Golden State Killer Busted by DNA. But Are Tests 100% Accurate?

Advanced DNA testing and a genealogy database reportedly
provided police with a breakthrough in the infamous Golden
State Killer case^[1]. Joseph James DeAngelo,
72, was arrested Wednesday by the Sacramento Sheriff on charges of
murder. Police link the alleged Golden State Killer to 12
homicides, 45 rapes, and 120 home invasions in the 1970s and ’80s
in California.

Investigators used DNA from crime scenes that had been stored
for decades and plugged the genetic profile of the suspected
assailant into an online genealogy database. Officers found distant
relatives of DeAngelo’s and traced their DNA to his front door,
The
New York Times reported^[2].

Although police say they have a range of evidence in the case,
they are touting the strength of what they claim is a “100% DNA
match.” In response to social buzz around the case, we wanted to
explore the concept of a 100% DNA match. What certainty can DNA
testing bring to a case?

Short answer: Not really 100%. The longer answer: While DNA is
extremely good at identifying individuals, mistakes have happened.
Accurate identification is dependent on a number of factors,
including the quality of the DNA sample, the number of genetic
markers analyzed, whether the sample was prepared properly, and the
ability of those doing the analysis to interpret the results.
(Learn
more about forensic science^[3].)

A DNA match identifies the potential source of the DNA, but it
says “nothing about the activity involved in the DNA coming to be
where it was found,” Sue Pope, a DNA expert, told The
Guardian.

DNA (deoxyribonucleic acid) is found in every cell in our
bodies, from heart to skin, blood to bone. It contains genetic
information that helps determine physical characteristics such as
eye color. DNA is made up of billions of letters of the genetic
code and, amazingly, 99.9 percent of that code is exactly the same
in every human. The 0.1 percent of the code that’s different in
each of us is what make us genetically unique (with the exception
of identical twins).

DNA testing uses a dozen or more genetic markers in the code to
identify the important similarities or differences between people.
The more markers used, the greater the accuracy, but also the cost
of testing. The probability of the DNA profiles of two unrelated
individuals matching is on average less than 1 in 1
billion^[4].

A sample can be from any part of the body, since the DNA is the
same. In the lab, the DNA is separated and millions of copies are
made using a method called polymerase chain reaction or PCR. These
DNA copies are cut into chunks at specific locations to make them
easier to analyze and thus create a DNA fingerprint. The result is
then compared to the DNA fingerprint from another sample to see if
they match.

Even advanced DNA testing, which allows the recovery of minute
traces of DNA, cannot prove how the DNA got to a crime scene. We
are constantly shedding skin cells or leaving traces on door
handles, on money, or even by shaking a hand. DNA can hang around
in a room for months or blow out the window and be taken miles
away.

“You could detect a single cell on a knife blade, but that
doesn’t mean anything—it might have arrived there long before the
crime or been transferred there by chance,” John Butler, special
assistant to the director for forensic science at the National
Institute of Standards and Technology told
Frontline^[5].

In the case of the alleged Golden State Killer, as in all other
cases, it is up to the prosecution to present a coherent argument
that convinces a judge and jury of a defendant’s guilt. Juries are
asked to weigh the sum of evidence in each case, and to interpret
the facts as best they can within the confines of the law. It’s not
an easy job. DNA evidence can provide valuable information—though
there are some limitations.