Frequently Asked Questions about Forensic DNA
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 individual identification.
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 of
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, and
- 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 Genome
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, then finding that the
"suspect" in a crime is type O really doesn't reveal very much.
If, in addition to being type O, the suspect is a blond, and blond hair is
found at the crime scene, then 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, then
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 a number
of 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 and 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.
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
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 analysis using many more probes, and raising the
odds against coincidental matches.
What are some of the
DNA technologies used in
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 is one of the original 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.
PCR (polymerase chain reaction) 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
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
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 the 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
Some Interesting Uses of DNA Forensic Identification
Using DNA fingerprinting techniques akin to those used to solve crimes and
settle paternity suits, scientists at the University of California, Davis,
have discovered that 18 of the world's most renowned grapevine varieties, or
cultivars, including varieties long grown in northeastern France such as
Chardonnay, the "king of whites," and reds such as Pinot and Gamay noir, are
Banks for Endangered Animal Species
Declining Grizzly Bear Population
Snowball the Cat
A woman was murdered in Prince Edward Island, Canada. Her estranged husband
was implicated because a snowy white cat hair was found in a jacket near the
scene of the crime, and DNA fragments from the hair matched DNA fragments from
Snowball, the cat belonging to the husband's parents. M. Menotti-Raymond et
al., "Pet cat hair implicates murder suspect," Nature, 386: 774, 1997.
- Angiosperm Witness for the Prosecution
The first case in which a murderer was convicted on DNA evidence obtained from
a plant was described in the PBS TV series, "Scientific American Frontiers." A
young woman was murdered in Phoenix, Arizona, and a pager found at the scene
of the crime led the police to a prime suspect. He admitted picking up the
victim, but claimed she had robbed him of his wallet and pager. The forensic
squad examined the suspect's pickup truck and collected pods later identified
as the fruits of the palo verde tree (Cercidium spp.). One detective
went back to the murder scene and found several Palo Verde trees, one of which
showed damage that could have been caused by a vehicle. The detective's
superior officer innocently suggested the possibility of linking the fruits
and the tree by using DNA comparison, not realizing that this had never been
done before. Several researchers were contacted before a geneticist at the
University of Arizona in Tucson agreed to take on the case. Of course, it was
crucial to establish evidence that would stand up in court on whether
individual plants (especially Palo Verde trees) have unique patterns of DNA. A
preliminary study on samples from different trees at the murder scene and
elsewhere quickly established that each Palo Verde tree is unique in its DNA
pattern. It was then a simple matter to link the pods from the suspect's truck
to the damaged tree at the murder scene and obtain a conviction. [WNED-TV (PBS
- Buffalo, N.Y.)]
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)
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 January 2003, the database contained more than a million DNA
profiles in its Convicted Offender Index and about 48,000 DNA profiles collected
from crime scenes but which have not been connected to a particular offender.
As more offender DNA samples are collected and law enforcement becomes better
trained and equipped to collect DNA samples at crime scenes, the backlog of
samples awaitning testing throughout the criminal justice system has increased
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, seethe U.S. Department of Justice's
Through DNA Technology.
More on CODIS
Ethical, 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 a person and their families including susceptibility to
particular diseases, legitimacy of birth, and perhaps predispositions to certain
behaviors and sexual orientation. This 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 --criminal or
otherwise. Although the DNA used is considered "junk DNA" (STRs-single tandem
repeated DNA bases), in the future this information may be found to reveal
personal information such as susceptibilities to disease and certain behaviors.
Who is chosen for sampling is also 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 U.S., arresting people on less
than probable cause just to obtain DNA evidence raises the question of Fourth
Amendment violations against unreasonable search and seizure.
Practicality also is a concern. 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 where the evidence would have been useful
for conviction .
Potential Benefits of DNA Databanking Arrestees
(According to Howard Safir, NYC Police Commissioner, 1999)
- Most major crimes involve people who also have committed minor offenses.
- Innocent people currently are incarcerated for crimes they did not commit;
if samples had been taken at the time of arrest, these individuals would have
been excluded early in the investigative process.
- Moving the point of testing from conviction to arrest would result in
savings in investigation, prosecution, and incarceration.
- Investigators would be able to compare other cases against the arrested
person's DNA profile, just as with fingerprints.
Mitochondrial DNA Explained
Every cell contains both nuclear and mitochondrial DNA. Nuclear DNA is found
within the nucleus of the cell and is composed of two sources of DNA: the egg
and the sperm. This type of DNA defines us as individuals and is most often used
in forensic or paternity cases. The nuclear DNA of a forensic specimen from a
crime scene is compared to a specimen from a suspect to see how similar they
are. In terms of a paternity suit, the nuclear DNA of the child is compared to
the nuclear DNA from the father to see if the father contributed to the child's
Mitochondrial DNA (mtDNA) is contained in the mitochondria of the cell. The
mitochondria are organelles located outside the nucleus in the cytoplasm of the
cell. These organelles are responsible for energy transfer and are basically the
"powerhouses" of the cells. The CIL uses this form of DNA because it preserves
well in bones and many of the casualties that we are attempting to identify do
not have blood samples on file (unlike the modern military). This form of DNA is
in short strands and therefore does not mutate or change form very quickly - it
is relatively stable and can be compared across several generations.
Mitochondrial DNA is only passed along the maternal line - so if we want to
compare a sample from a casualty individual we have to obtain a blood sample
from the mother or any of the siblings who would share the same sequence of
mtDNA as the mother. If nieces or nephews were to contribute DNA samples, only
the child of a sister would contain the proper sequence since a brother's child
would obtain his or her mtDNA from his mother who would not be a blood relative
of the deceased in question.
The chart below can help you determine whether you are an eligible mtDNA
The number of eligible donors of blood (MtDNA) continues to decline, making
these samples very important to future identifications. All maternal relatives
of WWII, the Korean War, Cold War, and Vietnam War KIA (BNR) casualties are
encouraged to contact the appropriate service and arrange blood (MtDNA)
Contact the appropriate service casualty office (see number below).
Missing persons Branch 1-800-531-5501
Mortuary Affairs 1-800-531-5803
Casualty and Memorial Affairs Operations Center
Missing Person Section (POW-MIA Affairs)
U.S. State Department (for civilian unaccounted-for)
Combined DNA Index System
The Combined DNA Index System (CODIS) is the FBI's national databases of genetic
identification codes. Each DNA sample is stored as a 13 digit number.
CODIS consists of two sub-databases. The forensic index contains DNA evidence
found at crime scenes. The offender index contains the DNA profiles of known
offenders of sex offenses and other violent crimes. CODIS is primarily a
national database for DNA data accumulated at local and state levels. All 50
states participate. In order to decrease the number of irrelevant matches, the
convicted offender database requires all 13 CODIS STRs to be present for a
profile upload. Forensic unknown profiles only require 10 of the STRs to be
present for an upload.
As of November 2005, 124,200 forensic profiles and 2.8 million offender profiles
have been accumulated, making it the second largest DNA databank in the world
behind the United Kingdom. As of the same date, CODIS has produced over 27,700
matches to requests, assisting in more than 29,600 investigations.
The CODIS server itself is hidden at an unknown location.
There are many privacy and ethical concerns accompanying CODIS. The foremost is
that CODIS represents the holy grail of eugenics. The database could be
interpreted to yield the criminal gene. Such a discovery would have tremendous
potential for abuse. CODIS does not have such a capacity. First, it does not
store actual genetic information- just short identification codes. Secondly, the
identification codes are created from so called "junk DNA". This DNA is not
believed to have any function or influence on human characteristics.
The growing public approval of DNA databases has seen the creation and expansion
of many states' own DNA databanks. California currently maintains the third
largest DNA databank in the world (naturally, as CODIS contains all states'
databank information). Political measures such as California Proposition 69
(2004), which increased the scope of the databank, have already met with a
significant increase in numbers of investigations aided.