"The Use of DNA Fingerprinting in Forensic Science" by Donald Bowling
Upon arriving at the crime scene, a detective finds a small piece of blood stained cloth on the ground five feet away from a murder victim. The cloth does not match the clothing the victim is wearing. At this point the detective picks up the cloth with a pair of forceps and places it in a bag labeled "biological evidence." This is the first step in a long process that this cloth, with its valuable treasure chest of DNA, will take until it is successfully "fingerprinted" in a lab. Several months later the fingerprinted evidence will appear on the witness stand exhibited by a forensic scientist. DNA in recent years has become more widely used by police in areas of violent crime and in solving paternity cases. There is no doubt that DNA is a valuable resource to forensic science, but we must consider how this technology actually works, and if it is accurate and practical enough to have wide spread implications on the legal system.
Deoxyribonucleic acid (DNA) is the building block of all life (Windholz). It is the special set of instructions which makes life possible as well as unique. Every person on earth has his or her own special variation of the DNA molecule due to the unique mixing of genes from their parents. The only identical set of DNA between individuals is in identical twins (Raloff). Within a human cell there are 23 pairs of chromosomes. They hold some fifty to 100 thousand genes. Each one of the genes has two possible configurations which give a great amount of variability that is useful for DNA fingerprinting. Besides the genes, the chromosomes are made up of 98% junk DNA which codes for no particular traits (Schefter 62). Often this part of the chromosome is highly variable in its molecular make up. The variation of the makeup in the junk DNA as well as in genes is based on the arrangement of molecular sub-parts called nucleotides. DNA fingerprinting is based upon getting a copy of these unique variations in arangement of nucleotides. The familiar double helix shape of the DNA molocule is due to the physical and chemical laws by which the four types of nucleotides bond. The nucleotides are arranged on the DNA molocule in pairs (Adenine to Thymine and Cytosine to Guanine) like the rungs of a ladder in the famous double helix pattern (Windholz) . Experts say if the complete arrangement of nucleotides in the forty six chromosomes was printed out on paper it would take up 1000 books each 600 pages long (Schefter 62).
It is impossible as well as impractical for the whole genetic code of an individual to be examined. The techniques used today are either based on the type of genes a individual has (dominant or recessive) or on the amount of reoccurrence of a certain pattern of nucleotides in the genetic material. In the case of the Polymerase Chain Reaction (PCR) technique, the gene types of an individual are matched to obtain a DNA fingerprint, while in the case of Restriction Fragment Length Protocol (RFLP) method the repitition of nucleotide patterns within a chromosome are measured to obtain a fingerprint. No matter which test is used, the genetic material to be fingerprinted can come from a variety of sources including: blood, saliva (containing epithelial cheek cells), and semen or other less likely samples such as skin, bone, teeth or hair. In any case the sample must contain nucleated cells (red blood cells have no nucleus thus they can not be used) (Connors 80). Upon arrival at a lab, the blood or other biological material is removed from the host material by a neutral solvent solution. It is centrifuged at 1,600 revolutions per minute until the DNA containing cells settle to the bottom. At this time these cells are placed into a solution of enzymes which eat away the cell membranes and the nucleus, freeing the genetic material contained within (Schefter 60). A lab technician proceeds to either do PCR or RFLP analysis of the genetic material.
Polymerase Chain Reaction (PCR), is based upon the exponential duplication of a single stand of DNA. Twenty cycles of replication would give one million copies of the original DNA. This mass duplication takes place in an instrument called a thermal cycler which causes the DNA to be copied by providing the appropriate enzymes and nucleotides at the proper cycle of cooling and heating in a controlled environment. Once the strand has been mass replicated, the new copies are put on a sheet of nylon with gene probes, which bind to certain genes. Upon finding the specific gene, the probes attach to that fragment of DNA. For each gene probe an indicator solution is then added. When this indicator locates a probe which is attached to a "target gene," a blue dot appears on the nylon strip. If the indicator does not find its "target," no color appears (Schefter 62). This series of blue dots and blank misses is the "DNA fingerprint." The PCR method of sequencing has many advantages over RFLP sequencing. The fact that it is able to duplicate a small amount of sample many times over means it requires a very small sample size, about the size of the tip of a pin, approximately fifty cells, compared to the five thousand to fifty thousand needed for RFLP ("Science"). The PCR method is also quicker than RFLP; it only takes two or three weeks where RFLP takes months. However, there is a trade off for this small sample size and quickness in that it is considerably less accurate than the RFLP method. The probability of two individuals being matched by PCR is between 1/100 and 1/2,000 (Schefter 90).
Restriction Fragment Length Protocol is based on an enzyme called HAE III which finds a certain
string of nucleotides, such as CCGG,and cuts the DNA string between the given C and G
sequence ("Science"). This process in the end makes many peices of DNA of variable lengths. The
variation of sizes in the pieces of DNA is then utilized in a process known as electrophoresis. In
this process the DNA solution is put at one end of a plate containing an agarose gel made from
seaweed. Due to the Jell-O like colloid structure of the agarose gel, the pieces of DNA, depending
on their size, move at different rates through the colloid mixture when an electric current is passed
through, the fragments are deposited at different distances along the plate (Schefter 62). After one
night of electrophoresis, the gel is taken off and bathed in a solution containing four types of
radioactive probes each of which bind to a different terminating nucleotide on the end of the cut
up molecules (Raloff). The radioactive tag called Phosphorus 32 (P32) is then able to be exposed
to a piece of X-ray film which is placed on top of the gel (Schefter 62). Once exposed, the film
has stripes of white at different locations on it, indicating how far the different pieces of DNA
have moved across the plate. This pattern is what is known as the classic "DNA fingerprint."
Because of the weakness in the radioactivity of P32, it may take up to two weeks to significantly
expose a piece of X-ray film. The film must be separately exposed for each of the five (or more)
areas that are being tested, each taking an additional week. After the process is complete, the lab
technician and another expert examine the film to determine a match. The film may also be
scanned into a computer for precise measurements (Schefter 63). In all, the RFLP process may
take up to three to four months (Raloff). The sample required for this test has to be about the
size of a dime ("Science") but the trade off for the time and larger sample size is a great increase
in accuracy. The probability of two people matching the same pattern in RFLP method ranges
from 1/10,000 to 1/100,000 (Schefter 90). Some experts in the O.J. Simpson trial place the
exclusionary accuracy of this technology in the neighborhood of 1/170 million. This is a
substantial amount of accuracy when compared to the chances of a person swimming in the ocean
being attacked by a shark around 1/100 million (Reibstein and Fotte 44).
The RFLP procedure uses parts of the "junk" DNA of a chromosome known as Variable Number
Tandem Repeats (VNTR's). These are repeats of certain patterns of nucleotides over and over
again in the DNA strand. The number of repeats an individual has on a given chromosome is
highly variable. The number of repeats is the primary trait measured in RFLP fingerprinting. The
length of the given pattern determines where the individual piece of DNA will settle in the gel and
the number of DNA fragment that settle in a given area help determine the thickness of the band
on the film (Schefter 64).
The previously test methods where discussed based on the use of nucleic DNA (obtained from the nucleus of a cell). In August of 1996 the FBI began using a new type of DNA known as mitochondrial DNA. This DNA is found in an organelle within the cell known as the mitochondria. Due to the lack of nuclear DNA in hair, bone, and teeth, mitochondrial DNA is often used to identify these sources of biological evidence. This DNA is only passed on from mother to child through the egg cell (Wilson). Mitochondrial DNA has two highly variable areas which mutate at a given rate from individual to individual. Mitchell Holland a biochemist at the Armed Forces Institute of Pathology claims that the most frequent sequences in these areas occur in 3% of the population (Cohen 22). Most of the time 600 base pairs of mitochondrial DNA can be used to gain a near positive identification of an individual. Because this DNA is only passed along maternal lines makes it very useful in paternity cases involving orphans or in establishing the ethnic association of an individual in the case of an unknown suspect. This method is also used to identify the remains of soldiers found in Vietnam after being in the jungle for years (Wilson).
In the use of forensic DNA, there are many factors which can affect the accuracy of a given test,
and thus affect the acceptability of the evidence in a court of law. The normal rates of error and
probability of two individuals having the same fingerprint must be considered in any test. For
example, the PCR method is not accurate enough to convince a jury beyond a reasonable doubt
with a 1/100 rate of accuracy. However, in many cases this technology can be used as a cheap,
quick exclusionary method of innocent, wrongfully arrested suspects. Another thing which has
been recently acknowledged as a great factor in calculation of the statistical accuracy value of a
given test is the ethnic association of the assailant. As different ethnic groups have different rates
of genetic closeness within the population, the statistical rates of accuracy of test vary greatly.
The National Academy of Sciences endorses a method known as the ceiling method of population
genetics for calculating the accuracy in a given test within a specific ethnic group. It calls for a
calculation of the occurrence of certain genetic traits among a 100+ person sample. After the
clinical trials are done, a conservative estimate is made on the chances of two individuals sharing
the same DNA fingerprint (Raloff). Many experts however think that this method of population
genetics is very invasive in racial related areas and may lead to eugenic like outlook by many
A second factor affecting the accuracy of a DNA fingerprint is police incompetency. The nature
of biological evidence requires it to be searched for diligently. The searching must be done both at
the crime scene and on the victim. This is a tedious process, and throughout the process cross
contamination must be prevented. Most of the investigative police force today composed in the
majority by police officers who are 40+. These policemen are not accustomed to the use of DNA
evidence and now must be retrained. It is important that everyone on the police force be trained,
from the patrolman who arrives at the scene first and who must preserve the evidence, to prevent
contamination, to the chief of police who is ultimately responsible. This is a very expensive
proposition, but it is necessary for the evidence to be qualitative and for it to be admissible in
court (Connors 13-14).
A third factor that affect the accuracy of DNA fingerprinting is that of the laboratory procedures
and accuracy. Statistics have shown that the number one DNA lab in the country is the FBI crime
laboratory. This lab has been using DNA since its introduction into the US it has handled 21,621
cases. In accuracy the FBI is followed by private and state laboratories. But until recently, there
have been no standards for accuracy or a regulatory system to inspect the lab techniques or
standards. A regulatory agency must be established as well as standards for uniform testing
(Connors 32). Laboratories should also use every method possible to avoid contamination of test
samples as well as the equipment used in the testing. This especially holds true in the case of PCR
testing in which such small samples are used. Even a small skin cell from a lab worker could cause
the wrong DNA to be amplified causing detrimental results ("Problems"). Sterile conditions are
necessary to get an accurate fingerprint. Such things as fresh gloves at all stages of lab
preparation, a laminar flow cabinet with ultraviolet lighting, and dedicated vessels with reagents
for each stage of the testing, can reduce chances of contamination ("PCR"). These are only a few
methods to help insure quality lab work but all necessary precautions must be taken to get
qualitative results which will be admissible in court.
DNA has many practical applications in the area of law not only can it be used for the typical rape
and murder cases. Recently new applications have been found for DNA evidence in the area of
paternity suits and wildlife law enforcement. In an article in National Wildlife, illegally poached
materials are traced from the carcass of a dead animal to the market place in which pieces of the
animals are being sold (Freind 23). DNA was also used in the prosecution of the World Trade
Center bomber. By finding cheek cells underneath a stamp mailed to the center before the
bombing, forensic scientists were able to extract DNA which proved valuable in prosecution
As well as its use in the conviction of criminals, DNA has recently taken an important role in freeing wrongly convicted criminals. Recent appeals of some rape cases using DNA evidence have led to the release of 25% of the defendants. The National Institute of Justice thinks that 25% or more convicted rape offenders are really innocent due to the lack of DNA evidence at the time of their conviction. It may not be possible, though, to prove many of these men innocent because police often discard evidence after a short period of time. However, it is possible to extract viable DNA from available samples many years old (Connors 10).
Since the first use of DNA evidence in England (1886) by Dr. Alec J. Jeffereys who coined the
term "DNA fingerprinting", there have been many factors which have affected the use of DNA
evidence in courts (Connors 22). Most of the states allow DNA evidence in their courts. States
that do not allow DNA evidence are Maine, North Dakota, Rhode Island and Utah (Connors 24).
Futhermore, DNA has been accepted as official evidence in federal trials by the Supreme Court in
the case State (WVA) v. Woodall (Connors 22). Despite previous acceptance of DNA evidence in
most states, a special hearing, called a Frye hearing, is still required to admit scientific evidence
into a trial. Frye hearings weigh the accuracy and consistency of the scientific evidence to be
presented before the jury. The hearings are still necessary because the court proclaimed in Hayes
v. US., that "DNA has not yet reached the level of stability of other forms of identification such
as (traditional) fingerprinting" ("Science"). Many legal experts agree that exclusion of DNA
evidence from the courtroom by special admissibility rules will force dependence on inferior types
of evidence such as eyewitness testimony. (Connors 5).
If the court were to exclude DNA evidence, the judicial system would have to rely on inferior
polymorphic proteins in identifying biological evidence. As biological evidence is a must in
paternity and rape cases, either DNA or polymorphic proteins must be used as methods of
identification. The polymorphic analysis methods, which are similar to blood typing, have a higher
rate of error than DNA. The chances of two individuals having the same types of polymorphic
proteins are much closer than the 1/100 ratio of PCR testing. Furthermore, evidence subjected to
polymorphic analysis must be carefully preserved and must be collected in a large enough amount
for significant analysis. The DNA molecule is also much more stable than polymorphic proteins
which degrade over time thus producing erroneous results in polymorphic protein analysis
Another area which affects the court's consideration of DNA evidence is expert testimony. Expert
witnesses are powerful when testifying and have the tendancy to overshadow other witnesses.
These experts are often unable to put technical information into layman terms. For these reasons it
is sometimes necessary for the judge to exercise Rule 403 which allows the judge to have greater
control over the expert than he would a lay witness, thus restraining the expert witness in conduct
(Connors 4). Experts often disagree on the accuracy of "DNA fingerprinting." The quoted
accuracy of a given test varies greatly from expert to expert, as well as from trial to trial. These
differences of opinion can affect the consideration of DNA evidence by trial juries. In the O.J.
Simpson trial, the Nobel laureate, Kary Millis, who discovered the PCR method, came under
personal attack due to his reservations that the scientific community has been involved in fraud
involving falsification of results. Mr. Millis's knowledge of advanced scientific developments and
his admitted use of drugs were also brought into question. All the contiversy surrounding the
expert witnesses may have contributed to the dismissal of DNA evidence in the case (Cohen 22).
With the recent events of the O.J. Simpson trial, DNA evidence has been put more in the spotlight
than ever. The defense before the trial even said that the results of the DNA fingerprinting would
have a great bearing on their strategy. If there were no match of the blood to Simpson either at
the crime scene or the victim's on his possessions, the defense would ask for immediate dismissal
of the charges against Mr. Simpson (Schefter 90). However, a match was found resulting in the
defense's attack of the prosecution's evidence. First, the handling of DNA from the crime scene
to the lab was put under strict scrutiny. As well, the technology, conditions, and procedures
within the laboratory where scrutinized (Connors 14). Second, during cross examination one of
the prosecutions witnesses admitted to the mislabeling of collected evidence. Third, the police
department was accused of planting the biological evidence at the scene. Fourth, the reputation
of one of the leading private DNA testing laboratories, Cellmark Labs, was attacked by the
defense. Fifth, in return for the supposed foul ups on the behalf of the prosecution the defense
wanted to do their own private testing of the biological evidence, which was not allowed
(Schefter 90). The mistakes made by the prosecution and the inferior presentation of the DNA
evidence resulted in dismissal of much of the this evidence. The rest of the DNA evidence was
disregarded by the jurors due to the large amount of conterversy surrounding it.
DNA evidence has much to promise in the future of forensic science. New methods to increase
accuracy and to reduce the price of "DNA fingerprinting"are being developed. With better police
training and the development of laboratory standards for quality, the use of DNA fingerprinting
promises to be as common place as traditional fingerprinting. Juries may soon come to expect
DNA evidence in all cases. One thing which must be remembered is that DNA is not the cure for
all evidence of the courtroom. "DNA is just a piece of the puzzle used to convict. It does not
prove beyond a reasonable doubt that a person was at the crime scene, nor does it prove guilt"
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Freind, Tim. "DNA Fingerprinting: Power Tool." National Wildlife Oct. / Nov. 1995: 16-23.
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"PCR: DNA Amplification Protocol." Internet.
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http://www.washington.edu/fingerprinting/problems.html. 13 Mar. 1997.
Raloff, J. "Panel OK's DNA Fingerprints in Court Case." Science News 25 Apr. 1992. Proquest
Reibstein, Larry, and Dona Fotte. "These Genes Fit Him Too Well." Newsweek 22 May 1995:
44-45. Proquest CD-ROM.
Schefter, Jim. "DNA Fingerprints on Trial." Popular Science Nov. 1994: 60+. SIRS Researcher
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http://www.govtech.net/1995/gt/oct/dna_deba.htm. 13 Mar. 1997.
Wilson, John. "Expert Says Hairs in Victim, Bed Match Ware." Chattanooga Free Press 29 Aug.
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