FREDERICK R. BIEBER
Harvard Medical School
SCIENCE AND TECHNOLOGY OF FORENSIC DNA
PROFILING: CURRENT USE AND FUTURE DIRECTIONS
DNA-based Human Identity Testing and its Forensic Applications
Introduction
While DNA-based genetic analysis has a long-standing and important role in research, medical diagnostics,
and in patient care, the central role of DNA profiling in forensic investigations has been emphasized in the
aftermath of the September 11, 2001 attacks on the U.S. DNA-based genetic profiling has been a key tool
for direct and indirect identification of hundreds of victims from these attacks on the U.S. Furthermore,
typing of relatives of criminal terrorist suspects has been suggested as a method for indirect identification of
recovered remains of suspected terrorist operatives whose identity is uncertain.
The utility of genetic profiling based on inherited DNA variation has come of age and has now gained
almost universal acceptance as a forensic tool that is central to the proper and complete investigation of
many civil and criminal matters. Such applications include use in civil matters involving claims of patient
sample or nursery mix-ups, paternity testing, as well as probate and immigration disputes. DNA profiling of
crime scene evidence and comparison to known samples from victims or suspects provides a powerful
exculpatory or exclusionary tool that has proven fundamental in resolution of many investigations of felony
crimes. Study of close relatives of missing persons and unknown soldiers provides an indirect method to
assign identity to recovered remains that would otherwise be unidentifiable. Various types of data banks
and databases of the DNA samples or DNA profiles are now in widespread use, allowing linkage of
evidence to possible suspects, for victim identification, and family reunifications.
Application of current and future technologies offers exciting opportunities as well as some troubling
dilemmas and challenges. Practitioners and society must consider the nature and extent to which
technology and data sharing should be used in the interest of public safety. This chapter provides
background technical information on current DNA-based laboratory methods used in human identity
testing, an overview of computerized searching of forensic DNA databases and a view to their future use.
Problematic areas relating to the science and applications are addressed.
History of Forensic Identity Testing
Human forensic identity testing can trace its modern origins to the late 19th century when several individuals
in Argentina and in Europe, including British physician/geneticist Sir Francis Galton, recognized the utility
of fingerprint ridge pattern analysis for forensic use (see Cole S, in this volume). Galton, a cousin of
Charles Darwin, was an early Mendelist who, with others, recognized that digital fingerprints analysis could
be used for identification (Galton, 1892; Cole, 1998). Galton’s own studies of the various fingerprint
patterns on the volar surfaces of the distal phalanges revealed that even monozygotic twins have distinctive
patterns. The use of such fingertip ridge analysis by the French and British police gained widespread
adoption in Europe and Latin America in the late 19th century, and such methods rapidly spread to other
police and investigative agencies around the world.
During the first decades of the twentieth century, laboratory methods were developed for discrimination of
genetically determined variation in human red blood cell antigens. The central importance of these
discoveries became clear with their use for serotyping those needing blood transfusions due to illness,
during surgery, and especially during wartime or other armed conflicts. The human blood groups were
quickly recognized as useful genetic markers for the study of human population genetics, and their forensic
utility in paternity disputes, nursery mix-ups, and criminal investigations was quickly recognized.
For most of the past century blood-group antigen typing, and other laboratory techniques for detection of
inherited variants in blood cell enzymes or serum proteins were widely used for paternity testing in both
civil paternity and criminal paternity. While the discriminating power of blood group typing does not
compare with that of DNA profiling, it was used throughout the world before the introduction of
contemporary DNA-based genetic profiling. These blood-grouping techniques also were used for medical
applications such as typing and cross matching for selection of suitable donors of blood and blood products,
bone marrow, and whole-organ transplantation. During this same period, forensic applications of these
techniques to tissue, blood, saliva, semen and other body fluids from both evidentiary and known samples
obtained as dried stains or as liquid or solid samples at crime scenes have been introduced into courtrooms
throughout the world (Saferstein, 2000). It was not until the mid-1970s that methods to detect individual
genetic variation at the DNA level were introduced, rapidly changing medical research, diagnostics, and
forensics.
Since 1975, rapid technological changes have allowed highly discriminating DNA profiling to be
accomplished using trace samples found at crime scenes. These laboratory methods include techniques for
DNA amplification, fragment separation and direct sequencing. In addition computerized storage and
searching of DNA profiles from known individuals and from crime scene samples has permitted
investigation of crimes across jurisdictional boundaries. These issues will be discussed more fully in the
sections below.
Even as DNA has competed vigorously with digital fingerprinting for the role as the gold standard in
identity testing, both methods of “fingerprinting” have come under fire in the courts. For example,
challenges to certain aspects of fingerprint analysis have recently occurred. These challenges have asked
whether the theory, methods and interpretation of these fingerprint patterns meet the requirements outlined
in the Federal Rules of Evidence and those set forth under Daubert by the U.S. Supreme Court (see
Daubert vs. Merrill Dow Pharmaceuticals, 113 S. Ct. 2786 (1993). In a recent ruling by the U.S. District
for the Eastern District of Pennsylvania, Hon. J. Curtis Joyner denied defense motions to exclude fingerprint
evidence and granted the government’s motion to exclude the defense witnesses. U.S. vs. Byron Mitchell,
CR No. 96-00407; and Imwinkelried in this volume). On March 13 of this year, Senior U.S. District Justice
Louis H. Pollack of the Eastern District of Pennsylvania reversed his previous decision in U.S. v. PlazaLleera, Acosta and Rodriguez that barred fingerprint experts from telling juries that two fingerprints are a
match. His ruling outlined his reversal and his conclusion”… that the standards which control the opining
of a competent fingerprint examiner are sufficiently widely agreed upon to satisfy Daubert’s requirements.”
(U.S. v Plaza, Acosta, Rodriguez).
Genetic Variation at the DNA level and Its Utility as a Biological Identifier
DNA contains a chemical code that governs production of proteins, enzymes and other nucleic acids. DNA
is divided and packaged into chromosomes that reside in the nucleus of individual cells. During each cell
division these chromosomes, and the DNA contained therein, replicate and divide assuring that the two
daughter cells contain the same set of genes as the parent cell. During gametogenesis (formation of sperm
and egg) half of the chromosomes (and DNA) are transmitted to each offspring, in accordance with the 19 th
century discoveries of Gregor Mendel. In addition to genes inherited from both parents and packaged in
the chromosomes in the nucleus of all cells, there is additional genetic material inherited only from mothers
or from fathers. For example, mitochondria are transmitted in the cytoplasm of a mother’s egg cell to all of
her sons and daughters- they are not normally transmitted by fathers. Mitochondria are essential in cellular
energy production and replicate and segregate during each cell division, assuring that daughter cells each contain
multiple mitochondria. Multiple mitochondrial chromosomes (each consisting of a single DNA molecule) exist in each
of the many extranuclear organelles known as mitochondria, which exist in many copies per cell. Similarly, genes
located on the Y chromosome are transmitted from fathers only to their sons.
FIGURE 1.
PEDIGREE SHOWING DIFFERENCE BETWEEN NUCLEAR AND MT DNA
TRANSMISSION IN FAMILIES
The DNA base pair sequence is normally identical in all cells in an individual. The “genetic code,”
common in all living organisms, allows the specific DNA sequence of the AT or GC pairs to encode the
chemical message that will determine the specific chemical structure of gene products - either RNA
molecules or proteins. Current estimates suggest that the 3 billion base pairs of DNA in humans encode
approximately 30,000 genes that produce an even greater number of functional gene products. A host of
additional molecules (e.g. spliceosomes, chaperones) are involved in the complex process of protein
synthesis, and alternative splicing of the DNA transcripts increases dramatically the actual number of gene
products encoded by a more limited number of genes. These gene products, in turn, govern most other
chemical and physiological processes. Mutations and other genetic variants occur commonly in the
functional genes, forming the basis for certain heritable diseases, or advantageous traits, depending upon the
specific nature of the DNA alteration, the other repertoire of genes, and upon the environmental condition
of the organism.
Variation in Non-Coding DNA sequences
In addition to DNA sequences that encode functional gene products, a large proportion of many animal and
plant genomes contain sequences that do not appear to encode functional genes and may have other
functions. DNA sequence variation between individuals in these non-coding regions allows comparison of
the similarities and differences among members of the same (or different) species. Depending upon where
in the genome they occur, DNA sequence differences in the non-coding regions between individuals may
have no functional importance or may also confer selective advantage or disadvantage to the individual
organism.
Geneticists have documented that while over 98% of the human genome appears to be very similar in most
individuals, considerable variation exists in the DNA of the non-coding regions (see Lander et al., 2001;
Venter et al. 2001). This genetic variation, transmitted from parent to child, is well studied by those
interested in heritable disease, and by forensic scientists and others concerned with use of genetic markers
to aid in identity testing. Inherited variation in specific regions (genetic loci) is analysed to identify the
specific variant DNA lengths or sequences (known as “alleles”) present in a particular DNA sample
extracted from a patient or, in the forensic context, a known individual or item of evidence containing a
biological fluid or tissue. Variant regions of forensic interest exist in the DNA comprising the autosomes
(22 pairs in humans), the so-called sex chromosomes (a single XX pair in females, a single XY pair in
males) and in the DNA of the cellular pool of mitochondrial chromosomes (mtDNA).
Contemporary Forensic DNA Profiling
There are many uses for forensic profiling of inherited DNA variants in matters pertaining to law and the
courts. In addition to the well-known use of these techniques in murder and sexual assault investigations, in
which DNA is extracted from crime-scene evidence and the DNA profiles compared to those obtained from
known individuals (i.e., victims or suspects), all of the methods and procedures described in this chapter can
be utilized in identification of bodies from natural or accidental disasters (e.g., identification of plane-crash
and other victims from the 9/11/01 attacks on the U.S.), identification of war-crime victims (e.g., Kosovo,
Bosnia), reunification of family members separated by war, natural disaster, or political oppression (e.g. ,
Argentina), and identification of “unknown soldiers”. Other applications include animal forensics (e.g., in
poaching and bird-smuggling cases and endangered-species identification), and the study of human origins
via population genetics (Balazs et al., 1989; Helmuth et al., 1990; Comey and Budowle, 1991; Galbraith et
al., 1991; Krane et al., 1992; Alford et al., 1994; Hammond et al., 1994; Kimpton et al., 1994; Hochmeister
et al., 1995; Huang et al., 1995; Lygo et al., 1996; Sensabaugh and Kaye, 1997).
Scientists interested in genetic variation at the DNA level in animal and plant populations utilize a number
of methods for study of DNA samples. First, chemical and physical extraction methods are capable of
removing intact DNA molecules from various tissues, even those compromised by environmental or storage
conditions (see Bieber, 1998; Lee, 2001). The chemical methods utilize detergents to remove other cellular
or chemical components of the tissue or fluid sample while physical methods utilize special synthetic
membranes to physically bind DNA molecules from individual samples. Second, the polymerase chain
reaction (PCR) allows amplification of trace amounts of extracted DNA into a quantity sufficient for
analysis. Third, automated methods for direct determination of the base-pair sequence of DNA are now
available and a variety of laboratory methods allow detection and comparison of length differences in
certain defined regions of the genome that are known to exhibit stable inherited variation. Computerized
digital storage of the resulting DNA profile (i.e., the specific length or sequence of DNA identified at one or
more loci) allows rapid comparison of sample profiles from evidence or from certain individuals whose
profiles have been stored in a central database.
1st publicly recognized use of DNA profiling in forensics
The first major publicly recognized forensic use of DNA-based genetic profiling involved study of DNA
polymorphisms (i.e., variations) in crime scene evidence in England in the mid-1980s. Alec Jeffries, a
professor at the University of Leicester, utilized multilocus DNA probes to study DNA extracted from
crime-scene evidence samples (obtained at autopsy from two teenage rape/homicide victims) and compared
the patterns produced to those from DNA obtained under court order from a confessor to one of these two
crimes. Jeffries' laboratory findings appeared to exclude the (false) confessor as a source of the DNA in
both of the evidence samples, and, at the same time, documented the apparent genetic similarity of the
profiles from the two crime scenes, indicating a single source of both of the evidentiary seminal stains (i.e.,
a serial killer). The search for the perpetrator of these two sexual homicides involved a voluntary blood
donation (or "blooding") of adult males within the region in which the murders occurred (Wambaugh,
1989). This process eventually identified the perpetrator of the murders.
This dramatic use of modern DNA-based methods for forensic testing led to replacement of the older
serological blood typing methods in virtually all laboratories in developed countries by the late 1990s. It
also drew attention to the possibility of typing large numbers of individuals for elimination as suspects, and
raised the idea of computerized storage of the DNA profiles of large numbers of individuals for use in
investigations of unsolved crimes. In the United States, the first apparent use of PCR-based forensic DNA
typing was to confirm that two autopsy samples were derived from the same person (Pennsylvania v.
Pestinikis).
Admissibility of and Challenges to Forensic DNA Evidence in the Courts
While DNA-based laboratory methods have gained widespread acceptance in most areas of biological and
medical research and in medical diagnostics, vigorous (and often successful) challenges to DNA-based
identity testing results have been made in courtrooms across the United States and elsewhere (Coleman and
Swenson, 1994; Billings, 1997). These challenges have been based on issues surrounding the collection,
transport, and preservation of evidentiary samples, chain-of-custody documentation, and matters pertaining
to State and Federal rules of evidence (see Imwienkelried in this volume; Billings, 1992; Wooley and
Harmon, 1992; Aldhous, 1993; Thompson, 1993; Scheck, 1994; Miles et al., 1995; Bieber, miles, mcle). In
many State courts, novel scientific testing must meet the so-called "Frye test" or "Frye Standard." This
refers to a landmark 1923 case (Frye v. U.S.) in which the Washington D.C. Appeals Court ruled that
scientific evidence and theory must meet a general acceptance standard in the scientific community before
such evidence can be admissible in court. This concept of "general acceptance" has led to hundreds of
decisions in the State and Federal courts with regard to DNA-based forensic testing, the PCR-based
methods, and the foundations of human population genetics in the calculation of the combined match
probabilities. Since the U.S. Supreme Court's 1993 ruling (Daubert v. Merrill Dow Pharmaceuticals),
many states have adopted the so-called "Daubert standard," which allows the Courts more latitude, with the
judge as the "gatekeeper" in deciding which new evidence may be helpful to the finder of fact (i.e., the
jury). Issues of admissibility, qualifications of laboratory personnel, compliance with laboratory standards,
statistical interpretation of results, and laboratory proficiency testing continue to be contentious issues in
many court cases in which DNA-based identity-testing results are offered into evidence (see Imwinkelried,
this volume). Indeed, as this chapter was been written, admissibility challenges were in progress in
Hennepin County, MN over the matter of whether DNA testing should be admissible when generated using
unpublished PCR primer sequences that are held as proprietary intellectual property by the manufacturer.
The rigorous challenges to DNA evidence have in a meaningful way altered the landscape of admissibility
of all types of forensic evidence and have increased the scrutiny placed on collection and transfer analysis
and interpretation of all forensic evidence.
In spite of the so-called “DNA wars” fought in the courtrooms, it is important to underscore the role of
DNA-based genetic-identity testing as an exculpatory tool in addition to its potential exclusionary value
(see M. Berger in this volume). As many as 30% of all DNA-based paternity test results exclude the tested
male as the biological father of the referent child. Similarly, DNA-based forensic testing frequently
excludes suspects, defendants, and even persons already convicted and incarcerated as sources of important
evidentiary blood, body-fluid, or tissue samples. Already, over 100 [now at 108] post-conviction
exonerations have now occurred in the United States using DNA typing that was performed months or years
after convictions have occurred. Several of these post-conviction exonerations have involved death-row
inmates (see National Institute of Justice, 1996).
Current Methods for Forensic DNA Analysis
Several laboratory methods are utilized for forensic DNA profiling. Current methods utilize the polymerase
chain reaction to amplify trace amounts of DNA for later analysis using protocols to detect DNA sequence
or length variation. The methods, techniques, and laboratory practices used in forensic DNA analysis are
similar and in some cases identical to those used for research and for clinical diagnostic protocols. What is
notable about their use in forensic identity testing is the application of the high discriminating power of the
profiling systems for purposes of genetic exclusion and their central role in contemporary criminal
investigations, inheritance, and immigration disputes. Also unique are the many implications in society
because of the controversial role of DNA-based profiling in matters pertaining to DNA data banks that hold
computerized records of DNA profiles of convicted felons, crime scene evidence, and military personnel
(Herrin, 1993; Imwinkelreid , in this volume).
The Polymerase Chain Reaction (PCR)
Once intact DNA has been extracted from a known or evidentiary source, most current laboratory methods
for forensic DNA analysis take advantage of a laboratory method known as the polymerase chain reaction
(PCR). PCR is used for amplification of specific regions of the genome in the specific organism tested.
PCR methods allow amplification (or copying) of trace amounts of evidentiary DNA isolated from any
number of plant and animal species. The PCR process mimics the normal cellular processes used in the
replication of DNA molecules in living cells and organisms. After 25-35 cycles of PCR-based DNA
amplification, millions of copies of the original source DNA molecules have been created, yielding
sufficient DNA to allow detection of length or sequence variation that existed in the original biological
sample. Validated PCR-based methods include tests for genetic variation in either the DNA sequence or in
the length of certain defined DNA segments. These inherited variations are known as sequence
polymorphisms and length polymorphisms, respectively.
Indirect Detection of PCR-Amplified DNA Sequence Variation - The Reverse Dot-Blot
Method
An important laboratory procedure, developed in the late 1980s and used widely since the early 1990s,
detects DNA sequence polymorphisms using an indirect method referred to as the “reverse dot-blot”
system. In this system, DNA molecules are labeled with chemical tags during PCR amplification. These
tagged PCR products are then hybridized to a membrane containing fixed probes specific for the different
possible alleles at particular genetic loci. This method first was developed by scientists at the Cetus
Corporation and then by Roche Molecular Systems (Saiki et al., 1989; Reynolds et al., 1991). With the
commercially available kits (Applied Biosystems, CA), it is possible to amplify alleles at six distinct genetic
locations (or loci) simultaneously in a single multiplex reaction. As little as 0.5 ng of DNA can be typed
using this method (e.g., 40-50 cells from a single hair root, or a single drop of blood). The kits contain all
the necessary reagents, including primers for PCR and the typing strips. Because PCR methodology
accomplishes efficient amplifiation of even trace amounts of DNA, great care is needed to reduce the
chance of introducing extraneous DNA from other sources into the process. A separate area is therefore
typically reserved for the preamplification and post-amplification steps of the PCR process (see Bieber,
1998; Holland and Bieber, 2000).
Detection of Genetic Variation in Length of PCR-amplified DNA Products
In the 1990s PCR-based systems were developed to detect DNA length polymorphisms in forensic samples.
Throughout the human genome, there are DNA sequences that are repeated side-by-side (i.e., in tandem) at
various locations (loci) throughout the chromosomal DNA. They are not located within coding regions of
functional genes and, at a given locus, the specific number of these repeat units varies in the population,
making these markers useful for forensic purposes. Some of these loci are termed Variable Number of
Tandem Repeats (VNTRs) and smaller repeat units (2-7 Watson-Crick base-pair repeats) are termed Short
Tandem Repeats (or STRs). STRs (and VNTRs) are stable and inherited as Mendelian traits. PCR primers
can be designed to amplify STRs, such that the amplified PCR products can be separated and their size
determined. At any given locus, any individual could have one or two STR alleles, depending upon
whether they inherited the same or different sized repeat units from each parent. Some STR variation also
occurs on the human Y chromosome and these markers can help distinguish male contributors to
evidentiary stains.
In forensic applications, laboratories most commonly perform the multiplex PCR amplification of between
six and 16 separate autosomal STR loci along with the X- and Y-linked amelogenin alleles. Commercial
kits allow detection of inherited variation at a single genetic locus or at multiple loci simultaneously (socalled multiplexing). Because the STR PCR amplification products are relatively small (i.e., less than 400
bp in size), STR genetic typing has found widespread forensic use because degraded DNA samples often
preclude use of previous methods requiring more intact DNA. Visual comparison between the allelic ladder
and amplified samples of the same locus allows rapid and precise assignment of alleles. Results can also be
recorded in a digitized format, allowing direct comparison with stored databases.
Detection and typing of variation in length of the PCR-amplified four and five base pair STRs and of the
X- and Y-chromosome specific amelogenin alleles, is based on the migration of the PCR-amplified alleles
in relation to a laboratory sizing standard used for comparison (known as an “allelic ladder”). PCRamplified alleles can be resolved using several laboratory methods, all involving separation of the DNA
fragments based on their size (i.e., the number of repeats plus flanking DNA) (see Budowle, 2001; Butler,
2001).
INSERT FIGURE HERE
Additional DNA Variation only Inherited from Mothers or Fathers
While the DNA profiling methods described above apply to both males and females, two additional
categories of forensic DNA profiling have widespread application to forensic identity determination. These
methods are distinctive in that they identify DNA variations inherited only from mothers (mtDNA) or only
inherited from fathers to sons (Y-chromosomes).
Study of Mitochondrial DNA (mtDNA) Inherited from Mothers
First, DNA known as mitochondrial DNA (mtDNA) is transmitted almost exclusively by mothers to their
offspring in the cytoplasm of the egg cell. This mtDNA is different than and replicates independently of the
nuclear DNA described previously. Ample mtDNA sequence variation exists that can be extremely useful
in population studies and for forensic profiling. This variation is detected by direct sequencing of the DNA
base pairs in variable regions of the mitochondrial chromosome (Holland, parsons, smith).
Because mtDNA is found in extranuclear cell components, certain tissues, including bone and hair, contain
ample mtDNA but no nuclear DNA. This becomes very useful when such tissue is the only available tissue,
or when nuclear DNA is compromised due to post-mortem decay or to other detrimental environmental
effects on nuclear DNA. Thus, in some compromised biological crime scene or disaster scene evidence,
mtDNA analysis is possible while traditional nuclear DNA analysis is not. Because mtDNA is maternally
inherited, all maternal relatives will typically have the same mtDNA profiles. This is convenient for kinship
analysis or for victim identification when a known comparison sample for a decedent is unavailable for
study. While mtDNA profiling allows certain types of family reconstruction to be accomplished, mtDNA
profiling (by direct sequencing of the mtDNA) would not distinguish full brothers (or sisters) with the same
mother.
Genetic Variation at Y-Chromosome Loci Inherited by Sons from their Fathers
In addition to polymorphic loci located on the autosomes (non-sex determining chromosomes), length
polymorphisms at numerous Y-chromosome specific STR loci have been identified (see Butler, 2001).
Considerable DNA variants exist on the Y-chromosome, transmitted by males normally only to their sons.
This variation, in the form of “Y-STR variants“ can be detected using the same PCR-based methods
described above for variants on the non-sex chromosomes. The human Y-chromosome contains gene
sequences critical to normal testis differentiation (and therefore to “maleness”), in addition to non-coding
sequences that can be typed by STR analysis or by direct sequencing. Once many such Y-linked STR
“loci” are typed, a so-called haplotype (or “haplogroup”) can be identified that would be similar in all male
descendents of a particular man. Because little genetic recombination occurs between the X and Y
chromosomes in males, the Y-specific DNA is transmitted basically intact from fathers to sons, generation
after generation (N.B. This method of Y-chromosome STR analysis was used to define a possible link
between President Thomas Jefferson and Sally Hemings. As Thomas Jefferson did not have a namesake
son survive to reproduce, it was necessary to locate male-line descendants of Jefferson’s paternal uncle,
Field Jefferson, and the sons of Thomas Jefferson’s sister, whom some consider as possible fathers of Sally
Hemings’ children.)
Y-chromosome STR analysis has proven quite useful addition to the standard panel of autosomal loci used
in forensic analysis. In particular, Y-chromosome profiling is useful in investigations of cases involving
sexual assault in which a DNA mixture of a female victim and one or more male contributors is found.
Additional Y-specific DNA sequence variation can be examined using SNP analysis (see below) or by
direct DNA sequencing. Numerous typing systems are now available for determination of allelic variation
at several loci along an individual Y chromosome (known as the “Y-STR haplotype”) in a given human
DNA sample (see Butler, 2001; Sinha, 2000).
There will be a limited number of Y-STR (or Y-SNP) haplotypes and, therefore, such DNA profiling cannot
distinguish all males from one another, most certainly not males who are directly related from a common
direct paternal line. Nevertheless, Y-STR haplotyping and mtDNA sequencing, along with routine nuclear
DNA profiling can be used alone or in combination, depending upon the particular circumstance. For
example, in mass disaster investigations, Y-STR analysis can determine whether the unidentifiable deceased
body (i.e., the source of a particular DNA sample) is excluded or included as the possible relative (e.g., son,
father, brother) of a surviving or deceased male whose DNA reference source is available.
Statistical Interpretation of DNA Profiling Results
The objective of the PCR-based forensic DNA profiling is to compare the alleles representing the genetic
types present in the DNA extracted from the evidence to genotypes of DNA extracted from known blood or
tissue. After careful laboratory analysis, usually one of the three following interpretations is made assuming
DNA could be extracted from the evidence.
Inclusion: Known or reference standard sample alleles are present in the evidentiary or questioned sample.
Exclusion: Known or reference standard sample alleles are not present in the evidentiary or questioned
sample.
Inconclusive: No conclusion can be made as to possible source of the DNA extracted from the questioned
sample.
Depending upon the specific evidentiary sample, mixtures may be identified and the interpretation of the
evidence may be complicated by presence of alleles either found or not seen in both known and questioned
samples.
Once interpretation of a DNA inclusion is made, it becomes crucial to perform the appropriate statistical
interpretation of the data. The “finder of fact” needs to know how common or how rare a particular DNA
profile is. In practice, this calculation permits an estimate of whether the profile that matches the evidence
and the victim/suspect could be frequently encountered (e.g., found in one of 10 individuals) or that it is so
rare that for practical purposes it should be considered unique or individualizing.
In essence, the calculations use the so-called “product rule” to allow multiplication of the expected
frequency of a DNA profile at each locus with one another, to produce an estimate known as the “point
estimate of the combined match probability“. These statistical estimates are derived from knowledge of the
frequency in various populations of the specific alleles found in the key evidentiary samples. Thus, a
particular multi-locus DNA profile would be expected less frequently if there were rare alleles than if there
were common alleles. Similarly, a DNA profile based on typing of 13 independent loci would be expected
to be seen in an unrelated individual less frequently than would a 5 or 6-locus profile. Simply put, the less
common the allele(s) identified, and the larger the number of loci typed, produce a “match” statistic that
would predict a smaller chance of observing the particular profile in a randomly selected unrelated
individual.
Thus, for example, if a sample was matched to an individual, based on two loci, where the alleles observed
at each locus occurred with a frequency of one in ten individuals in the population, then the calculated
random match probability would be “1 in a 100”. To change the scenario a little, if the calculation were
based on 13 loci, where each allele occurred with a frequency of one in ten individuals, the calculated
random match probability would be “1 in 10 trillion.” Actual estimates generated can range from “1 chance
in 10” for a single locus profile to “1 chance in 700 quintillion or less” for a 16 locus profile with
uncommon alleles.
These statistics provide an estimate of the expected frequency of a particular profile (seen in both an item of
evidence and in a known source) in an unrelated randomly selected individual. It might be assumed that a
very rare multi-locus DNA profile generally would likely be considered by jurors to be more incriminating
than a common one. Because of the multiplication step using the product rule, the number of independent
loci at which results are available dramatically changes the frequency estimates from the thousands to the
billions, quadrillions, or even quintillions. These estimates vary according to differences in frequencies of
alleles in different human populations, and can even differ within populations (this is known as “population
substructure”).
The statistical calculations of DNA profile match probabilities can be highly contentious in the courtroom,
as they typically estimate the frequency of multi-locus STR profiles as being so rare that, for example, “only
1 in 7 quadrillion randomly selected individuals would be expected to have a certain profile found in both
the evidence and a known source“. This essentially becomes a source identity statement (i.e., “the DNA at
the crime scene comes from John Smith”) and while it has been argued that such statements are unduly
prejudicial, they are based on sound statistical estimation methods using databases of allele frequencies in
well-defined human populations. In fact, so-called “John Doe warrants” have been issued for the
“owner” of a certain specified multi-locus STR profile in instances in which a key evidentiary DNA profile
is obtained but the unidentified suspect remains at-large. This strategy attempts to avoid statute of
limitation problems in cases where the perpetrator was unseen or unidentified.
Which Methods to Use?
Considerations affecting which of the various laboratory methods to use include the nature, size and
condition of the samples, and the number that will be analyzed. The PM+DQA1 test, based on dot-blot
techniques, is the least technically demanding to perform, both in terms of experience and equipment
requirements. A reasonably high throughput is possible (50 samples can be amplified and hybridized in a
day by a single technologist) and the equipment requirements are minimal. This factor, together with its
relatively high power of discrimination (Sensabaugh and Blake, 1993) as a result of using 6 different loci,
made the PM+DQA1 test particularly suitable for screening large number of samples. The PCR products
are small, ranging from ~250 to 150 bp; hence this test will also perform well when the DNA in the sample
is degraded, a very common occurrence in forensic samples.
However, the traditional dot-blot methods are not ideal for analysis of DNA mixtures and the STR-based
systems are more useful. Detection of low copy number samples will increase the sensitivity, but not
necessarily the specificity of the analysis. Typing more loci increases the power of discrimination, thereby
increasing the opportunity to exclude the falsely accused donor of a forensic sample, while also making the
estimate of the combined match probability more incriminating when there is a “match” at all typed loci to a
known reference DNA sample.
Although the amount of DNA template required for typing the D1S80 locus and the STR/amelogenin loci is
about the same as that needed for the PM+DQA1 test, the former two tests require a higher-quality DNA
and thus may not always perform as robustly when the DNA template is degraded. As the demand for
greater volume of testing increases, newer technology along with more robotics has become incorporated
into the forensic laboratories. STR systems (e.g., array detection of single nucleotide polymorphisms
(SNPs) and mass spectroscopic analysis) have already replaced the dot-blot systems in most laboratories in
the U.S., Canada, and Europe.
Critical Scientific Parameters for Reliable Forensic DNA Analysis
Two National Research Council Reports (1992, 1996), several FBI DNA Advisory Board (DAB)
Guidelines (see FBI, 1998), and numerous independent validation studies (see Bing, Budowle) have
detailed the general scientific practices necessary for sound forensic DNA analysis. These reports, along
with the individual laboratory and analyst experience form the basis for current use and interpretation of
forensic DNA evidence.
General Laboratory
Chain of custody, quality control, quality assurance, and care in sample handling and transport are
particularly relevant in forensic investigations, where attention to detail in sample collection and legal chain
of custody of all samples must be maintained (Lee et al., 1993; TWGDAM, 1995, FBI DAB). While small
samples and sample mixtures are occasionally present in human genetics research and in diagnostic clinical
genetics, these two challenges are often the rule in DNA-based forensic testing. Also, mathematical
approaches to interpretation of results are a key factor in the interpretation of results and in their acceptance
in the courts (Steinberger et al., 1993; Weir, 1996).
As a general rule, no forensic sample is processed unless the technologies have been validated and
performed according to the DNA Advisory Board Standards, using standard control reference material
supplied by or traceable to NIST. Other laboratory considerations include maintenance and calibration of
equipment, assay of the quantity of DNA amplified, use of methods to ascertain that DNA amplification has
occurred, and use of appropriate controls (TWGDAM, 1995, DAB,’98)
VALIDATION STUDIES AND POPULATION STUDIES
Adherence to high standards and use of quality equipment and reagents is clearly important (CPHG, UNITS
8.2 & 9.2). Careful validation studies of the reagents and commercially available "kits" used in forensic
genetic typing have been performed by a number of groups, indicating that the results of such typing, when
performed correctly according to appropriate protocols, are reliable and trustworthy (Cotton et al., 1991;
Budowle et al., 1992; Lander and Budowle, 1994; Budowle et al., 1995a,b). Specific genetic loci typed in
forensic DNA typing must therefore behave in accordance with known biological rules (e.g., Mendel's laws,
Hardy-Weinberg Equilibrium), and must be independent of one another in order to use the “product rule”
for statistical estimation of expected profile frequencies (National Research Council 1992, 1996; also see
CPHG, UNIT 1.4). The two National Research Council Reports make specific recommendations in regard
to the laboratory and population-genetic aspects of forensic testing and data analysis. There is also an
abundance of journal literature on these subjects (Lander, 1989; Budowle et al., 1991; Evett and Gill, 1991;
Gill et al., 1991; Edwards et al., 1992; Sajantila et al., 1992; Lewontin, 1993; Richards et al., 1993; Weir
and Evett, 1993; Neeser and Liechti-Gallati, 1995; Weir and Buckleton, 1995; Evett et al., 1996; Micka et
al., 1996, other refs too,,,,DAB standards).
LABORATORY AND PERSONNEL ACCREDITATION
Several professional agencies have been involved in accreditation and certification of laboratories and
personnel involved in genetic testing and human identity testing. These include the American Board of
Medical Genetics (ABMG) and the American Board of Pathology (ABP), which certify doctoral-level
scientists and physicians in several areas including clinical molecular genetics and molecular genetic
pathology (involving required training and exam questions on paternity testing and forensics). Other
agencies include the American Association of Blood Banks (AABB), the American Board of Criminalistics
(ABC), and the American Society of Crime Laboratory Directors (ASCLD).
LABORATORY ACCREDITATION AND QUALITY ASSURANCE
Forensic laboratories in the U.S. followed guidelines promulgated by the Technical Working Group on
DNA Analysis Methods (aka TWGDAM) in the early 1990s. In late 1995, in accordance with the Federal
DNA Identification Act of 1994, the Director of the FBI appointed a DNA Advisory Board (DAB) and
charged its members with promulgation of updated recommendations and policies on quality-assurance
standards for forensic DNA testing laboratories. The DAB produced several new recommendations,
approved by the FBI Director, that define minimum quality-assurance standards and place specific
requirements on the forensic laboratories. These policies are mandatory for receipt of federal funding and
participation in CODIS (FBI/DAB, 1998). Quality control and proficiency testing are also addressed,
including minimum education and training requirements for laboratory personnel. The DAB dissolved in
late 2000 in accordance with the Federal statute. Recommendations or changes dealing with quality
improvement and other technical matters will, in the future, be made by the Scientific Working Group on
DNA Analysis Methods (aka SWGDAM). Several proficiency-testing programs are available for forensic
and paternity testing, including those administered by the College of American Pathologists (CAP). CAP
offers regular surveys (external open proficiency tests) in many areas of medical diagnostics, including
genetic testing, and in paternity testing and forensics.
CAVEATS IN THE INTERPRETATION OF FORENSIC DNA TESTING
Laboratory forensic geneticists are in a unique position in the search for truth in cases involving genetic
identity testing results. While attorneys for the prosecution and the defense are clearly advocates for both
justice and for their clients ("the people" and “the defendant”, respectively), the scientist/DNA analyst
called as an expert witness has the distinct and important role as an educator to the judge in the pretrial
hearings and to the finders of fact (i.e., the jurors) in the jury trial. As an expert witness, the forensic
scientist is asked to offer fact testimony as well as opinion testimony. This is a special role in the
adversarial justice system and should, in this author’s opinion, be exercised in a cautious and conservative
fashion. In reality, this does not always occur and several forensic scientists have been investigated for
allegedly falsifying or fabricating data, results, and trial testimony (see NY Times, OK City Tribune).
Several examples underscore the importance of understanding the caveats in interpretation of DNA
evidence and the role of the expert witness as an unbiased participant in forensic casework. For example,
there are several cases involving DNA-typing evidence linking an individual suspect or victim who happens
to be a monozygotic twin (i.e., an "identical twin") to a person who is the source of crime-scene evidence.
Clearly, analysis of nuclear DNA polymorphisms cannot distinguish between monozygotic twins (or
triplets). Thus, a so-called "DNA match," while inclusionary, would not necessarily be probative.
Similarly, DNA patterns from close relatives (especially those from genetic isolates or those conceived by
consanguineous parents) have a greater chance of matching than do those from randomly selected, unrelated
individuals, and this is often important to consider in the statistical analyses (Budowle, 1995).
Conversely, a "non-match" (or apparent exclusion) could be misleading to the finders of fact (jurors). For
example, DNA typing of blood samples obtained from suspects/victims who were bone-marrow recipients
after the time the crime-scene evidence was collected will not match the patterns/alleles identified from
DNA extracted from peripheral-blood lymphocytes obtained after the marrow transplant.
It is crucial to note that the finding of a particular DNA profile at a crime scene does not provide any
information about when and how it was deposited. Conversely, not finding a particular DNA profile at a
scene does not indicate that the “owner” of that profile was not there at the scene. While most geneticists
recognize that DNA typing cannot determine when or how the evidence was deposited at a particular
location, this is not as obvious to the judge, lawyers, members of the jury, and the interested public.
Evidence collection and chain-of-custody protocols are crucial to help insure that the scientific data are both
reliable and probative. Degraded samples and contamination or admixture with nonhuman DNA (e.g.,
contamination with bacterial, animal, or plant DNA) present special challenges to the forensic DNA analyst.
Compiling and Searching of DNA Databases
In a practical sense, banking of DNA samples and DNA profiles existed before the interest in forensic DNA
registries. These repositories of human tissue or DNA include the heel-stick blood spot cards obtained in
the first days of life from all live born infants in the U.S. These blood spot cards are collected for genetic
disease screening by the Departments of Health, to allow prompt identification and timely treatment of
severe but treatable inherited metabolic and genetic disease (Reilly, 1997). Furthermore, hospital pathology
departments around the world routinely archive paraffin-embedded tissues from surgical biopsies and from
autopsy studies conducted for diagnostic and prognostic testing. These tissue blocks are often retrieved
from storage for retrospective DNA-based interrogation after the DNA has been extracted from the
deparaffinized tissue. Several states have offered parents the chance to prepare and keep blood spot cards
on their children and other family members, storing a blood spot on filter paper, a lock of hair, etc. in a way
allowing future DNA testing should the child be lost, runaway, or otherwise displaced. It is the practice of
the entire U.S. military to obtain and store fingerstick blood samples on special paper for later use as
military “dog tags”.
Searching Forensic DNA Databases
All 50 U.S. states have either statutory legislation providing for obligatory DNA banking of blood or saliva
samples from those convicted of certain felony crimes. Federal legislation is recently in force covering U.S.
Federal territories, buildings, the U.S. military and the District of Columbia. Other countries, including
Canada and Great Britain, have regional or national DNA data banks containing the profiles of offenders or
of crime scene evidence.
Under the provisions of the enacted legislation, blood or other tissue samples are obtained for DNA
extraction and multiple genetic loci are typed (the number of loci typed in such cases varies between
countries – in the U.S. 13 STR loci are typed). Typing results (multi-locus DNA profiles) are stored in a
computerized database for future comparison to DNA profiles from evidentiary samples from unsolved
crimes (crime scene index samples). Similarly, profiles from unsolved crimes can be compared to those in
the databank of known offenders (offender index). In the U.S., individual states search their data against
those in a central national index at the Federal Bureau of Investigation (FBI). In the U.S., this whole
system, known as CODIS (Combined DNA Index System), is designed to link offenders or unsolved cases
to one another and thus can identify possible suspects in neighboring or distant jurisdictions (U.S.
Department of Justice, 1996).
Since the inception of CODIS and the various state-operated DNA databases, hundreds of case-to-case or
case-to-suspect "hits" (i.e., DNA matches) have been reported, with one state (Florida) now reporting
several new "hits" each week. Given the well-known high degree of criminal recidivism, particularly in
sexual-assault cases, DNA databases hold promise for identification of more perpetrators than would be
possible without such coordinated efforts (McEwen and Reilly, 1994; Scheck, 1994; McEwen, 1995).
In consideration of the effectiveness of DNA database searching in the criminal justice system, it is
important to consider that costs be measured not simply by the number of so-called matches or “hits”, but
also in the many benefits from exonerations or DNA eliminations. Indeed the elimination of someone as a
suspect based on DNA profiling results can save hundreds if not thousands of hours of wasted investigative
time and removes uninvolved parties from unnecessary intrusion from law enforcement personnel.
Lower Stringency Searching and The Matter of Siblings
Genetically related siblings typically share one or both parents. Thus, in the case of full-siblings, it would
be expected that sharing of DNA profiles would occur much more commonly than among unrelated persons
(beyond that of sharing mtDNA and Y-chromosome profiles, as discussed above). This expectation is
supported by data collected on sibling and non-sibling DNA profiles in the U.S. Comparing the DNA
profiles of full siblings indicates the expected higher degree of allele sharing and locus identity compared to
unrelated individuals (Bourke, Ladd, Bieber). Our data demonstrate that full siblings born to unrelated
parents have identical STR profiles at an average of four loci, compared to identity at less than a single
locus among unrelated individuals. Our data set included a sib pair with identity a nine of the CODIS loci.
In a quality control search of a DNA database, a colleague has informed us of discovery of a pair of inmate
brothers who reportedly share identical DNA profiles at ten STR loci (their parents are closely related).
These observations in siblings have important implications for forensic geneticists, as it becomes important
to consider the matter of brothers in cases in which complete multi-locus DNA profiles are not obtained
(e.g., due to DNA degradation). Also, in a search of a DNA data bank, a high degree of allele sharing can
provide an important investigative lead (i.e., possibly implicating a brother) even in the absence of a
complete profile match of crime scene evidence against a registry of convicted offenders. Thus, it is
important for DNA data bank administrators to have a system to notify law enforcement when a high degree
of allele sharing is found in a computer search, even if it is not a complete “match” at all loci. In reality,
sibling issues are not the rule in forensic investigations, and low-stringency database searches would usually
lead to too many “partial profile hits” to be of any practical use in the majority of investigations. If very
large numbers of SNPs are assayed, discriminant function analyses will be helpful in further study of the
utility of reduced stringency searching of forensic samples against offender profiles.
Surname Searching as a Forensic Tool?
Interestingly, males generally “inherit” yet another trait from their father, other than their nuclear or Ychromosome DNA profile - their family name or surname. Thus, database searching of similar surnames
could be considered in an investigative search of possible suspects, once the Y-chromosome DNA
haplotype profile is identified in forensic evidence (see Kayser et al, 2002). For example, in a hypothetical
investigation, once a specific Y-chromosome DNA haplotype is identified in key forensic evidence, a
database search might identify a haplotype as one commonly found in males named “Adamsky”, or
“Baker”, or “Smiley”.
Individuals with the same surnames might be targeted for investigation or
questioning, or even for court-mandated DNA profiling. There would certainly be civil liberties concerns
about this concept, relating to the issue of lack of individualized suspicion as a basis for any involuntary
DNA profiling based only on the circumstance of having a certain last name.
While surname searching based on Y-chromosome haplotype DNA profiling might seem an implausible
idea, in theory, it could be a highly efficient search strategy in cases of rare DNA profiles associated with
uncommon surnames. However, one major limitation of this concept is evident - because of alternative
name spellings or name changes, variant surnames could also be included in such a hypothetical search
strategy. Depending upon the rarity of the profile or of the surname, the number of such potential suspects
could be small or enormous. Another example of a serious limitation of surname searching would be the
common issue of non-paternity or of unknown paternity. Unknown or disputed paternity is very common in
some countries and non-paternity alone (e.g. one/both partners may not be biological parents) has been
estimated to occur in up to 15% of pregnancies, regardless of religious or socioeconomic class. These
factors add considerable complication to the hypothesized use of surname searching algorithms.
How our law enforcement and public safety officials, forensic scientists and concerned public would react
to such theoretical, yet very possible, search strategies is difficult to predict. In the immediate aftermath of
high profile crimes there is often a public outcry for use of whatever means necessary to solve a crime.
Also of note in the aftermath of the September 11, 2001 attacks on the U.S. were the initial displays of
support for national identification cards, increased gun control and other government intrusions on privacy
and constitutional freedoms. Whether such public displays of support will be short lived is uncertain, as is
the constitutionality of such database search strategies. Nevertheless, it should be noted that once
legislatively mandated practices intended to increase public safety are in force, they rarely are reversed
legislatively and are seldom ruled impermissible by the courts.
Database search of DNA from “consent samples” and from military personnel:
One area of particular interest will be the extent to which so-called consent sample DNA profiles are
searched against unsolved prior or future crime scene evidence. Consent samples (sometimes referred to as
“elimination samples”) are those provided voluntarily by those questioned by peace officers investigating
unsolved crimes. These volunteers could include family members, spouses, and friends of victims. Once
eliminated as a contributor of the crime scene DNA profile, those providing the samples may have the
understanding that their samples will be destroyed and that their DNA profiles are not searched against the
database of unsolved crime scene DNA profiles.
In 1994 a sexual assault conviction was overturned (Canada, see R. v Borden[1994] 2
S.C.R.145,92C.C.C.(3d)404,33 C.R.(4 th)147) on the basis of lack of informed consent from an individual
contacted about providing a DNA sample as an “elimination sample” that was later found to match key
evidence in a second case in which he was not identified by the victim. The Supreme Court of Canada
found that in order for a person to waive their constitutional right to be secure against unreasonable
seizure, the person must be possessed of the requisite informational foundation for a true relinquishment
and that the consent form must make clear the scope of the investigation. More recently (1999) the
Supreme Court of Canada (R. v. Arp [1994]3S.C.R.339, 129C.C.C.(3d)321,20C.R.(5 th)1) ruled that there
was no violation of an individuals constitutional right to be secure against unreasonable seizure, when an
individual consented to providing a DNA sample as an “elimination sample” in a homicide case and the
police subsequently obtained a search warrant to seize the sample for a different investigation. The Court
found that the police have an obligation only to disclose the anticipated purposes known at the time the
consent is obtained. Further, if there is no limitation or restriction on the use of the evidence by the
individual or the police, then it is admissible for use in another investigation.
With regard to the U.S. military, all enlisted and commission military personnel must provide blood
samples which are preserved on special blood spot cards which are then stored, as the modern “dog tags”,
for use in the event that the individual is killed, injured or missing in action. The blood spot cards provide a
source of a reference DNA sample to be used in identification of “the unknown soldier”, or as in the case of
the 9/11 Pentagon attack, to return the remains of the victims to their families. However, it is not widely
appreciated that, under certain specified conditions (i.e., judicial court order) access to these cards could be
ordered in order to obtain DNA profiles in military investigations of criminal activity. Thus, this blood card
“bank” could be interrogated for forensic investigations in a manner different from the original intent of the
collection. Indeed, some have quietly advocated DNA profiling and database searching of such “banks” in
a mass search (or “sweep”) if a military person is a suspect in a crime. Such actions would most certainly
be challenged in the military courts. Also, there would undoubtedly be harsh criticism of such proposals by
the 2 million + members of the U.S. military at the idea that their “DNA dog tag” bank be searched in a way
similar to that of a registry of convicted offenders. While offensive (to this author), such a search would not
be that different to searching computerized records of digital fingerprints taken “voluntarily” from all
military members, virtually all sworn law enforcement officers, and many other individuals employed in
sensitive positions involving security clearances.
Summary Conclusions and A View to The Future of Forensic DNA Profiling- The
Genetic Eyewitness
The utility of DNA variation in forensic investigations has been amply demonstrated during the past decade.
Applications include civil paternity testing, medical diagnostics, and forensic testing, as well as use in
identification of victims from war and mass casualties. Its use as an exculpatory tool and as a powerful
inclusionary evidence cannot be ignored and provides, in some cases, the make or break evidence freeing
incarcerated inmates or sealing the fate of defendants in jury trials. Interpretational challenges involving
complex DNA mixtures will continue to require the expertise of experts as will development of novel
methods for even more rapid and cost effective technology for DNA profiling.
Advances in biotechnology will continue to improve the array of laboratory methods used for forensic
profiling of human and non-human DNA. Profiling of SNPs, use of microarrays for mass screening of
hundreds of loci, and development of robotics will reduce the costs in time and labor needed to perform this
testing. Lack of qualified personnel or funding will continue to require crime labs to send out samples to
commercial contractors. Technical advances will allow DNA extraction, profiling, and searching a database
of known offenders without the long delays now encountered in some jurisdictions.
New methods, along with development of miniaturized kits, have led some to speculate about applications
of forensic DNA profiling for use in the field. Indeed, we are not far from the day where miniaturized crime
scene kits could permit DNA extraction, analysis and computer database searching right at the crime scene.
While possible, it is unclear whether such actions would be desirable or sensible, or whether such field
applications would conform to requirements for standardized protocols in laboratories that would conform
to DAB or SWGDAM requirements.
Storage of evidence and protocols for retrospective DNA profiling of previously adjudicated cases will
continue to challenge the courts and the manpower of personnel in the criminal justice system.
Despite the remarkable capabilities of modern crime laboratories, fiscal imperatives often prevent optimal
use of forensic DNA profiling of searching the existing databanks. For example, in individual cases funding
shortages typically limit the number of evidentiary exemplars from being thoroughly examined. What effect
this has on the result of individual cases is unknown. In old unsolved cases, this lack of DNA extraction
obviously precludes the DNA profiles from being entered into the crime scene index. Because of these
funding and staff shortages, searching the DNA profiles against the profiles obtained from other solved or
unsolved cases, or against the profiles of known offenders, cannot be performed. Thus, many unsolved or
cold cases languish in the archives of crime labs, waiting for that tenacious investigator, committed forensic
scientist, or concerned family member to reactivate the case. Fortunately, in the U.S., federal funding has
allowed for some backlog reduction to be accomplished, but other factors prevent such action in many
cases. This is indeed unfortunate in light of the recidivistic nature of many offenders. Moreover, the
powerful exculpatory power of DNA profiling and the possibility of exoneration remind us of the need not
to ignore certain cases in which DNA profiling was never performed.
Several practical matters account for the tremendous backlog in working old unsolved cases that might
benefit from modern DNA analysis. The first is a shortage of qualified examiners in the crime labs. Even
though advances in computer robotics and sample tracking software have eliminated many hours of tedium
in the handling and processing of samples, the initial examination of evidence, selection of which exemplars
to test, and the interpretation of results requires highly skilled individuals, whose work will be scrutinized in
the courts. Second is the fact that difficulties with proper evidence storage may prevent successful
extraction of DNA years later. Once adjudicated, case crime scene evidence is very often stored properly in
crime labs or police storage facilities under carefully controlled conditions. However storage practices are
highly variable and sadly key evidence that may have been untested may be more haphazardly stored in less
than ideal conditions, or even discarded, preventing current or future technologies from being applied in
retrospective analysis. Very recently, legislative proposals have been offered in several U.S. states to
require that all evidence that might contain DNA be stored indefinitely.
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Additional Forensic DNA Bibliography
Laboratory Publications:
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Laboratory Abstracts:
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Marijuana Samples for Forensic Applications and Narotics Enforcement. American Academy of
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of Forensic Sciences Annual Meeting, Reno, NV.
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Forensic Sciences Annual Meeting, Orlando, FL.
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Annual Meeting, Seattle, WA.
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Figure 3. Electropherogram showing fluorescent peaks at nine STR loci plus the amelogenin locus. As
single individuals can ordinarily have no more than two alleles at a single locus, the finding of 3 or
more peaks at several loci provide evidence of a DNA mixture with two or more contributors. The
PCR product peak heights vary considerably, making assignment of a specific number of contributors
and unequivocal interpretation of the profiles of the individual contributors problematic. The presence
of both an X and Y peak at the amelogenin locus indicates that one or more contributors is male.
Figure 3. Electropherogram showing DNA profile comparison in evidence and known victim sample and
questioned evidence sample at 3 different STR loci (D3S1358, vWA, and FGA). Note that both samples
have similar DNA profiles at these 3 loci (i.e., alleles 14 and 17 at locus D3S1358; allele15 at the vWA
locus, and alleles 20 and 21 at the FGA locus). This would be considered a 3-locus “match” between these
two specific samples. Typing additional loci would be needed to obtain a complete profile.
Known victim sample