It’s my understanding that DNA matching is arguably the best and most reliable of the forensic sciences. One reason is that it’s based on scientific knowledge and ideas that were developed independently of its forensic uses. Scientists spent decades studying DNA: How it’s structured, what it’s made of, how it replicates, how it combines in sexual reproduction, and how it is that we each come to have the DNA that we do.
In the 1800’s, when Charles Darwin published his theory describing how organisms evolve over many generations, he didn’t have a good explanation of how traits of organism combine with each other: If children are a mix of their parents’ traits, then after a few hundred generations, why aren’t we all medium height, medium build, and beige?
At about the same time (and apparently unknown to Darwin) Gregor Mendel published research showing that some traits of organisms seem to pass from generation to generation in discrete chunks — you’re either albino or not, you either can smell hydrogen cyanide or you can’t. (Mendel worked with plants, but those are human traits that are now believed to follow strict Mendelian rules.) This implied that however traits passed from parents to children, they passed in discrete chunks — perhaps with many chunks combining to determine fuzzy traits such as height or skin color — although Mendel had no idea what those chunks were.
By the end of the 1800’s, scientists were studying a peculiar microscopic substance found in living cells, and over the course of the 20th century, they began to suspect that Mendel’s hereditary chunks were pieces of this molecule, which came to be called deoxyribonucleic acid, or DNA. Eventually they were able to prove that DNA was the mechanism of heredity, and to determine its biological behavior at the molecular level. It explained everything that Darwin and Mendel had observed about heredity.
Biologists began using DNA to study the relationships between species of animals, and doctors began hunting for bits of DNA — genes — that correlated with (as possibly caused) diseases and conditions with known hereditary components. Scientists and engineers developed methodologies and tools for studying DNA, and their data went into publications and databases. They accumulated a lot of knowledge about DNA and how to work with it.
By the time DNA began to be used forensically to match biological samples to individual people, it had about a century of history and billions of dollars of research behind it.
Except…not every aspect of forensic DNA analysis is based on that science. Technicians can now recover DNA from tiny amounts of foreign skin cells found on a victim. But even if that identifies an individual, what exactly does it mean?
I wrote last year about a case here in Cook County where a guy was acquitted of sexual assault even after his DNA was found on the victim. Part of the problem with the case was that the DNA was found on the victim’s lips, and the lab could not tell what kind of cells it came from — it could have been from saliva, skin, or even hair.
(A bigger problem was that the DNA was so degraded as to be very nearly meaningless. Instead of the usual one-in-millions odds, the DNA expert said the odds were better then one-in-a-dozen. This is little better than saying he was tall and had dark hair. Hundreds of thousands of people in the area would also have matched.)
Now PDgirl offers two astounding examples of misleading DNA transfer:
…millionaire guy is murdered in his home. They are able to extract unknown DNA from on the guy’s fingernails. The assumption is that the DNA must be that of the killer. They run the DNA through a database and get a hit to a local man named Lukis Anderson. Mr. Anderson is arrested and charged w/ murder and faces the death penalty. He spends 5 months in jail, awaiting a resolution on the case.
However, it is impossible for Mr. Anderson to have been involved in the crime because on the night the man was killed, Mr. Anderson was in a hospital due to severe intoxication. He had been brought to the hospital by paramedics earlier that evening. Â Airtight alibi if there ever was one. So how did his DNA get on the dead guy’s fingernails?
Well, according to the theory that the prosecutors have put forth, after finally conceding it wasn’t possible that Mr. Anderson was the killer, is that the most likely explanation is that the DNA was unintentionally transferred by the paramedics.  You see, the paramedics that had taken Mr. Anderson to the hospital earlier in the evening were the same paramedics that responded to the crime scene and that handled the millionaire’s body.
Imagine if the paramedics had decided not to transport Anderson, so he hadn’t been in the hospital at the time of the killing. He might be in prison now because the forensic techs were so good at recovering DNA.
The second example is even stranger, but at least no one got arrested:
Not concerning enough for you? Well, what about Germany’s “Phantom of Heilbronn,” a notorious female serial killer who linked by her DNA to 40 different crimes, and yet somehow continued to manage to evade police during her crime spree that left police baffled for 2 years? She was also linked by DNA to several cold cases. Except for the devious serial killer turned out to be an innocent woman who worked at the factory where the cotton swabs used to collect DNA evidence were made. The Phantom of Heilbronn never actually existed
A similar problem in Austria a few months earlier was also traced to contaminated swabs, although in that case the police were using swabs that hadn’t been certified for DNA testing.
(DNA traces are fairly durable, and DNA itself is not a pathogen, so disinfection protocols that protect human health are not intended to eliminate all traces of DNA, which is probably why sterile swabs and trained paramedics have both been implicated in accidentally transferring DNA traces.)
Modern laboratory techniques can pull usable DNA samples from fewer than a dozen cells. Since your body has about one and a half trillion skin cells and sheds them by the millions every day, it seems likely that you are leaving theoretically detectable traces of yourself everywhere you go, and someday your freedom may depend on forensic experts having a real understanding of just how easy it is to transfer traces of your DNA around the environment.
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