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How does forensic identification work?
Any type of organism can be identified by examination of DNA sequences
unique to that species. Identifying individuals within a species is less
precise at this time, although when DNA sequencing technologies progress
farther, direct comparison of very large DNA segments, and possibly even
whole genomes, will become feasible and practical and will allow precise
To identify individuals, forensic scientists scan 13 DNA regions,
or loci, that vary from person to person and use the data to create
a DNA profile of that individual (sometimes called a DNA fingerprint).
There is an extremely small chance that another person has the same
DNA profile for a particular set of 13 regions.
Some Examples of DNA Uses for Forensic Identification
- Identify potential suspects whose DNA may match evidence left at crime
- Exonerate persons wrongly accused of crimes
- Identify crime and catastrophe victims
- Establish paternity and other family relationships
- Identify endangered and protected species as an aid to wildlife officials
(could be used for prosecuting poachers)
- Detect bacteria and other organisms that may pollute air, water, soil,
- Match organ donors with recipients in transplant programs
- Determine pedigree for seed or livestock breeds
- Authenticate consumables such as caviar and wine
Is DNA effective in identifying persons?
[answer provided by Daniel Drell of the U.S. DOE Human
DNA identification can be quite effective if used intelligently. Portions
of the DNA sequence that vary the most among humans must be used; also, portions
must be large enough to overcome the fact that human mating is not absolutely
Consider the scenario of a crime scene investigation . . .
Assume that type O blood is found at the crime scene. Type O occurs
in about 45% of Americans. If investigators type only for ABO,
finding that the "suspect" in a crime is type O really doesn't reveal
If, in addition to being type O, the suspect is a blond, and blond hair
is found at the crime scene, you now have two bits of evidence to
suggest who really did it. However, there are a lot of Type
O blonds out there.
If you find that the crime scene has footprints from a pair of Nike Air
Jordans (with a distinctive tread design) and the suspect, in addition
to being type O and blond, is also wearing Air Jordans with the same tread
design, you are much closer to linking the suspect with the crime
In this way, by accumulating bits of linking evidence in a chain, where
each bit by itself isn't very strong but the set of all of them together
is very strong, you can argue that your suspect really is the right person.
With DNA, the same kind of thinking is used; you can look for matches
(based on sequence or on numbers of small repeating units of DNA sequence)
at many different locations on the person's genome; one or two (even
three) aren't enough to be confident that the suspect is the right
one, but thirteen sites are used. A match at all thirteen is rare
enough that you (or a prosecutor or a jury) can be very confident
("beyond a reasonable doubt") that the right person is accused.
See some recent articles about statistical analysis on this topic:
How is DNA typing done?
Only one-tenth of a single percent of DNA (about 3 million bases) differs
from one person to the next. Scientists can use these variable regions
to generate a DNA profile of an individual, using samples from blood,
bone, hair, and other body tissues and products.
In criminal cases, this generally involves obtaining samples from crime-scene
evidence and a suspect, extracting the DNA, and analyzing it for the presence
of a set of specific DNA regions (markers).
Scientists find the markers in a DNA sample by designing small pieces
of DNA (probes) that will each seek out and bind to a complementary DNA
sequence in the sample. A series of probes bound to a DNA sample creates
a distinctive pattern for an individual. Forensic scientists compare these
DNA profiles to determine whether the suspect's sample matches the evidence
sample. A marker by itself usually is not unique to an individual; if,
however, two DNA samples are alike at four or five regions, odds are great
that the samples are from the same person.
If the sample profiles don't match, the person did not contribute the
DNA at the crime scene.
If the patterns match, the suspect may have contributed
the evidence sample. While there is a chance that someone else has the
same DNA profile for a particular probe set, the odds are exceedingly
slim. The question is, How small do the odds have to be when conviction
of the guilty or acquittal of the innocent lies in the balance? Many
consider this a matter for a jury to take into consideration along with
other evidence in the case. Experts point out that using DNA forensic
technology is far superior to eyewitness accounts, where the odds for
correct identification are about 50:50.
The more probes used in DNA analysis, the greater the odds for a unique pattern
and against a coincidental match, but each additional probe adds greatly to
the time and expense of testing. Four to six probes are recommended. Testing with several more probes will
become routine, observed John Hicks (Alabama State Department of Forensic Services).
He predicted that DNA chip technology (in which
thousands of short DNA sequences are embedded in a tiny chip) will enable much
more rapid, inexpensive analyses using many more probes and raising the odds
against coincidental matches.
What are some of the DNA
technologies used in forensic investigations?
Restriction Fragment Length Polymorphism (RFLP)
RFLP is a technique for analyzing the variable lengths of DNA fragments that
result from digesting a DNA sample with a special kind of enzyme. This enzyme,
a restriction endonuclease, cuts DNA at a specific sequence pattern know as
a restriction endonuclease recognition site. The presence or absence of certain
recognition sites in a DNA sample generates variable lengths of DNA fragments,
which are separated using gel electrophoresis. They are then hybridized with
DNA probes that bind to a complementary DNA sequence in the sample.
RFLP was one of the first applications of DNA analysis to forensic investigation.
With the development of newer, more efficient DNA-analysis techniques, RFLP
is not used as much as it once was because it requires relatively large amounts
of DNA. In addition, samples degraded by environmental factors, such as dirt
or mold, do not work well with RFLP.
Polymerase chain reaction (PCR) is used to make millions of exact copies of
DNA from a biological sample. DNA amplification with PCR allows DNA analysis
on biological samples as small as a few skin cells. With RFLP, DNA samples would
have to be about the size of a quarter. The ability of PCR to amplify such tiny
quantities of DNA enables even highly degraded samples to be analyzed. Great
care, however, must be taken to prevent contamination with other biological
materials during the identifying, collecting, and preserving of a sample.
Short tandem repeat (STR) technology is used to evaluate specific regions (loci)
within nuclear DNA. Variability in STR regions can be used to distinguish one
DNA profile from another. The Federal Bureau of Investigation (FBI) uses a standard
set of 13 specific STR regions for CODIS. CODIS is a software program that operates
local, state, and national databases of DNA profiles from convicted offenders,
unsolved crime scene evidence, and missing persons. The odds that two individuals
will have the same 13-loci DNA profile is about one in a billion.
Mitochondrial DNA Analysis
Mitochondrial DNA analysis (mtDNA) can be used to examine the DNA from samples
that cannot be analyzed by RFLP or STR. Nuclear DNA must be extracted from samples
for use in RFLP, PCR, and STR; however, mtDNA analysis uses DNA extracted from
another cellular organelle called a mitochondrion. While older biological samples
that lack nucleated cellular material, such as hair, bones, and teeth, cannot
be analyzed with STR and RFLP, they can be analyzed with mtDNA. In the investigation
of cases that have gone unsolved for many years, mtDNA is extremely valuable.
All mothers have the same mitochondrial DNA as their offspring. This is because
the mitochondria of each new embryo comes from the mother's egg cell. The father's
sperm contributes only nuclear DNA. Comparing the mtDNA profile of unidentified
remains with the profile of a potential maternal relative can be an important
technique in missing-person investigations.
The Y chromosome is passed directly from father to son, so analysis of genetic
markers on the Y chromosome is especially useful for tracing relationships among
males or for analyzing biological evidence involving multiple male contributors.
The answer to this
question is based on information from Using
DNA to Solve Cold Cases - A special report from the National Institute of
Justice (July 2002).
Some Interesting Uses of DNA Forensic Identification
DNA Forensics Databases
National DNA Databank: CODIS
The COmbined DNA Index System, CODIS, blends computer and DNA technologies
into a tool for fighting violent crime. The current version of CODIS uses
two indexes to generate investigative leads in crimes where biological
evidence is recovered from the crime scene. The Convicted Offender Index
contains DNA profiles of individuals convicted of felony sex offenses
(and other violent crimes). The Forensic Index contains DNA profiles developed
from crime scene evidence. All DNA profiles stored in CODIS are generated
using STR (short tandem repeat) analysis.
CODIS utilizes computer software to automatically search its two indexes for
matching DNA profiles. Law enforcement agencies at federal, state, and local
levels take DNA from biological evidence (e.g., blood and saliva) gathered in
crimes that have no suspect and compare it to the DNA in the profiles stored
in the CODIS systems. If a match is made between a sample and a stored profile,
CODIS can identify the perpetrator.
This technology is authorized by the DNA Identification Act of 1994.
All 50 states have laws requiring that DNA profiles of certain offenders
be sent to CODIS. As of August 2007, the database contained over 5 million
DNA profiles in its Convicted Offender Index and about 188,000 DNA profiles
collected from crime scenes but not connected to a particular
offender. (source http://www.fbi.gov/hq/lab/codis/clickmap.htm).
As more offender DNA samples are collected and law enforcement officers become
better trained and equipped to collect DNA samples at crime scenes, the
backlog of samples awaiting testing throughout the criminal justice system
is increasing dramatically. In March 2003 President Bush proposed $1 billion
in funding over 5 years to reduce the DNA testing backlog, build crime
lab capacity, stimulate research and development, support training, protect
the innocent, and identify missing persons. For more information, see the
U.S. Department of Justice's Advancing
Justice Through DNA Technology.
More on CODIS
Ethical, Legal, and Social Concerns about
The primary concern is privacy. DNA profiles are different from fingerprints,
which are useful only for identification. DNA can provide insights into
many intimate aspects of people and their families including susceptibility
to particular diseases, legitimacy of birth, and perhaps predispositions
to certain behaviors and sexual orientation. This information increases the potential
for genetic discrimination by government, insurers, employers, schools,
banks, and others.
Collected samples are stored, and many state laws do not require the
destruction of a DNA record or sample after a conviction has been overturned.
So there is a chance that a person's entire genome may be available —regardless
of whether they were convicted or not. Although the DNA used is considered
"junk DNA", single tandem repeated DNA bases (STRs), which are not known
to code for proteins, in the future this information may be found to reveal
personal information such as susceptibilities to disease and certain behaviors.
Practicality is a concern for DNA sampling and storage. An enormous backlog of over half a million
DNA samples waits to be entered into the CODIS system. The statute of limitations
has expired in many cases in which the evidence would have been useful for conviction.
Who is chosen for sampling also is a concern. In the United Kingdom, for example,
all suspects can be forced to provide a DNA sample. Likewise, all arrestees
--regardless of the degree of the charge and the possibility that they may not
be convicted--can be compelled to comply. This empowers police officers, rather
than judges and juries, to provide the state with intimate evidence
that could lead to "investigative arrests."
In the United States each state legislature independently decides whether
DNA can be sampled from arrestees or convicts. In 2006, the New Mexico
state legislature passed Katie's Bill, a law that requires the police
to take DNA samples from suspects in most felony arrests. Previous New
Mexico laws required DNA to be sampled only from convicted felons. The
bill is named for Katie Sepich, whose 2003 murder went unsolved until her
killer's DNA entered the database in 2005 when he was convinced of another
felony. Her killer had been arrested, but not convicted, for burglary
prior to 2005.
Opponents of the law assert that it infringes on the privacy and rights of the
innocent. While Katie’s Law does allow cleared suspects to petition to
have their DNA samples purged from the state database, the purging happens only
after the arrest. Civil liberties advocates say that Katie's Bill still raises
the question of Fourth Amendment violations against unreasonable search and
seizure and stress that the law could be abused to justify arrests made on less
than probable cause just to obtain DNA evidence.
As of September 2007, all 50 states have laws that require convicted sex offenders
to submit DNA, 44 states have laws that require convicted felons to submit
DNA, 9 states require DNA samples from those convicted of certain misdemeanors,
and 11 states—including Alaska, Arizona, California, Kansas, Louisiana,
Minnesota, New Mexico, North Dakota, Tennessee, Texas, and Virginia—have
laws authorizing arrestee DNA sampling.
Potential Advantages and Disadvantages
of Banking Arrestee DNA
- Major crimes often involve people who also have committed other offenses.
Having DNA banked potentially could make it easier to identify suspects,
just as fingerprint databases do.
- Innocent people currently are incarcerated for crimes they did not
commit; if DNA samples had been taken at the time of arrest, these individuals
could have been proven innocent and thereby avoided incarceration..
- Banking arrestees' DNA instead of banking only that of convicted criminals
could result in financial savings in investigation, prosecution, and
- Arrestees often are found innocent of crimes. The retention of innocent
people's DNA raises significant ethical and social issues.
- If people’s DNA is in police databases, they might be identified as
matches or partial matches to DNA found at crime scenes. This occurs
even with innocent people, for instance, if an individual had been at
a crime scene earlier or had a similar DNA profile to the actual criminal.
- Sensitive genetic information, such as family relationships and disease
susceptibility, can be obtained from DNA samples. Police, forensic science
services, and researchers using the database have access to people’s
DNA without their consent. This can be seen as an intrusion of personal
privacy and a violation of civil liberties.
- Studies of the United Kingdom’s criminal database, which retains the
DNA samples of all suspects, show that ethnic minorities are over represented
in the population of arrestees and are, therefore, overrepresented in
the criminal DNA database. This raises the concern of an institutionalized
ethnic bias in the criminal justice system.
- Even the most secure database has a chance of being compromised.
DNA Forensics Links