Some truths are axiomatic. They’re ideas we hold as being so solidly true, we never bother to examine them too deeply.

The concept of “DNA evidence” as the fool-proof smoking gun is one of these axioms. We understand that your personal DNA encoding is responsible for determining your sex, hair color, eye color and even your risk of having a heart attack. We know that, when a person interacts with a crime scene, there’s a distinct possibility that they may leave behind substances from which DNA can be sampled and a match found. There’s even new technology aimed at potentially reconstructing your face from DNA, to help police search you out.

But is DNA evidence presented in court really so iron-clad? What exactly constitutes a “match” by forensic standards, and how close can that match be to the true criminal? These questions were put to the test just recently in the appeals trial for Howard Wright, charged with the 1995 rape and murder of Patricia Daggett. The conviction was ultimately upheld on a 3-2 vote, but the two judges that dissented point out that the DNA evidence was nowhere near as conclusive as they contend then Assistant District Attorney Sandra Doorley made it seem. Indeed, based on court documents, it very clearly was not. The dissent reads in part:

As the majority notes, the People “presented evidence that . . . defendant’s DNA could not be excluded from various pieces of evidence recovered [from the victim’s vehicle].” At trial, the People’s forensic expert, who analyzed defendant’s DNA sample, described the two types of DNA testing used in this case—mitochondrial DNA analysis and YSTR DNA analysis…

The People’s forensic expert acknowledged the two above-mentioned types of DNA analysis at trial, but she did not speak at length about a third type of DNA analysis—autosomal..

How could an analysis of DNA evidence not point solidly to a single individual? The dissent notes that the sample could not have ruled out the victim’s husband, nor could it rule out the second alleged accomplice in the crime, Christopher Gifford. And what are these three types of DNA testing?

The DNA of DNA forensic testing

To unravel this mystery, we first need to understand that not all DNA in your body is the same. There are two types: nucleic DNA and mitochondrial DNA.

Nucleic DNA is a combination of information from your genetic parents, which is passed to you, and is responsible for the you-building that we all understand. Mitochondrial DNA is passed more or less unchanged from your genetic mother to you. Mitochondrial DNA does not contain enough information to be used to pinpoint a specific individual, but can be used to broadly exclude individuals who simply do not match.

That leaves us with nucleic DNA. But here again, there are two different types of testing, YSTR and autosomal. YSTR tests are performed on the Y chromosome and therefore only possible in male individuals. But since the Y chromosome is passed intact from father to son, all individuals of a specific paternal line – which could potentially be quite vast – will have similar or exactly the same YSTR results.

Autosomal DNA testing is the closest thing in the forensic arsenal to the “smoking gun.” In autosomal testing, 22 out of the 23 pairs of chromosomes that make you what you are can be tested. The 23rd pair is, of course, the sex pair for which YSTR is used. Here is the evidence which sometimes boasts a 1-in-a-billion accuracy.

Then what of Mr. Wright’s sentence?

Regardless of which method of testing is used, the results boil down to a statistical analysis and professional judgement of the facts of the case. And none of these types of evidence is easy to get in the first place. DNA like all living material is delicate and easily destroyed at the scene of the crime, by investigators or even by the lab.

Mitochondrial and YSTR testing can certainly be used to exclude a suspect from consideration, assuming that their DNA signatures differ significantly enough. But they don’t do much to identify a specific suspect, especially as in this case, where multiple persons involved in the crime all fit the same basic profile. Only autosomal DNA testing can positively identify a single individual, but that requires a working, undamaged sample of all 22 required pairs.

The dissent for Mr. Wright’s appeal says that the People’s forensic expert never spoke “at length” about autosomal DNA evidence. Was that because that evidence was not conclusive, or not possible to obtain? Nobody says.

Other evidence presented in the case, as recounted by the appeal ruling, is deeply suspicious but circumstantial. Wright was seen in Daggett’s car with her prior to the murder; the prosecution proved that Daggett was raped in the car; eye witnesses saw Wright in Daggett’s car the next day. None of these facts would have been usable for a conviction on their own, without DNA evidence. But the DNA analysis was itself dubious and inconclusive: it implicates three potential suspects, one of whom was never on trial.

The real problem here is not whether the DNA matched one individual suspect. The real problem seems to be that the fate of our justice system was in this case settled by a 3-to-2 decision in which the majority seems to whistle past a very large graveyard. The majority waves a fairly vague hand at the DNA evidence and concludes, based on circumstantial evidence, that a “reasonable person” might have found Mr. Wright guilty. In the absence of the DNA, is that really true?

If sympathy for the convicted is hard to come by, it is worth considering the implications of a 3-to-2 decision in the opposite direction. Had the flimsiness of the DNA evidence been the thing that overturned the conviction of Howard Wright, how many more men in how many more jails might have been set free, possibly erroneously, on the basis of dubious forensic evidence gathering?