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How DNA Profiling Works


Use of DNA Profiles in Law Enforcement
Touch DNA

DNA is often in the news, but one of the most recent stories included a new term: touch DNA. Although it's new to the media, touch DNA has been in existence for several years. DNA is usually recovered from bodily fluids such as blood and semen, which are often located by the stains they leave behind. Touch DNA involves recovering DNA from skin cells left by the perpetrator.

In the JonBenet Ramsey case, investigators scraped clothing that JonBenet had been wearing. There was enough evidence in two different places to create a DNA profile that matched one already created from blood -- both of which belong to a male not related to JonBenet. This convinced prosecutors that the Ramsey family could not have been responsible for JonBenet's death.

Once the profile is created, what's next? It really depends on how the DNA profile is to be used. If it was created from DNA recovered in a criminal investigation, prosecutors in the United States will enter it into CODIS, the Combined Data Index System. CODIS is a computer program maintained by the FBI, which operates databases across the country. These databases contain more than five million profiles. CODIS contains several different indexes:

  • The­ Offender Index contains the profiles of people convicted of various crimes. The crimes that result in inclusion in the Offender Index vary depending on the state, and they range from certain misdemeanors to sexual offenses and murder.
  • The Arrestee Index contains profiles of people arrested for committing specific violent felonies. The exact crimes also vary by state.
  • The Forensic Index contains profiles taken from crime-scene evidenc­e, including blood, saliva, semen and tissue.
  • The Missing Persons Index consists of two indexes: Unidentified Persons, which contains the profiles recovered from the remains of unidentified persons and Reference, which contains profiles of relatives of missing persons. These two indexes are periodically compared to each other to determine if a missing person's remains have been recovered.

    CODIS DNA profile
    Photo courtesy: Federal Bureau of Investigation (FBI)
    This is an example of a DNA profile in the FBI's CODIS database.

CODIS uses algorithms to compare 13 different STR locations, plus one that determines the gender of the person in question. It has rules and safeguards to protect the privacy of people whose profiles are in the database. The matching algorithms -- which must be confirmed by an analyst--can produce leads for law enforcement or even identify a potential assailant. The downside of using CODIS is that it's only as strong as the number of profiles included, and there is a backlog of more one million profiles to be entered.

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Prosecutors can also use DNA experts to match profiles while building cases where there's a high degree of certainty of the assailant. However, DNA profiling is being used more and more for people convicted prior to its common use, which began in the late 1980s. Since the early 1990s, convicted criminals have been able to use the latest DNA profiling technology as part of their appeals process. Most states have laws explicitly describing the rights convicted criminals have to DNA testing. In some cases, people can request additional testing anytime, while in others, they must do so within a few years of their conviction.

­Attention­ to post-conviction DNA testing really began with a 1996 National Institute of Justice report that spotlighted 28 people convicted of rape and murder who had been exonerated due to later DNA testing. Since 1989, more than 218 convicted criminals have been released after DNA testing proved their innocence. The true perpetrator was identified in 84 of those cases [source: The Innocence Project].