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Emily Moore
Irving Litofsky
FSCI500
October 14, 2013
Trace DNA Analysis
Introduction
The advent of DNA analysis in the mid-1980’s revolutionized the field of forensic
science. Until the development of DNA profiling, fingerprints had been the gold standard
in forensic identification. Since its genesis, researchers have continually developed new
methods to improve the quality of DNA profiles as well as techniques to obtain profiles
from smaller and smaller amounts of DNA. DNA found in its smallest quantities is
known as trace DNA.
Trace DNA analysis describes the methods and procedures for collecting,
preserving, and analyzing DNA recovered from a crime scene in very minute quantities.
Although the terminology is sometimes used interchangeably, trace DNA analysis can be
further classified into two different types – touch DNA analysis and low copy number
(LCN) DNA analysis.
History of DNA Analysis
DNA molecules are long chains composed of millions of units called nucleotides.
There are four different nucleotide bases (adenine, guanine, cytosine, and thymine) that
combine in different sequences to encode the information for cell division, protein
synthesis, and controlling cellular processes. The earliest method for creating a DNA
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profile used non-coding regions of DNA that contained consecutive repeating sequences
of 10 to 100 nucleotides (Houck and Siegel 263). The number of times that a particular
sequence repeats itself differs greatly between individuals, so the regions, or loci, that
contain these repeating units are termed variable number tandem repeats (VNTR). Dr.
Alec Jeffreys discovered that adding enzymes, called restriction enzymes, to a sample of
DNA would result in different size fragments depending on a person’s genetic make up.
Restriction enzymes are specialized proteins that cut DNA at a particular sequence. Due
to the fact that individuals have variable numbers of nucleotides in between the regions
cut by the restriction enzymes, the length of the fragments are highly variable and can be
used to identify people or match crime scene DNA to that of a victim or suspect. The
differences in fragment lengths are called restriction fragment length polymorphisms
(RFLP).
RFLP analysis begins with DNA extraction. The DNA must be separated from the
cells and isolated for analysis. Restriction enzymes then cut the DNA at particular
sequences, resulting in numerous fragments of different lengths. Next, gel electrophoresis
separates the fragments by size. The DNA sample is then loaded into one end of a slab of
agarose or polyacrylamide gel, and an electrical current is run through the gel. DNA
molecules have a negative charge and are attracted to the positively charged cathode at
the opposite end of the gel. As they move through the gel matrix, smaller fragments will
move more easily through the pores in the gel while the larger pieces of DNA get trapped
by the gel and move slower. As a result, a banding pattern will emerge in the gel, with
each band representing DNA fragments of different lengths. The fragments that migrate
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the farthest are the smallest fragments, and those that travel the least distance are the
largest.
Once the VNTRs are separated according to size, additional measures are taken in
order to visualize the banding pattern. Analysts use a technique called Southern blotting
to transfer the DNA fragments from the gel onto a nylon membrane (Houck and Siegel
264). Once stabilized and fixed onto the nylon membrane, probes are added to locate and
visualize the invisible bands of DNA. Each of the four nucleotide bases (adenine,
guanine, thymine, and cytosine) will only form a bond with one other base. Adenine and
thymine always bond together and cytosine and guanine always bond together. The
probes are short pieces of DNA consisting of complementary nucleotides that can form a
bond with the VNTRs. Single locus probes allow visualization of one VNTR at a time,
but Jeffries originally used multi-locus probes that were capable of binding several
different VNTR sites simultaneously (NFSTC). Unbound probes are washed off the
membrane with a suitable solvent before the DNA is visualized. Probes can be
radioactively labeled and used to develop photographic or x-ray film, or they can be
tagged by chemicals that will react with a chemiluminescent substrate to produce a
visible glow in the location of the DNA bands.
While historically important for generating the earliest DNA profiles used in
criminal investigations, modern crime laboratories rarely use RFLP analysis of VNTRs.
The procedure is extremely laborious and time-consuming. Additionally, radioactive
probes are hazardous to the health and safety of laboratory technicians (although this
problem can be avoided by using chemiluminescent probes instead). The main
disadvantage, however, is that large samples of un-degraded DNA are necessary to obtain
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results. Modern techniques have been developed to detect smaller quantities of DNA and
even generate profiles from old or partially degraded DNA samples.
In 1983, Kary Mullis developed a method for making additional copies of DNA,
called polymerase chain reaction (PCR) (Houck and Siegel 267). The ability to make
millions or billions of copies of a DNA molecule in a relatively short period of time
revolutionized not only forensic DNA analysis, but the field of molecular biology as well.
Whereas RFLP analysis only works on large samples of un-degraded DNA, PCR-based
methods for DNA analysis (including trace DNA analysis) can take small, even minute
samples and amplify them until there is sufficient DNA material for analysis.
In the late 1990,’s DNA analysis underwent another major transition. Molecular
biologists discovered DNA sequences similar to VNTRs, but consisting of smaller
repeating segments known as short tandem repeats (STRs). STRs have consecutive
repeating units of only 2 to 6 base pairs. Although not as discriminatory as RFLPs, their
short lengths and the number of available loci made STRs ideal for amplification and
analysis by PCR (NFSTC). Additionally, three or more STRs could be analyzed
simultaneously, a method called multiplexing, saving time and making analysis much
more efficient. According to Bill Tilstone, the Director of Instructional Technology and
Education for the National Forensic Science Technology Center, “STR analysis is the
current method of choice for DNA testing in crime laboratories and yields results that are
nearly equivalent to individualization” (NFSTC).
Although there are many modifications to address different issues that can arise
during DNA analysis, the basic steps for STR PCR DNA profiling are the same. The first
step is to extract and quantitate the DNA from the source cells. In order to extract the
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DNA from the cells, different chemicals are applied to denature and hydrolyze proteins.
This breaks them apart, helping to release the DNA and allowing them to be removed
from solution. Additional chemicals are added to prevent enzymes present in the cell,
called nucleases, from digesting the DNA and destroying it. Once the DNA has been
isolated from the denatured proteins and additional cellular debris, it is purified to remove
inhibitors that could disrupt amplification by PCR and then concentrated if necessary.
Quantitation is subsequently required to determine the amount or concentration of DNA.
This is an important step to in ensure quality assurance and is most often accomplished
by quantitative PCR (qPCR).
Quantitative PCR uses fluorescent dyes and monitors their intensity for each PCR
cycle. Because DNA is replicated exponentially, nearly doubling each cycle, the original
quantity can be extrapolated by determining the cycle number when the fluorescent
intensity exceeds a defined threshold. This method is accurate, precise, efficient, and
specific to human DNA (NFSTC). Other commonly used methods of quantitation include
yield gels, spectrophotometry, fluorometry, slot blot hybridization, and commercial kits
such as AluQuant™ (NFSTC).
Amplification, the second step in a basic STR DNA analysis, is accomplished by
PCR, “an enzymatic process similar to DNA replication in cells” (NFSTC). Each cycle
doubles the original amount of DNA and the resulting products are referred to as
amplicons. During the first stage of PCR, the DNA is heated to a temperature that will
separate the two strands and form single-stranded DNA. The DNA is then cooled so that
primers can anneal, or attach themselves to the DNA in a region near the targeted STR.
Adding more than one primer and targeting multiple STRs at the same time is referred to
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as multiplexing. This is done in the laboratory to save time and increase the efficiency of
DNA analyses. Once the primers anneal, the temperature is then raised again, and an
enzyme called Taq polymerase adds the complementary base pairs to the single-stranded
DNA template, forming a new molecule of identical DNA sometimes referred to as an
amplicon. Taq polymerase is different from the polymerase found in humans. It is from a
bacterium, Thermus aquaticus, that lives in an extremely warm environment. This makes
Taq polymerase capable of withstanding the drastic temperature changes during each
cycle of PCR without degrading. As few as 30 cycles can generate approximately1 billion
amplicons, each containing a copy of the target DNA sequence (NFSTC).
Once the target STRs have been amplified, they are separated by capillary
electrophoresis. Capillary electrophoresis works on the same basic principles as gel
electrophoresis, but small capillary tubes, through which the DNA travels at different
speeds depending on size and charge, replace the gel slabs. The capillaries are “hollow
fused silica tube[s] with an internal diameter of 50-100 µm and 25-75cm in length”
(NFSTC). Capillary electrophoresis has gained popularity in modern crime labs and is
used for analyzing gun shot residues, explosive residues, drug samples, and pen inks, in
addition to DNA.
DNA samples are introduced into the capillary by electrokinetic injection. This
occurs when the capillary is submerged in the sample and negatively charged DNA
particles move in the tube via an electromotive force. Sample stacking concentrates the
DNA fragments inside the tube, improving the efficiency and electrophoretic resolution.
As with gel electrophoresis, an electric field acts on the DNA particles, moving them
from the source vial, through the capillary, towards the positively charged cathode. Along
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the way, the fragments (each containing a different STR) separate according to size.
Smaller fragments move more quickly through the capillary while larger fragments
remain in the capillary longer. As the fragments move through the capillary, they
eventually pass through a detector. Fragments containing different STRs are labeled with
fluorescent dyes prior to entering the capillary electrophoresis instrument. The detector
operates by shining a laser onto the sample, causing the dyes to become excited. The light
that they emit is measured spectroscopically and the intensity of the fluorescence is
recorded in relative fluorescent units (RFUs). RFUs versus time are plotted on a graph
called an electropherogram. The time correlates to the size of the fragments (with the
shortest fragments reaching the detector first), and the RFUs correlate to the amount of
DNA present (larger amounts of DNA have more RFUs).
Analysis of the electropherograms is done with the use of computer software and
interpreted by an experienced DNA analyst. DNA fragments are sized by comparing
them to internal size standards, DNA pieces of known fragment length, which are run
through the capillary at the same time as the evidence DNA. Genotypes, which describe
the genetic make up of the alleles, are determined at each STR locus based on the length
of the fragments. One peak on an electropherogram at a particular locus indicates that a
person is homozygous for that STR, meaning that the genetic code for that locus is the
same on each of the two matching chromosomes on which it is found. Two separate
peaks at a particular locus indicate two different alleles on the corresponding
chromosomes. Thirteen loci and amelogin are analyzed in a typical DNA
electropherogram and compared to population frequencies for each allele at each locus.
The possible combinations of different alleles at each locus are so varied that they can
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individualize a person with an extremely high degree of certainty. Depending on the
number of alleles and loci that match, the probability that any person other than the
suspect would share the same DNA profile can be small enough to assume individuality.
Current Uses and Improvements Made for Trace DNA Analysis
Improvements in methods of extraction and amplification of DNA samples have
allowed analysis on tiny amounts of DNA, called trace DNA. Extraction techniques have
become sensitive to smaller and smaller quantities of DNA. The target quantity of DNA
for most commercial kits that analyze the 13 loci used in the Combined DNA Index
System (CODIS) and amelogin is approximately 1 nanogram (NFSTC).
Organic extraction is the preferred method for isolating DNA from biological
samples in most forensic crime laboratories. Organic extraction involves dissolving a
sample or biological stain in deionized water mixed with ethylenediaminetetraacetic acid
(EDTA) and a buffer solution. The EDTA prevents nucleases from breaking down and
destroying the target DNA, and the buffer helps to weaken the cell membrane so the
DNA can pass through (NFSTC).
Denaturation and hydrolysis of proteins in organic extraction is accomplished by
the addition of a detergent, proteinase K, and dithiothreitol (DTT). The detergent serves
to destroy and fragment the cell membrane and separate the DNA from histone proteins
that bind it together. The detergent also causes the proteins to lose their shape, decreasing
their ability to dissolve in water and making it easier to separate them from the DNA.
Proteinase K and DTT further degrade the histone proteins, freeing the DNA.
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The DNA is separated from the denatured proteins by the addition of a
mixture of phenol, chloroform, and isoamyl alcohol. The phenol is immiscible with
water, which means that the two will not mix. Instead, phenol forms a layer on top of the
aqueous solution containing the DNA. The proteins become trapped in the organic layer,
the phenol, and the aqueous solution containing the DNA can be removed and purified.
Purification involves multiple steps, and it attempts to removes any remaining
contaminants, especially PCR inhibitors that may coelute with the DNA.
Reducing inhibitors is especially important when dealing with very small or trace
quantities of DNA. Analysts want to maximize the amplification by PCR to prevent
extraneous peaks, missing alleles, and incomplete profiles that cannot be analyzed. Large
samples of contaminated DNA can simply be diluted with additional solvent, thereby
reducing the concentration of inhibitors while retaining enough DNA to obtain a full
profile. With trace quantities of DNA, bovine serum albumin (BSA) is frequently used to
stabilize the DNA and reduce inhibition by compounds found naturally in body fluids
(hemin from blood, for example). Samples may also be re-extracted to purify the DNA
sample using “Chelex resin, phenol chloroform, Thiopropyl Sepharose 6B (Sigma)
extraction beads, or magnetic beads (DNA IQ™)” (NFSTC).
New methods employed during PCR may also help to increase the yield of
product DNA and help to prevent the loss or uneven replication of certain alleles.
Increasing the amount of Taq polymerase may increase the yield and minimize the effects
of inhibitors. Shorter primers, called mini primers, have been shown to attach better in
some situations. Additionally, primers designed to attach closer to the STR region can
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improve success of PCR for degraded samples and help to counter the effects of
inhibitors.
According to research conducted by Ray Wickenheiser in 2002, full DNA profiles
can often be obtained from trace samples as small as 100 picograms of purified DNA, an
amount far below the recommended one nanogram (445). Each cell in the human body
contains approximately 5 picograms of DNA, so full profiles can sometimes be obtained
from as few as 20 cells. Considering that “the average human being [sheds]
approximately 400,000 skin cells daily,” DNA analysis from touched objects has become
a reality in the last decade (Wickenheiser 445). This type of trace DNA analysis is often
referred to as “touch DNA.” Wickenheiser claims that using standard laboratory
procedures without modification, “a trace DNA profile is obtained for approximately 30
to 50% of exhibits tested” (445). Wickenheiser reports that full profiles have been
recovered from epithelial cells deposited on touched objects from crime scenes ranging
from the ordinary (ex. firearms, knife handles, steering wheels, cigarette butts) to the
unexpected (ex. Contact lens found in a vacuum cleaner, shoelaces, machined washed
blue jeans, urine in snow, and various food items).
One of the most important considerations in recovering sufficient touch DNA to
result in a complete profile is knowing where to swab and in which order different areas
of an object should be swabbed. Logic dictates that evidence collectors would want to
swab areas that are most likely to have been touched, such as the handle of a knife or the
gear shifter in a car, but it is not always that simple. Often times, it may be necessary to
swab different areas of a single object separately to avoid mixed profiles. For example, in
a murder case where the victim was strangled by an electric cord, separating the cord into
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zones aided investigators in identifying a suspect. As would be expected, the area of the
cord that had been wrapped around the victim’s neck yielded an electropherogram
representative of the victim’s DNA with the suspect’s DNA present only as a minor
profile. Sampling the zones furthest from the center prevented the suspect’s DNA from
being masked by the victim’s and yielded a profile for the offender (Wickenheiser 444).
Another important factor that evidence collector’s must consider is the order in
which they swab various zones. On an object that could potentially have multiple sources
of DNA, this is very important. Wickenheiser suggests an approach that categorizes
different types of evidence based on their relative quantities of DNA. Category I evidence
includes rich sources of DNA, such as tissue samples or semen. Categories II and III
contain evidence with intermediate relative amounts. Evidence classified as Category IV
contains only small traces, such as handled objects or clothing. When processing a piece
of evidence such as a gun with blood and tissue present from blowback, Wickenheiser
recommends processing areas such as the handle or trigger first to attempt to collect DNA
from Category IV areas before they may be contaminated by DNA from lower category
areas (447).
Low copy number (LCN) DNA analysis is similar to touch DNA analysis in that
it deals with very small samples of DNA, less than 200 picograms. Sometimes the terms
are used interchangeably, but there is a distinct difference between the two methods of
trace DNA analysis. Emily Head, a scientist employed by the Alcohol, Tobacco, and
Firearms (ATF) laboratory in Beltsville, MD specializes in touch DNA analysis.
According to Head and various other sources, “LCN describes techniques that are done
post-extraction to increase your yield” whereas touch DNA analysis simply describes the
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analysis of DNA left on a surface without significant modifications to standard
procedures. The ATF laboratory does not perform LCN analysis because (as will be
discussed in the research below), LCN analysis of the profiles often generates difficult
and unreliable data.
Regardless of the debate over its validity, many forensic laboratories currently
perform LCN analysis. They employ various techniques to try to increase the yield of
DNA post-extraction. One of the simplest ways that laboratories accomplish this is by
increasing the number of PCR cycles. Each cycle doubles the number of amplicons, and
the sample is increased exponentially. By raising the number of cycles from between 28
and 30, which is the standard, to 34 and above, more copies can be produced (Gill 229).
Sometimes, additional Taq polymerase is added after 28 cycles to replace any that may
have begun to degrade from repeated exposure to rapid heating and cooling (van
Oorschot et al. 6).
Another method that attempts to increase the yield during PCR is the use of
primers made from “synthetic nucleotides with stronger binding capabilities than
standard nucleotides” (van Oorschot et al. 6). These primers may be more effective at
binding to the template strands and providing the starting point for Taq polymerase to
elongate the chain and form the new amplicon.
Whole genome amplification (WGA) is a method for copying the entire sample of
DNA, rather than just the sequences that contain the STRs of interest. Employing WGA
allows the scientist to increase the amount of the complete DNA sample, “producing
hundreds of nanograms form picogram input amounts” (van Oc et al. 7). Once the entire
genome has been amplified, a second round of PCR can amplify the specific target
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sequences that contain the STRs for analysis by capillary electrophoresis. Additional
modifications to PCR to try to increase yield in LCN samples may include reducing the
volume of PCR, thereby increasing the concentration of DNA template, using different
DNA polymerases, or adding BSA.
LCN methods also include post-amplification techniques for purifying and
manipulating the amplified product before STRS are separated and detected by capillary
electrophoresis. Filtration, silica gel membranes, and enzymatic hydrolysis will
“[remove] salts, ions, and unused [deoxyribonucleoside triphosphates (dNTPS)] and
primers from the [PCR] reaction” (van Oorschot et al. 8). Concentrating the solution
containing the PCR amplicons allows for more efficiency and greater amounts of DNA
loaded into the capillary for analysis. Scientists can further increase this amount by
increasing “the time or voltage, or both, of the electrokinetic injection” (van Oc et al. 8).
Once loaded into the capillary electrophoresis instrument, dyes that are capable of higher
intensity fluorescence can aid in the detection of minute quantities of trace DNA,
visualizing peaks on the electropherogram that may otherwise have been below the
threshold for detection.
Current Research
Research indicates that touch DNA may yield usable profiles even from objects
that have been handled by more than one person, such as the steering wheel of a car with
multiple drivers. The epithelial cells deposited on an object are not fixed firmly to the
object, but rest lightly to the surface. When an object is touched, skin cells containing
DNA from the “previous contributor will often be replaced by subsequent contact by a
second individual” (Wickenheiser 449). One case cited by Wickenheiser discusses an
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armed robber who entered a bank and wrote a hold-up note using a pen from the bank
teller counter. Numerous bank customers had used the pen previously. When swabbed
and analyzed for touch DNA, the sample produced a mixed profile, but the suspect’s
DNA was the major contributor and was “easily separated from a number of minor trace
profiles” (444).
A separate study was conducted to determine whether touch DNA could be
deposited on a surface by secondary transfer. Secondary transfer would occur if a person
touched an object that contained DNA from a previous contributor and acted as a vector
to move that DNA to a second object. This would create issues for forensic investigators
if DNA found at a crime scene may have arrived on an object by means of secondary
transfer; it would imply that a person who may not have been present or had no contact
with the particular object had been at the crime scene or touched the object personally.
While research shows that secondary transfer is possible, it is not a concern for criminal
investigations. When secondary transfer occurs, the vector individual will deposit far
more DNA than the transferred sample and would clearly be the dominant profile if
analysis results in a mixed profile (Wickenheiser 449).
Full profiles with peaks from only a single donor are most likely to be obtained
from larger samples of DNA. Research shows that a variety of factors influence the
amount of touch DNA that can be recovered from an object. Forensic scientists have
demonstrated that the length of time that the substrate is in contact with the donor has no
impact on the amount of DNA recovered. Transfer seems to be instantaneous and is not
increased by longer handling times. Two factors that have been shown to contribute are
the individual touching the object and the surface texture of the substrate. Some
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individuals naturally shed more cells than others. These individuals leave larger numbers
of epithelial cells on a surface and are called “sloughers.” Individuals who are poor
donors, leaving fewer cells on touched objects are “non-sloughers” (Wickenheiser 446).
Sufficient amounts of trace DNA for analysis are most likely to be found on rough,
porous substrates. Their surface texture is conducive to dislodging and trapping epithelial
cells. Smooth, porous objects (which are generally the best substrates for fingerprint
recovery) do not adhere cells as readily.
Numerous validation studies have been published evaluating methods to try to
obtain profiles from LCN samples. Many issues that have been identified and will need to
be addressed in future research. One of the issues identified by Bruce Budowle in a 2009
study is stochastic amplification. When the starting sample of DNA is too low, “primer
binding may not occur equally for each allele at a locus during the first few cycles” (210).
As PCR progresses and the DNA sample is amplified exponentially, the end result is that
certain alleles are amplified more than others, giving an unbalanced final product. This is
stochastic amplification and it can cause numerous other effects, including allele drop
out, heterozygous peak imbalance, and stutter.
Allele drop out occurs when “a sample is typed and one or more alleles are not
present” (NFSTC). As a result, no peak for that allele will appear on the resulting
electropherogram. An individual who is heterozygous for a particular locus will appear to
be homozygous if one of the alleles does not appear on the electropherogram, which
leads to ambiguity in interpretation of the graph. Furthermore, if the amount of amplified
DNA is too minute to pass the threshold of detection, there may be no peaks detected at
all.
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Theoretically, the two alleles for a particular locus should be present in equal
amounts because each allele is represented once in the genome; therefore, ideally, the two
peaks for a heterozygous locus should be the same height. In reality, this is not always the
case and two peaks are generally considered heterozygous if the height of the smaller
peak is at least 70% of the height of the larger peak (NFSTC). Stochastic effects resulting
from the unbalanced replication of small or degraded DNA samples frequently lead to
much greater imbalances and debate over how such peaks should be interpreted.
Stutter is “a by-product of the amplification of STR loci whereby a minor product
one repeat smaller than the primary allele is generated” (NFSTC). When utilizing LCN
methods and increasing the number of PCR cycles with small or degraded samples,
replication becomes less efficient and can result in numerous stutter peaks, which
correspond to lengths that do not accurately reflect any of the STRs. Not only does this
make analysis more difficult, but some stutter peaks may become so amplified that they
“may actually exceed the height/area of the associated allelic peak” (Budowle 212).
These stutters are sometimes referred to a “false alleles” because they indicate the
presence of an allele that is not actually present at the particular STR locus (Gill 230).
In addition to the concerns about stochastic effects, many validation studies also
refer to the fact that in trace quantities of DNA, very small amounts of contamination
may have a major impact on the results of an analysis. DNA from contamination may be
amplified along with the crime sample, resulting in peaks on the electropherogram that
should not be associated with the crime scene sample. These peaks are often referred to
as “allele drop-ins” (Budowle 213). Additionally, with extremely small samples, there is
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often insufficient material to repeat the analysis to check for contamination, bringing up
quality control concerns.
LCN typing is not without its value. It can be very useful “for developing
investigative leads” and “in the identification of human remains” (Budowle 215). These
are situations where low quality profiles can provide limited information that may assist
investigators. Due to its low success rate and the high risk of stochastic effects and
contamination, it is not strong evidence to unequivocally link a suspect to a case.
Future Applications
Trace DNA analysis is already a valuable forensic tool, but better education and
training for the collection and preservation of trace DNA and modification of current
latent print processing procedures will increase the amount of DNA available for
laboratory analysis. In addition, development of technology for improving the efficiency
of PCR for trace samples and the statistical analysis of mixed or incomplete profiles will
advance the science in the next few years.
Research has already identified techniques that collect and preserve the highest
yields of DNA and minimize the chances of obtaining a mixed profile. Crime scene
investigators, whether sworn officers or civilian evidence technicians, must receive more
instruction on proper sampling and preservation of trace DNA samples. This should
include training on swabbing different zones separately to avoid mixed profiles, and
swabbing in order of areas that likely contain the least DNA to areas with a greater
probability of having DNA rich evidence. As the courts continue to rely on the value of
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DNA evidence and the technology continues to improve, the attention generated by high
profile cases and breakthroughs will publicize the need for training.
Recent success in obtaining trace DNA from fingerprints may have a major
impact on the way fingerprints are processed, both at the crime scene and the lab.
Fingerprint powders are commonly used to visualize latent prints on evidence at the
crime scene, but their application with a brush sweeps away epithelial cells that could be
loosely attached to the fingerprint residue. Alternative methods that are non-destructive
have been identified and may replace destructive methods so that both fingerprint and
DNA evidence may be recovered. For example, trace DNA profiles can be generated
even after prints have been visualized by cyanoacrylate fuming and vacuum metal
deposition (Wickenheiser 446). Research completed at the Department of Forensic
Services in Washington, D.C. in the summer of 2013 demonstrated that both full and
partial profiles were generated after lifting latent fingerprints with gel lifters and then
swabbing and analyzing the DNA according to standard operating procedures (Cook).
Further research and development is necessary to maximize the potential of methods that
visualize latent prints without preventing subsequent touch DNA analysis of the same
substrate.
Improvements to the PCR process will also improve the quality and quantity of
product amplified from trace amounts of DNA, reducing stochastic effects and increasing
the probability of generating high quality, complete profiles. In addition, just as STRs
were an improvement over RFLPs, even smaller sequences known as single nucleotide
polymorphisms (SNPs) may improve trace DNA analysis. SNPs look at variation at a
single point in the DNA code, requiring only very small portions of DNA to be amplified.
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New fluorescent dyes or other markers will also lower the detection threshold, and
counter the effects of allele dropout.
Lastly, future research will aid analysts in interpreting mixed and partial profiles.
Analysts need an appropriate statistical model for determining the probative value of a
mixed profile or a trace DNA profile that shows evidence of allele drop-out, allele dropin, stutter, and heterozygous peak imbalance. Although these profiles will never be
considered as valuable as a profile showing clear peaks at all loci, their evidentiary value
may increase if analysts could speak with more certainty to the strength of the correlation
between a suspect and a partial profile.
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Works Cited
Budowle, Bruce, Arthur J. Eisenberg, and Angela van Daal. “Validity of Low Copy
Number Typing and Applications to Forensic Science.” Croatian Medical Journal
(2009): 207-217
Cook, Diamond. “Recovery of Touch DNA from Fingerprints on Gel Lifts.” Master’s
thesis, University of Strathclyde, 2013
Gill, Peter. “Application of Low Copy Number DNA Profiling.” Croatian Medical
Journal 42.3 (2001): 229-232. Web. 02 Oct 2013.
Head, Emily. “Forensics Talk.” Message to author. 22 Aug. 2013. Email.
National Forensic Science Technology Center. President’s DNA Initiative - DNA
Analyst Training. Web. 16 Oct. 2013
van Oorschot, Roland, Kaye Ballantyne, and R John Mitchell. “Forensic trace DNA: a
review.” Investigative Genetics (2010): 1-17. Web. 02 Oct 2013.
Wikenheiser, Ray. “Trace DNA: A Review, Discussion of Theory, and Application of the
Transfer of Trace Quantities of DNA Through Skin Contact.” Journal of Forensic
Sciences 47.3 (2002): 442-450. Web. 02 Oct 2013.