sample segments. Photographic film, which darkens when exposed to the radioactive markers, identifies the location of all marked sample segments. This film, then, becomes the DNA fingerprint that forensic specialists analyze.

Interpreting DNA fingerprints is a relatively simple matter of lining them up side by side and comparing them as to the presence or absence of different-length fragments . The more fragments the two samples have in common, the more likely it is that the samples came from the same person.