In a criminal case, when the police or prosecutors indicate that they have a DNA profile match between evidence at the crime scene and the suspect or defendant, they suggest that this evidence conclusively establishes a connection between the suspect or defendant and the crime scene. In a criminal trial, the prosecutor might tell the jury that the DNA profile match is clearly proof beyond a reasonable doubt, throwing out statistics like a one in a billion chance the DNA could match someone other than the defendant. And jurors, who watch television shows that briefly and incompetently discuss DNA issues, typically believe in the strength and credibility of DNA evidence.
However, studies by DNA analysts indicate that DNA profile matches are not as conclusive as prosecutors would have everyone believe. There are 13 locations, or loci, on a chromosome that can be matched. The FBI says that when there is a DNA match of 9 of the 13 loci, the chances are 1 in 113,000,000,000 that the match is unreliable, or that the crime scene DNA might match someone other than the suspect (with the exception of relatives). If these odds are considered credible, it is compelling, if not irrefutable, evidence of guilt to a jury. However, DNA analysts have examined various state DNA databases and found many more profile matches than the 1 in 113,000,000,000 claim would suggest possible.
In Arizona, a DNA analyst was reviewing the state database and found two individuals whose chromosomes matched at 9 of 13 loci, according to an article at Sfgate.com. The FBI, of course, says the odds of this occurring are 1 in 113,000,000,000. In other words, it is practically impossible. The analysts continued her research and found dozens of similar DNA profile matches.
Criminal lawyers around the country learned of these results and requested similar studies of their states’ DNA databases, despite strong opposition from the FBI who tried to prevent these results from being release and anyone else from searching DNA databases for matches. Criminal defense attorneys argued that these results cast doubt on the strength and credibility of DNA evidence. No one was saying that DNA is not reliable. The argument was that since DNA evidence is often considered sufficient to make an airtight case (and criminal defendants are often convicted on DNA evidence alone), if there is evidence that DNA evidence may not be as strong as police and prosecutors suggest, criminal lawyers have a right to explore that possibility and argue a more accurate assessment of DNA evidence to a jury.
In two states, criminal defense attorneys were successful in inquiring into similar DNA matches in the state databases. In Illinois, there were 903 DNA profile matches at 9 or more loci out of a database of 220,000 profiles. In Maryland, there were 32 DNA profile matches at 9 or more loci out a database of 30,000 profiles. Those results may indicate that the odds that crime scene DNA matches someone other than the suspect or defendant are nowhere near as remote as the FBI and prosecutors say, and jurors may believe.