Tagged: dna

Bacterial Fingerprinting? Don’t Hold Your Breath

Over the past couple of days, the news has been filled with stories about using microbes to identify suspects. Everyone has all kinds of bacteria all over their bodies, and whenever you touch something you leave a smudge of your bacteria behind. On Monday, researchers at CU-Boulder published a study where they swabbed computer keyboards, tested the DNA of the bacteria they found, and saw that those bacteria’s DNA more closely matched the bacteria on the computer users’ skin than the bacteria on other people’s skin.

That’s all the study found. The bacteria on your keyboard have DNA that more closely matches the DNA in the bacteria on your fingers, than that of bacteria on other people’s fingers. Frankly, although that’s a nifty result and the scientists deserve to be praised for their work, it’s really a very modest finding. Not exactly earth-shaking.

But as usual, the media took this modest finding and blew it way out of proportion. The study’s authors insist that the project “is still in its preliminary …

Coming Soon: Full-Genome DNA Analysis

This is amazing. Sequencing an entire human genome is now going to be cheap and fast. We predict this will be a game-changing technology for the use of DNA technology.

Right now, DNA evidence is looked at much like fingerprint evidence. With fingerprints, law enforcement doesn’t compare every single ridge and whorl to see if there’s an exact match. Instead, particular locations are compared, to see if those locations are the same. And there’s a lot of subjective interpretation that is needed to make that call. DNA evidence is no different. The entire DNA sequence is not compared. Instead, a handful of locations are compared, to see if the DNA at those locations is the same. And there’s a lot of subjective interpretation that is then needed to make that judgment call. This can call DNA evidence into question, particularly when there are mixtures, degraded samples or equipment glitches that create room for errors of judgment.

If one were to compare the entire genome, however — all 3.3 billion base pairs of it — there would be much less room for interpretation and error.

The problem is that sequencing an entire genome has, to date, been prohibitively expensive. The first genome was a massive undertaking. As of 2009, only 7 people’s genomes had ever been sequenced. The time and expense needed to compare the genomes from a bit of evidence and from a single suspect would take forever and cost a huge amount of money. It’s just not practical.

But if the new technology announced in this WSJ video is for real, all that could change very soon.

If the technology is available to analyze and compare all 3.3 billion base pairs rapidly and cheaply, the current system of comparing 9 or 10 or 13 loci will be woefully …

DNA Evidence: Good Science, Bad Results


A couple of weeks ago, we taught another CLE course for the good folks at West Legal Ed Center, in our “Hope for Hopeless Cases” series. This one was on ways to defend cases where the government is going to use DNA evidence to prove your client’s guilt. (Here’s a link.)

DNA evidence can be just devastating. The science is good, after all. And to a lot of potential jurors (and judges and lawyers, unfortunately), “science” is another word for “magic.” Which is another word for “I don’t have to understand how it works, all I know is that it must be so.”

This can often be a wonderful thing, when the science is used correctly, and for the limited purposes to which it is suited. When used correctly, DNA evidence can free the innocent, and help ensure that we really are only punishing the guilty.

The problem is, DNA evidence is all too often used wrong.

And when that happens, the wrong people can get convicted.


And now today we read a good article in the latest Washington Monthly called “DNA’s Dirty Little Secret: A forensic tool renowned for exonerating the innocent may actually be putting them in prison.” (Link here.)

It’s a good article, about the case of John Puckett, who was convicted in 2008 of an old murder from 1972. It was a brutal rape and murder, with about 20 suspects at the beginning, but the case went cold. Then in 2003 the police tested the DNA found in the evidence. It was old DNA and degraded, and it was also a mixture of multiple people’s DNA. The results were compared to California’s DNA database, and there was a possible match with Mr. Puckett. He hadn’t been a suspect in 1972, but based on this apparent match — and on nothing else — he was prosecuted and ultimately convicted. Jurors have since said that they convicted because of the statistical odds quoted to them at trial, and that if they had known the stats of false positives — which were one in three — they never would have trusted the government’s stats like that.

The article highlights the fact that DNA evidence may be based on good science, but by the time it gets to a jury it can be seriously flawed. Contrary to popular belief, DNA evidence is not objective. It involves a huge amount of subjective …

Ninth Circuit Bungles Math, Can the Supremes Fix It?

The “Prosecutor’s Fallacy” is one example of why we think Statistics should be a required course in college. Let’s say the police have the DNA of a rapist. Only 1 in 3,000,000 people chosen at random will match that DNA sample. Your DNA matches. At your trial, the DNA expert...