How does forensic
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 individual
To identify individuals, forensic scientists scan 13 DNA
regions 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
Some Examples of DNA Uses for Forensic Identification
- Identify potential suspects whose DNA may match evidence left at crime scenes
- Exonerate persons wrongly accused of crimes
- Identify crime and catastrophe
- Establish paternity and other family relationships
- Identify endangered
and protected species as an aid to wildlife officials (could be used for prosecuting
- Detect bacteria and other organisms that may pollute air, water,
soil, and food
- Match organ donors with recipients in transplant programs
pedigree for seed or livestock breeds
- Authenticate consumables such as caviar
Is DNA effective in
[answer provided by Daniel Drell
of the U.S. DOE Human Genome Program]
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 random.
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 very
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.
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 scene.
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 four (sometimes five) are used. A match at all five 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.
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
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
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 judges 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?
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.
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.
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 (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
daughters. 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
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
Some Interesting Uses
of DNA Forensic Identification
National DNA Databank: CODIS
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.
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
More on CODIS
Legal, and Social Concerns about DNA Databanking
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
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
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.
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 incarceration.
- 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.
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
- Even the most secure database has a chance of being compromised.
DNA Forensics Links