DOCTOR'S VIEWS ARCHIVE
DNA Testing - What Clinton Was Up
The Starr Report released to the Internet on Sept. 11, 1998 states
that: "Physical evidence conclusively establishes that the President
and Ms. Lewinsky had a sexual relationship." This "physical evidence"
was from DNA testing, a facet of forensic medicine.
Forensic medicine is a branch of medicine that uses medical
science in legal proceedings. Powerful methods of studying biologic
matter with genetic analysis of the basic molecules of inheritance
are at the forefront of forensic researchers.
Using DNA analysis, forensic medical specialists can precisely
match biologic material with persons who have left it at the scene of
Let us assume that a man's semen is alleged to be on a piece of
fabric. To learn whether the cloth is stained by the suspect's semen,
DNA testing can be done. Here is how it works:
- First, it must be shown that the stain contains semen. Semen
glows under ultraviolet light and also changes color when exposed to
- If these tests are positive, the lab technicians cut a swatch of
the fabric from the stain and dissolve the organic matter in the
stain by putting the swatch in a special solution.
- The technicians then look for sperm cells in the solution with an
ordinary low-power light microscope. The sperm look like little
- If there are visible sperm under the microscope, the techs next
extract DNA from the sperm. This is done with a mild detergent that
bursts non-sperm cells. A rinse with water removes broken cells. And
then a stronger detergent is used to burst the sperm and recover
- The lab then compares the DNA from the semen with the suspect's
DNA. A blood or saliva sample from the suspect will supply enough DNA
for the comparison.
- If the stain is small and the amount of DNA minute, a method
based on the polymerase chain reaction, or PCR, is employed to make
millions of copies of selected segments of the DNA. Note that PCR
does not change the DNA but merely amplifies the amount.
- The DNA from the semen is then compared with the suspect's. This
can be done using what are called restriction fragment length
polymorphisms, or RFLPs. To examine RFLPs, the DNA from the sample is
cut (restricted) by special enzymes. These restriction enzymes cut
different DNAs differently. If the restriction enzymes cuts the
suspect's sample and the semen sample into the same number of
fragments of the same length, then the semen may be the suspect's.
- The odds that the DNA in the semen is the suspect's are then
calculated. The odds may, for example, be one in 11 million that the
DNA in the semen came from another man, not the suspect.