Forensic Evidence: The Reliability of DNA Testing

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 What Is DNA Testing?

Although 99.9% of human DNA sequences are the same in every person in the world, there are still enough differences to allow one person to be distinguished from another. Using a method called DNA testing, also known as DNA profiling, scientists are able to analyze a long chain of DNA to identify specific “loci.” These loci are very similar when the loci of two closely related people are compared, but among people who are completely unrelated, the differences are much greater.

Thus, in criminal prosecutions, DNA evidence is often used to link a person to the scene of the crime and even to support the charge that they committed the crime. On the other hand, it can also be used by a defendant to prove their actual innocence.

The Innocence Project, a non-profit that uses DNA testing to exonerate people who have been wrongly convicted and promotes reforming the criminal justice system to prevent future injustice, has documented 375 criminal cases in which DNA analysis has resulted in the exoneration of a person who had been wrongly convicted of a crime. In some of these cases, the person had already served years of their sentence in prison. In fact, the average number of years that had been served by people who were proven innocent by the use of DNA testing was 14 years.

The pursuit of exoneration by convicted defendants who claim their innocence has led to proposals for the enactment of new laws in New York state, because currently people who claim their innocence can only get back into court if DNA evidence would prove their innocence. Apparently, this is feasible in only 10% of criminal cases.

How Reliable Are DNA Tests?

Courts have generally accepted the accuracy and value of DNA testing that is performed to professional standards. For example, courts have allowed prosecutors to search for suspects by interviewing people in the DNA database whose DNA is merely similar to a DNA sample found at the crime scene. In paternity cases, DNA profiling is usually decisive.

Crime shows on television represent DNA evidence as a highly reliable method of establishing the guilt of a criminal defendant. The reality, however, is somewhat less certain and more complicated. The complications originate in the fact that the usefulness of DNA depends on a number of factors. For example, the manner in which law enforcement collects, handles and tests a DNA sample affects its reliability. A DNA sample is only as reliable as the procedures used to preserve and test it. If these procedures are not careful and precise, or if they prioritize results over accuracy, then the DNA evidence that results may not be trustworthy and perhaps should not be relied on to convict a defendant of a crime.

DNA evidence is found and collected at the scene of a crime or later from evidence taken from the scene in the form of saliva, dead skin, blood stains, hair, semen and even tears. One question is whether there is enough of it to get a reliable test result. A sample can be damaged by exposure to heat or cold or other features of the environment in which it is kept. There can be a question as to the person whose sample it is. Does it belong to a suspected perpetrator or could it have come from someone who just happened to be in the vicinity?

However, exact probabilities of a match remain disputed. The FBI estimates that the odds of a match that results from a mere coincidence is only 1 in 108 trillion. Other estimates of the possibility of a random match are 1 in 113 billion, 1 in 10 billion, or 1 in 8192. To explain the variance, more and more loci are being discovered for use in matching. The bottom line is that the chance that a match would happen by chance alone is extremely low.

Recently, the California Supreme Court addressed a “cold hit” murder case – where DNA at the crime scene was matched with the DNA of a convict in the FBI database. The court allowed a “rarity statistic” to be communicated to a jury in a criminal trial. Specifically, the jury was told that there was only a 1 in 930 sextillion chance of finding the same DNA profile identified in a sample from a crime scene in the general population. In other words, it is extremely unlikely that a match of DNA samples results from mere chance.

In any event, all states and the FBI maintain DNA databases and all states permit the collection of a DNA sample from a person who is convicted of a crime for inclusion in DNA databases. Some states require defendants convicted of any felony to provide a DNA sample to the state, while other states, allow DNA collection only from defendants convicted of certain specific crimes, such as violent crimes, e.g. murder, rape, other sex crimes or burglary.

And many states allow their state law enforcement agencies to collect a DNA sample from any individual who is arrested or charged with a crime. Again, the goal is to build the state’s DNA database. In a few states, even people headed for parole or probation are required to provide DNA samples as a condition of release.

As mentioned above, the goal of all this collecting of DNA samples is to store them in DNA databases. The national DNA database in the U.S. is the National DNA Index System (NDIS). It contains DNA profiles that are submitted by participating federal, state, and local forensic laboratories. The FBI turns to the Combined DNA Index System (CODIS), a program used by the FBI to analyze DNA data. States store DNA samples in databases that are operated by either:

  • Private laboratories;
  • Government agencies tied to law enforcement, such as the Department of Justice and Department of Public Safety; or
  • Government agencies not tied to law enforcement, such as the Department of Health.

Some states automatically submit every DNA profile for entry in the national database, the NDIS. The laboratories that submit DNA data to NDIS must be accredited and submit to audits to show their compliance with FBI standards. Private laboratories do not qualify to submit DNA data to NDIS, because they lack these assurances of the quality of their work..

The fact is that all of this effort in connection with assuring quality in DNA sampling and testing and building of databases comes from the fact that DNA testing has proven to be an invaluable tool in criminal investigation and prosecution.

The statistics involved in molecular biology and DNA analysis is an expert topic that is beyond the grasp of the average juror. It can be challenging to simply grasp what the numbers tell us. Courts continue to deal with the issue of deciding which statistics should be admitted into evidence and communicated to a jury. In any event, a defendant who wants to contest a DNA match would need the services of an expert witness to mount a meaningful challenge to DNA evidence of their guilt.

Do I Need a Lawyer to Challenge the Reliability of a DNA Test?

If you have been charged with a crime, you want to consult an expert criminal defense attorney immediately. Your lawyer can tell you about your rights, analyze your case and help build a defense. Your lawyer can guide you through the process, explaining your options.

If DNA analysis is going to be a factor in your case, your attorney can help you challenge the DNA testimony, if necessary. A defense lawyer knows how to enlist expert witnesses to testify upon your behalf.

However if you are involved in a paternity case, a case about who is the legitimate father of a child, then you are a candidate for paternity DNA testing and a family law attorney can advise you.

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