Although 99.9% of human DNA sequences are the same in every person in the world, there is still enough of a difference in order to distinguish one person from another. Using a method called DNA testing, also known as DNA profiling, scientists analyze a long chain of DNA to identify specific “loci.” These loci are very similar when you are comparing the loci of two closely related people, but among people unrelated, the differences are much greater. Thus, in criminal prosecutions, DNA evidence is often offered to link the accused with being at the scene of the crime, but it can also be used by the defendant to prove their actual innocence.
How Reliable Are DNA Tests?
Courts have accepted the overall accuracy and value of DNA testing. For example, courts have allowed prosecutors to search for suspects by interviewing people in the DNA database who have merely similar DNA to that found at the crime scene, indicating family members.
However, exact probabilities of a match remain disputed. The FBI estimates that the odds of a coincidental match are 1 in 108 trillion. Other estimates are 1 in 113 billion, 1 in 10 billion, or 1 in 8192.To explain the variance, more and more loci are being discovered. Another reason why there might be so much variance is that the DNA actually being analyzed is but a chemical replication of the original. Statistics may also take into account human error and the probability of obtaining an uncorrupted DNA sample.
Recently, the California Supreme Court addressed a “cold hit” murder case – where DNA at the crime scene was matched with a convict in the FBI database. The court allowed a “rarity statistic” to be told to the jury – that there was only a 1 in 930 sextillion chance of finding the same DNA profile in the general population.
The statistics of molecular biology is an expert topic beyond the knowledge of most jurors. Simply understanding what the numbers mean can be difficult. Courts continue to grapple with determining which statistics should be proper to admit into evidence.
Do I Need a Lawyer to Challenge the Reliability of a DNA Test?
If you have been accused of a crime, you should speak with a lawyer immediately. An experienced criminal defense attorney will inform you of your rights, help you with a defense, and advise you as to what options may be available to you. If you are challenging DNA testimony, a defense lawyer will also be important in obtaining expert witnesses to testify upon your behalf. However if you're involved in a court case about who is the legitimate father of a child, then you are a candidate for paternity DNA testing and the services of a family law attorney.