Biology Reference
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an additional line of evidence. Genetic sexing studies use PCR to amplify genomic regions that
are specific to the X and Y chromosomes. If only X chromosome regions amplify, then the
genetic sex of the skeleton is female, and if both X and Y chromosome regions amplify then
the sample is genetically male. However, although this seems straightforward, genetic sexing
using degraded samples can be somewhat problematic. Contamination is hard to rule out.
Furthermore, it is possible that due to the damaged nature of any particular set of remains,
combined with the fact that any individual sample is expected to have more copies of the X
than the Y chromosome, a Y chromosomal fragment may fail to amplify and yield a false result
(i.e., resulting in a female versus a male sex estimate).
CASE STUDY: THE TRIALS AND TRIBULATIONS OF DNA
RESEARCH IN PRACTICE
The Missing Person Case of Leoma Patterson
The story of the missing person case of Leoma Patterson was originally published in 2007
by the Dr. Bill Bass and Jon Jefferson writing duo in their fiction topic
Beyond the Body Farm
and offers a good lesson of how degraded DNA studies can go wrong. By way of back-
ground, Leoma Patterson went missing in 1978, and five months later, a partial skeleton
was found that was tentatively identified as hers. In 2005, members of Leoma Patterson's
family had the skeleton exhumed from its grave, and found a forensic genetic testing
company in Nevada to test a single bone sample from the exhumed skeleton and compare
it to two family samples.
According to the company's one-page report, they examined ten locations on the mtDNA,
and detected the presence of two different DNA bases at three of the ten locations. In one
paragraph, the report stated that “[p]ossible explanations include that the sample is
degraded,” and cautioned that, “[i]t is not recommended to use this information for sole
identification or comparison purposes.” However, in the next paragraph, the report declared
that “[i]n comparison to the bone sample, [Leoma Patterson's two maternal descendants] can
be excluded as having the same maternal lineage.” The family, as well as Bass and Jefferson,
took these words at face value and believed that the skeleton was
not
that of Leoma Patterson.
Soon after, Bass and Jefferson contacted the primary author of this chapter for her opinion,
whichwas that not onlywas the report unclear, worse, it wasmisleading. Though the statement
in the first paragraph was correct, it was not accurate to say that Leoma Patterson's two
maternal relatives could be excluded. To explain, the report indicated that the extracted
DNA showed more than one DNA base at several (three) DNA locations (a condition that,
when naturally occurring, is termed
heteroplasmy
). Although humans occasionally show het-
eroplasmy, the chances of having three heteroplasmic sites within a few hundred bases of each
other (which is what the report showed) is very low. This could only mean that they had more
than one individual's sequence on their hands, and that the samples were in fact contaminated.
On the advice of the first author of this chapter, several skeletal samples were sent to
Dr. Jason Eshleman, a researcher with extensive experience in degraded DNA. He followed
proper decontamination procedures for the samples, attempted to extract DNA, and found
no DNA remaining in any of it. It later turned out that this was because the skeleton had
