How DNA Testing Works

DNA testing will determine whether the president’s semen landed on Monica Lewinsky’s dress. We know carefully conducted DNA testing is very precise, but how does it work?

1) In the Clinton case, the first order of business will be to establish that the stain contains semen. (The stain could be from McDonald’s, after all. Click here for an alternate explanation.) Semen glows under ultraviolet light. Semen also changes color when you expose it to a series of two chemicals. If the both tests come up positive, the FBI technicians will then cut a swatch of the fabric from the stain and dissolve all the organic matter by dunking the swatch in a liquid-filled test tube.

2) Next, the technicians will look for sperm cells in the solution with a low-power microscope (sperm look like tadpoles). Every cell in every plant and animal contains DNA, which is unique to the individual. If sperm are present in the semen, the technicians will grab DNA from the sperm’s interior without grabbing DNA from other cells in the stain (probably Monica’s skin cells). Introducing a mild detergent will burst non-sperm cells; rinsing with water will clear burst cells; and introducing a stronger detergent will burst sperm cells and spill their DNA. Sperm have a high DNA concentration, so this technique yields a lot of DNA to work with. If no sperm are detected, it is likely that the semen’s author had a vasectomy. Technicians can still extract DNA from non-sperm cells in semen, but they won’t get as much.

3) The FBI will then compare the seminal DNA from the stain with the president’s DNA. Either a blood or saliva sample from the president will do. Legal experts report that the court will almost certainly compel Clinton to give a sample, though the Constitution doesn’t say what to do if he refuses. No police officer will arrest the president for contempt of court, though Congress could impeach him for refusing.

4) Lastly, the technicians must match the seminal DNA with the president’s. One technique (restriction fragment length polymorphism) is to cut the sample DNA–a long stringy molecule–into many smaller fragments with a special chemical. This chemical cuts different people’s DNA differently. For example, one person’s DNA might yield seven fragments that are 500 units long and four that are 300 units long, while another’s might yield eight fragments of 600 units and three of 200 units. If the chemical cuts Clinton’s sample and the semen sample into the same assortment of fragments, then the semen is Clinton’s. A second technique (polymerase chain reaction, or PCR) is appropriate when the amount of DNA is limited (it all depends upon the size of the stain). In this process, technicians actually create millions of copies of certain critical sections of the DNA under examination. Like the first technique, PCR allows technicians to compare Clinton’s DNA with the DNA from the stain.

DNA tests show only whether two samples are genetically similar, not whether they are identical. Tests can register a positive when in fact the DNA samples are from different people, but the odds are one in several million. Still, if the test results point a finger at him, Clinton could argue that he’s the victim of an enormous coincidence. (Don’t laugh, Barry Scheck has made a career saying just this sort of thing.)

Depending upon the technique chosen by the FBI lab, the test could take several weeks or several days. 

Explainer thanks Don McLaren of the Washington State Patrol Crime Lab and Dr. George Riley of Genelex.