What is touch DNA?

Husband-and-wife forensic experts Max and Lucy Houck get to the bottom of this mystery:

The touch DNA method is so named because it analyzes skin cells left behind when assailants handle weapons, victims or something else at a crime scene. The technique of analyzing these minute samples for genetic information is relatively new, having appeared only about five years ago.

Touch DNA forensics rose to prominence this past summer, when various news outlets reported that police had used the technique to clear the family of JonBent Ramsey formally of any wrongdoing in her gruesome 1996 death. In fact, the prosecutor in the Ramsey case, Boulder County (Colorado) district attorney Mary Lacy, learned about touch DNA when she attended a course in the summer of 2007 at the West Virginia University Forensic Science Initiative, which one of us (Max) directs.

In the 1980s, to perform DNA analysis on a piece of evidence or on a victim, forensic investigators needed a blood or semen stain about the size of a quarter. The sample size fell in the 1990s to the size of a dime and then became: “If you can see it, you can analyze it.”

Touch DNA does not require you to see anything, however—nor does it require any blood or semen at all. Instead it takes just seven or eight cells from the outermost layer of skin. Naturally, the method has dramatically increased the number of items of evidence that can be used for DNA detection.

Here is how it works: Investigators recover cells from the scene and use a process called polymerase chain reaction (PCR) to make many copies of 13 locations on the DNA. They then mix in fluorescent compounds that attach themselves to those copies. What emerges is a highly specific forensic profile of the person whose cells have been found. The entire process takes a few days.

These 13 locations were carefully chosen because they are highly variable among people but do not give away specific information such as race or gender, nor do they reveal levels of personal health or presence of genetic disease. The chance of DNA profiles from two different people having the same genetic signature is vanishingly small.

The trick to finding these cells is context. The forensic specialist on the scene must try to ascertain (or guess) which objects the perpetrator handled and sample them with a cotton swab or a collecting blade. And with the backlogs of evidence common in forensic labs, such a time-consuming process is not always prudent. But in cases such as the Ramsey murder, which has tripped up authorities for more than a decade, it can provide information that leads to a killer—or at least exonerates the innocent.

Why do our eyelids get so heavy when we are tired?

Mark A. W. Andrews, professor of physiology and director of the Independent Study Pathway at the Lake Erie College of Osteo pathic Medicine, replies:

Generally speaking, heaviness of the muscles around the eyes, including the levator muscles that open the upper eyelids, is similar to fatigue of any muscle of the body. Ocular and brow muscles are especially prone to fatigue because they are active for most of our waking hours. Over the course of the day, they gradually grow leaden with extended use, as our arms and legs do.

Such a feeling may be compounded by general fatigue, including a lack of sleep, or by specific muscle overuse related to long hours of focusing on, say, a computer monitor. Excess skin of the eyelid, or prolapsed fat pads underneath the eyes, makes an individual more prone to this sensation. Chronic allergies and sinus infections may also exacerbate the heaviness, and sun exposure may cause eyelid swelling and thereby increase the probability that the drooping will interfere with vision.

Although heavy eyelids do not typically indicate underlying medical issues, some conditions do cause drooping eyelids, or ptosis. A stroke or a muscular disorder such as myasthenia gravis or myotonic dystrophy can damage facial muscles or their nerves and cause ptosis, as can elective facial surgery or interventions such as Botox injections to the brow.

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