Only one-tenth of a single percent of DNA (about 3 million bases) differs from one person to the next. Scientists can use these variable regions to generate a DNA profile of an individual, using samples from blood, bone, hair, and other body tissues and products.
In criminal cases, this generally involves obtaining samples from crime-scene evidence and a suspect, extracting the DNA, and analyzing it for the presence of a set of specific regions of DNA (markers).
Scientists find the markers in a DNA sample by designing small pieces of DNA (probes) that will each seek out and bind to a complementary DNA sequence in the sample. A series of probes bound to a DNA sample creates a distinctive pattern for an individual. Forensic scientists compare these DNA profiles to determine whether the suspect's sample matches the evidence sample. A marker by itself usually is not unique to an individual; if, however, two DNA samples are alike at four or five regions, odds are great that the samples are from the same person.
If the sample profiles don't match, the person did not contribute the DNA at the crime scene.
If the patterns match, it means that the suspect may have contributed the evidence sample. While there is a chance that someone else has the same DNA profile for a particular probe set, the odds are exceedingly slim. The question is, How small do they have to be when conviction of the guilty or acquittal of the - innocent lies in the balance? Many judges consider this a matter for a jury to take into consideration along with other evidence in the case. Experts point out that using DNA forensic technology is far superior to eyewitness accounts, where the odds for correct identification are about 50:50. According to Eric Fischer (National Academy of Sciences), DNA-profiling reliability now falls between testing for blood groups and analyzing dermal (skin) fingerprints. The more probes used, the greater the odds for a unique pattern and against a coincidental match, but each additional probe adds greatly to the time and expense of testing. NRC2 recommends using four to six probes. Within a year, testing with several more probes will become routine, observed John Hicks (Alabama State Department of Forensic Services). He predicted that, within the next 3 to 5 years, DNA chip technology (in which thousands of short DNA sequences are embedded in a tiny chip) will enable much more rapid, inexpensive analysis using many more probes, raising the odds against coincidental matches.
Forensic scientists look forward to the day when DNA sequencing technologies have progressed to the point where direct characterization of very large DNA segments, and possibly even whole genomes, will become feasible and practical. Then base-by-base comparison of unique genomes finally will enable scientists to declare a perfect match.
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