A number of factors can introduce ambiguity into STR evidence,
leaving the results open to alternative
interpretations. To competently represent an individual
incriminated by DNA evidence, defense counsel must
uncover these ambiguities, when they exist, understand their
implications, and explain them to the trier-of-fact.
Degradation. As samples age, DNA like any chemical
begins to break down (or degrade). This process occurs slowly
if the samples are carefully preserved but can occur rapidly
when the samples are exposed for even a short time to
unfavorable conditions, such as warmth, moisture or
sunlight.
Degradation skews the relationship between peak heights
and the quantity of DNA present. Generally, degradation
produces a downward slope across the electropherograms in
the height of peaks because degradation is more likely to
interfere with the detection of longer sequences of repeated
DNA (the alleles on the right side of the electropherogram)
than shorter sequences (alleles on the left side).
Degraded samples can be difficult to type. The process of
degradation can reduce the height of some peaks, making
them too low to be distinguished reliably from background
"noise" in the data, or making them disappear entirely, while
other peaks from the same sample can still be scored. In
mixed samples, it may be impossible to determine whether
the alleles of one or more contributors have become undetectable
at some loci. Often analysts simply guess whether all
alleles have been detected or not, which renders their conclusions
speculative and leaves the results open to a variety of
alternative interpretations. Further, the two or more biological
samples that make up a mixture may show different
levels of degradation, perhaps due to their having been
deposited at different times or due to differences in the
protection offered by different cell types. Such possibilities
make the interpretation of degraded mixed sample particularly
prone to subjective (unscientific) interpretation.
Spurious Peaks. An additional complication in STR interpretation
is that electropherograms often exhibit spurious
peaks that do not indicate the presence of DNA. These extra
peaks are referred to as "technical artifacts" and are
produced by unavoidable imperfections of the DNA analysis
process. The most common artifacts are stutter peaks, noise
and pull-up.
Stutter peaks are small peaks that occur immediately
before (and, less frequently, after) a real peak. Stutter occurs
as a by-product of the process used to amplify DNA
from evidence samples. In samples known to be from a single
source, stutter is identifiable by its size and position.
However, it is sometimes difficult to distinguish stutter
bands from a secondary contributor in samples that contain
(or might contain) DNA from more than one person.
Noise is the term used to describe small background peaks
that occur along the baseline in all samples. A wide variety
of factors (including air bubbles, urea crystals, and sample
contamination) can create small random flashes that occasionally
may be large enough to be confused with an actual
peak or to mask actual peaks.
Pull-up (sometimes referred to as bleed-through) represents
a failure of the analysis software to discriminate between
the different dye colors used during the generation of
the test results. A signal from a locus labeled with blue dye,
for example, might mistakenly be interpreted as a yellow or
green signal, thereby creating false peaks at the yellow or
green loci. Pull-up can usually be identified through careful
analysis of the position of peaks across the color spectrum,
but there is a danger that pull-up will go unrecognized,
particularly when the result it produces is consistent with
what the analyst expected or wanted to find.
Although many technical artifacts are clearly identifiable,
standards for determining whether a peak is a true peak or
a technical artifact are often rather subjective, leaving room
for disagreement among experts. Furthermore, analysts
often appear inconsistent across cases in how they apply
interpretive standards-accepting that a signal is a "true
peak" more readily when it is consistent with the expected
result than when it is not. Hence, these interpretations need
to be examined carefully.
Spikes, blobs and other false peaks. A number of different
technical phenomena can affect genetic analyzers, causing
spurious results called "artifacts" to appear in the
electropherograms. Spikes are narrow peaks usually attributed
to fluctuation in voltage or the presence of minute air
bubbles in the capillary. Spikes are usually seen in the same
position in all four colors. Blobs are false peaks thought to
arise when some colored dye becomes detached from the
DNA and gets picked up by the detector. Blobs are usually
wider than real peaks and are typically only seen in one
color.
Spikes and blobs are not reproducible, which means that if
the sample is run through the genetic analyzer again these
artifacts should not re-appear in the same place. Hence, the
correct way to confirm that a questionable peak is an artifact
is to rerun the sample. However analysts, to save time, often
simply rely on their "professional experience" to decide which
results are spurious and which are real. This practice can be
problematic because no generally accepted objective criteria
have yet been established to discriminate between artifacts
and real peaks (other than retesting).
Threshold Issues: Short Peaks, "Weak" Alleles. When the
quantity of DNA being analyzed is very low (as indicated by
low peak-heights), the genetic analyzer may fail to detect the
entire pro?le of a contributor. Furthermore, it may be dif-
?cult to distinguish true low-level peaks from technical
artifacts. Consequently, most forensic laboratories have
established peak-height thresholds for "scoring" alleles. Only
if the peak-height (expressed in RFU) exceeds a standard
value will it be counted.
There are no generally accepted thresholds for how high
peaks must be to qualify as a "true allele." Applied Biosystems,
Inc., which sells the most widely used system for STR
typing (the ABI Prism 310 Genetic Analyzer(tm) with the
ProfilerPlus(tm) system) recommends a peak-height threshold
of 150 RFU, saying that peaks below this level must be
interpreted with caution. However, many crime laboratories
that use the ABI system have set lower thresholds (down to
40 RFU in some instances). And crime laboratories sometimes
apply their standards in an inconsistent manner from
case to case or even within a single case. Hence, a defendant
may be convicted in one case based on "peaks" that would
not be counted in another case, or by another lab. And in
some cases there may be unreported peaks, just below the
threshold, that would change the interpretation of the case if
considered.
Finding and evaluating low-level peaks can be difficult
because labs can set their analytic software to ignore peaks
below a specified level and can print out electropherograms
in a manner that fails to identify low-level alleles. The best
way to assess low-level alleles is to obtain copies of the
electronic data files produced by the genetic analyzer and
have them re-analyzed by an expert who has access to the
analytic software.