Daily Archives: May 8, 2009

More DNA

To answer a couple of questions that were asked in the comments section let me say that DNA profiling was developed in 1984, first entered a US courtroom in 1987, and by the mid-1990s was quite commonly used in criminal investigations. In the late 1990s the techniques of PCR and STR analysis, the combination of which allowed for profiles to be developed from extremely small samples, came into common use and currently these systems are automated in many of the more sophisticated labs. There is no special technique or equipment required in analyzing the so-called touch DNA as compared to any other DNA sample. It just requires that the DNA sample be of good quality and not necessarily of large quantity. The PCR technique allows the technician to produce as much identical DNA as is needed from the tiniest of samples, theoretically even a single cell.


Posted by on May 8, 2009 in DNA


Touch DNA

You can’t watch TV or read a book about a criminal case without coming across DNA as part of the discussion. DNA might or might not be found at the scene but everyone will ask “Where’s the DNA?” The simple reason is that DNA is the gold standard for identification or for proving that someone was indeed at a crime scene. We all know that DNA can come from blood and other bodily fluids and can be found on cigarette butts and soft drink cans and on licked stamps and envelopes. There was a famous case here in Southern California where the DNA of an abducted child was found on the carpet of the abductor’s van where the child had shed tears. The victim here was Danielle Van Dam and the killer was a miscreant named David Westerfield. Here is a link that will give an overview of this horrible crime:

Now there is a new technique that is known as “touch DNA.” What this basically means is that DNA can be obtained from an object that someone has touched. DNA has even been found in fingerprints. How is this possible? When we touch something we leave behind a print that is basically made up of oils and grime that are constantly deposited on our fingers. Everything we touch picks up a bit of oil and dirt and this mixes with the normal oils in our skin. When we then touch something these oils and dirt are left behind as a fingerprint. In addition to the oils, skin cells become part of the fingerprint and it is these that are the source of the DNA. Theoretically it takes a single cell to provide enough DNA for analysis. So if the perpetrator touches a weapon, or a doorknob, or almost anything he will leave behind a few of his skin cells. These cells are then extracted, amplified, and a DNA profile can be produced. You’ve heard stories of fingerprints being obtained from the inside of latex surgical gloves. This is possible though it is very unlikely. However, when someone pulls on and off a pair of these tight-fitting gloves, skin cells are almost always removed in the process. This means that the inside of the glove could contain skin cells and these in turn could be gleaned and analyzed.

Let’s say a perpetrator breaks into someone’s home and commits a murder. He then goes to the sink and washes any blood from his hand. He might then dry his hands on either a dishcloth or a paper towel which is then tossed into the trash. Skin cells are removed in fairly large numbers by the drying action. These could be found on the towel used and used to develop a DNA profile.

So you’re bad guy doesn’t need to bleed or get scratched by the victim or leave some bodily fluid behind at the crime scene, he merely has to touch something and your astute criminal investigator could find the bad guy’s DNA and solve the case.


Posted by on May 8, 2009 in DNA


Welcome to my blog

I have finally entered the blogosphere and want to welcome you to this new blog. It is a place where I will post thoughts and comments on forensic issues that will be of interest writers and will hopefully help them make their stories that much better. I’ll chat about an ongoing case that is in the news, or about something that I’ve read in a novel or seen on TV or in the newspaper, or might discuss some issue raised in a question to me from a writer. In each case I’ll try to explain the forensic science and medical science involved in the issue so that you can consider these issues for your stories. So I hope as we go along we can have some fun and also learn some useful stuff. Most of all I hope my ramblings will help the writers out there tell better stories and get them in print. Welcome aboard and I hope you’ll check back often.


Posted by on May 8, 2009 in Misc

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