For the Defense - Vol. 7, Issue 3 - 6
Trace DNA:
Are You Asking the
Right Questions?
F
orensic DNA analysis is only good at answering
one question: Who? It is not good at the other
questions of What?, When?, Where?, Why?, and
How? This article will not focus on how the Who?
is determined (which is an entirely separate matter),
but instead asks " How did it get there? " , " When did
it get there? " , and " Where did the transfer of DNA
happen? " . The defendant will be presumed to be
present. This article will take a deep dive into the
scientific literature, and the answers are terrifying.
The sensitivity of modern forensic DNA analysis
has reached a point where it is possible to get a
partial profile from approximately ten cells. Trace (or
" touch " ) DNA refers to the minute amount recovered
from a non-fluid biological sample. Generally, these
are epithelial (skin) cells that are transferred from a
person to an object, but could be in the form of saliva,
from individuals who cough/sneeze into their hand
or lick their fingers. DNA profiles obtained from such
a DNA sample may be complete or incomplete. Due
6 For The Defense l Vol. 7, Issue 3
Katherine L. Cross
and Arthur W. Young
to the high sensitivity, interpretation can become
complex as samples are often mixtures of multiple
individuals.
Primary (or direct) transfer is when a person
deposits DNA onto a surface, be it an object or even
another person's skin. Secondary transfer is when the
object or person touches another surface, which can
also be an object or person. Subsequent transfers
are called tertiary and quaternary transfers. In 2013,
a study found that liquid blood could migrate six
times, while dried blood and skin cells could only
migrate once.1
This study is not representative of
modern scenarios, however, as the skin cells came
from the rubbing of hands together over a surface;
modern scenarios usually deal with objects that have
been touched for long periods of time, which means
significantly greater quantities of available DNA.
The following cases and scenarios will illustrate
the complexities and nuances in evaluating the
For the Defense - Vol. 7, Issue 3
Table of Contents for the Digital Edition of For the Defense - Vol. 7, Issue 3
Contents
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For the Defense - Vol. 7, Issue 3 - Contents
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