One of the most striking developments in forensic DNA
testing in recent years is the testing of ever smaller biological
samples. Whereas the original DNA tests required a
fairly large amount (i.e. a blood stain the size of a dime) of
biological material to get a result, current DNA tests are so
sensitive that they can type the DNA found in samples
containing only a few cells. There is likely to be enough of
your DNA on the book you are reading right now for your
DNA profile to be determined by a crime lab.
The increasing sensitivity of DNA tests has affected the
nature of criminal investigations and has created a new class
of DNA evidence. Analysts talk of detecting "trace DNA,"
such as the minute quantities of DNA transferred through
skin contact. DNA typing is currently being applied, with
varying degrees of success, to samples such as doorbells
pressed in home invasion cases, eyeglasses found at a crime
scene, handles of knives and other weapons, soda straws,
and even single fingerprints.
These developments will bring more DNA evidence to court
in a wider variety of cases and may well open new lines of
defense. A key issue will be the potential for inadvertent
transfer of small amounts of DNA from one item to another,
a process that could easily incriminate an innocent person.
Studies have documented the presence of typeable quantities
of human DNA on doorknobs, coffee cups and other common
items. Studies have also documented the inadvertent
transfer of human DNA from one item to another. Primary
transfer occurs when DNA transferred from a person to an
item. Secondary transfer is when the DNA deposited on one
item is transferred to a second item. Tertiary transfer is
when the DNA on the second item is, in turn, transferred to
a third. There are published studies that document secondary
transfer of DNA (in quantities that can be detected by
STR tests) from items that people simply touched to other
items.
A recent study commissioned by a wealthy defendant was
used to show that tertiary transfer of DNA could have occurred
in a manner that falsely incriminated the defendant.
Dr. Dirk Greineder, a prominent physician and adjunct
Harvard Professor, was accused of killing his wife. A DNA
pro?le similar to Greineder's was found, mixed with his
wife's profile, on gloves and a knife found near the crime
scene. Greineder denied touching these items, which appeared
to have been used by the killer. But how did his DNA
get on them?
Greineder offered a two-pronged defense. First, he challenged
the conclusion that his DNA matched that on the
gloves, noting inconsistencies between his profile and the
profile on the gloves. The crime laboratory had shifted its
threshold for scoring alleles in a manner that allowed it to
count alleles that matched with Greineder, while ignoring
some that did not. And the lab had to evoke the theory of
"allelic drop out" to explain why some of Greineder's alleles
were not found.
Greineder's second line of defense is our focus here. He
argued that his DNA could have gotten onto the glove
through tertiary transfer. He and his wife had shared a towel
the morning of the murder-perhaps his DNA was transferred
from his face to the towel, and from the towel to his wife's
face. His wife was later attacked by a glove-wearing stranger
who struck her on the face, strangled her, and stabbed her,
in the process transferring Greineder's DNA from his wife's
face to the gloves and the knife. According to this theory, the
tell-tale extra alleles on the gloves and knife that matched
neither Greineder nor his wife were those of the killer.
To support the theory that his DNA could have been
transferred innocently to the instruments of murder,
Greineder commissioned a study. Forensic scientists Marc
Taylor and Elizabeth Johnson, of Technical Associates (an
independent laboratory in Ventura, California) simulated
the sequence of events posited by the defense theory: a man
wiped his face with a towel, then a woman wiped her face
with the towel, then gloves and a knife like those used in the
murder were rubbed against the woman's face. DNA tests on
the gloves and knife revealed a mixture of DNA from the
man and woman-exactly what was found in the Greineder
case. Taylor was allowed to present his findings to the jury.
Although the jury ultimately convicted Greineder (there was
other incriminating evidence besides the DNA), the case is a
good example of how the amazing sensitivity of contemporary
DNA profiling methods facilitate a plausible explanation for
what might at first seem to be a damning DNA test result.
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