Biology: Genetics: DNA Evidence

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Biology: Genetics: DNA Evidence


DNA evidence is an increasingly important part of the criminal justice system. It can convict perpetrators and exonerate the wrongfully accused. Whether identifying a recent crime victim or the remains of an ancient mummy, DNA fingerprints are a precise, scientific, and individual calling card. While science is constantly improving and expanding methods of DNA evidence analysis, legal systems are embracing scientific evidence and challenging long-held definitions of proof.

Historical Background and Scientific Foundations

In 1984 British geneticist Alec Jeffreys (1950–) invented the process of DNA (deoxyribonucleic acid) fingerprinting, or genetic fingerprinting, at the University of Leicester. This is the process of using DNA samples to distinguish and identify individuals. Forensic scientists who investigate crime scenes use DNA fingerprinting to identify suspects and victims by analyzing samples of blood, hair, saliva, semen, or tissue. Genetic fingerprinting looks most closely at a sample's minisatellites, sections of DNA consisting of a short series of bases (10–100 base pairs) that occur in hundreds of locations in the human genome.

Processing DNA Evidence

DNA samples can be taken from living persons in many ways, but the most common are hair with follicle attached, saliva (mouth or nose swab), blood, or vaginal swab. Such donation is usually voluntary, but it can be compelled by court order. DNA can also be collected from dead donors, usually in the form of blood, hair, or tissue. Samples from unknown donors are gathered when crime scene investigators and forensic scientists find hair, blood, and tissue at crime scenes. Once gathered, these are labeled, taken to the lab, catalogued, stored, and tested.

Forensic scientists use several methods of DNA fingerprinting. Most have a few steps in common, such as the separation of the DNA material to be analyzed from surrounding material, the suspension or mixing of the sample, and analysis and comparison of sequences, typically by a computer.

Restriction Fragment Length Polymorphism (RFLP) Analysis

RFLP analysis was one of the earliest forms of fingerprinting used in DNA evidence analysis. Its results were slow, however, and testing required the destruction of fairly large samples. If only minute trace evidence was found at a crime scene, such analysis was impossible.

RFLP analysis looked at variable number tandem repeats (VNTRs), the repeated sequences of genetic code base pairs. Every individual's DNA has several VNTR repeating sequences that have been inherited from one or both parents. The combination of patterns they form are unique.

DNA analysis through RFLP has substantial limitations. A sample greater than the size of a quarter is needed to complete RFLP analysis. Also, samples must be relatively fresh and not mixed with other contaminants. These two conditions severely limited the utility of RFLP, especially in processing crime scenes with only trace evidence.

Polymerase Chain Reaction (PCR) Analysis

Most PCR tests amplify specific regions of DNA using a cycling of higher and lower temperatures and a special enzyme, along with fluorescent labeled sequence-specific DNA primers. Other PCR tests involve cards that react with a sample and map sequence variations. These card-based systems are used less frequently because it is harder to determine a DNA footprint from a mixed DNA sample, which are common in rape and sexual assault investigations. Overall, PCR requires less time to get results and can be used on smaller, older, and more degraded samples than RFLP.

Short Tandem Repeats (STR) Analysis

STR, the most widely used method of DNA fingerprinting, incorporates several aspects of PCR, but primarily identifies repeat units in certain locations (loci) of DNA sequences. Many STR systems can process several samples at once, giving it considerable power of discrimination.

The Federal Bureau of Investigation established the Combined DNA Index System (CODIS), a database of convicted offenders. The program allows law enforcement agencies to match DNA profiles, which can then be used to link crimes and identify criminals. CODIS uses 13 core loci (DNA locations); thus, the odds that two random individuals could have the same CODIS loci DNA profile are as small as 1 in one billion.

Other Methods

Developed in the early 1990s, AmpFLP uses staining gels and an allelic ladder (in which different forms of a gene, or alleles, fall into different places on the ladder). Although it is not as precise as other methods, it is used today mostly in developing nations because it is inexpensive and easy to set up.

Mitochondrial DNA (mtDNA) analysis focuses on inherited maternal DNA markers. MtDNA analysis can use old, small, highly degraded samples. It can also use samples from hair, teeth, and bones. Analysis of mtDNA is often used in academic forensic work, such as genetically mapping relationships within and among various populations. Any directly related maternal relative can serve as a comparison sample.

Y-chromosome (Y-STR) analysis permits a researcher to differentiate several paternally related males when comparison samples are unavailable.

DNA Evidence in the Courtroom

Scientifically, all methods of DNA analysis produce highly reliable results. There remains a very slight chance that some random individuals may have similar DNA analysis profiles. The frequency of matching profiles is called the random match probability. While all methods of DNA analysis are similarly reliable, the random match probability can be affected by the rarity of the markers and the number of loci matched in two samples. For example, matching DNA profiles that both contain a combination of more rare markers is less likely to produce a match by chance.

However, presenting and refuting DNA evidence in court can introduce faulty interpretations and characterizations of DNA evidence testing. Random match probability is frequently discussed when DNA evidence that is recovered from a crime scene is used at trial. However, many forensic scientists assert that lawyers and the media often incorrectly state or misrepresent random match probability. The accurate statement that “a 1 in 7,000 chance exists that some particular person other than the defendant could produce genetic evidence like the actual genetic evidence recovered” is often mischaracterized as meaning that there is “only 1 chance in 7,000 that a person other than the defendant left the genetic evidence.”

Furthermore, studies have shown that juries are often confused by the presentation of DNA evidence—and the random match probability principle—at trial. One study showed that how an attorney discussed random match probability could significantly sway test jurors. Discussion of false positive laboratory error rates proved less influential among test jurors. Even though both are statistically very unlikely, a random match is significantly less likely to occur than a false positive lab error.

In 1987 serial rapist Tommie Lee Andrews became the first person in the United States to be convicted using DNA evidence. The following year, Timothy Spencer, dubbed the “Southside Strangler,” was the first person sentenced to death after a conviction based largely on DNA evidence. In 1989 DNA evidence overturned an appealed conviction. Despite the rapid integration of DNA evidence into rape and murder cases, the general public paid little attention to emerging DNA evidence analysis for several years.

The 1995 murder trial of former football player O.J. Simpson was the first time many Americans became familiar with DNA evidence. The prosecution introduced DNA evidence linking Simpson to the murder of his ex-wife and her friend, but defense attorneys asserted that the evidence was inaccurate, potentially tampered with, and not scientifically certain enough to be taken as concrete proof. Simpson was acquitted, but the controversial trial demonstrated the need for better DNA testing, clear and universal guidelines on the handling of DNA evidence, and a system by which DNA evidence should be introduced, questioned, and weighed in court.

Modern Cultural Connections

DNA evidence is a staple in popular culture and a central feature television of crime shows such as the CSI franchise, which attempt to follow DNA evidence through its long forensic process from field sample to lab analysis. The popularity of such shows reveals that much of the public is now familiar with DNA evidence and its connection to criminal law.

DNA evidence, when expertly processed, is a significant aid to criminal court cases, and many juries and courts find it highly persuasive. In the United States, however, it is weighed with other evidence presented at trial, so it cannot by itself guarantee a conviction. Before the introduction and widespread use of DNA evidence, the only ways to tie perpetrators to crimes were eyewitness accounts, photographs, videotape, fingerprints, and other less-reliable forms of identification that were prone to human error. DNA evidence has added much-needed precision to courtroom evidence and has been used in criminal cases from single murders to mass killings. For example, DNA evidence helped identify victims of genocide in the former Yugoslav Republics and helped build war crimes cases against the perpetrators.

Handled with scientific rigor and precision, DNA fingerprinting is highly accurate. It is, however, vulnerable to human error. Mishandled samples or inadequate testing may affect its reliability. However, failures of DNA evidence are increasingly rare absent misconduct, evidence tampering, or sample destruction.

DNA evidence is not used exclusively by governments, law enforcement officials, and prosecutors. In 1992 a group of civil rights lawyers and law student volunteers formed the Innocence Project at the Benjamin N. Cardozo School of Law at New York's Yeshiva University to help vindicate prisoners who claimed to have been wrongly convicted. The Innocence Project, now a series of legal clinics in law schools across the United States, selects cases based on the evidence used at trial, the likelihood of error, the availability of scientific evidence, and the proper use of the scientific evidence.

Many early Innocence Project cases involved convictions secured before widespread analysis of DNA evidence by crime scene investigators or before courts accepted it as reliable scientific evidence. However, Innocence Project legal experts also cite erroneous witness statements, government misconduct, false confessions, poor lawyering, and faulty forensic analysis as other significant contributors. Most wrongful convictions overturned by DNA evidence in the United States have been for rape and murder, carrying sentences varying from 10 years to death.

Beginning in the late 1990s, several states suspended prisoner executions until all pending death row cases could be reviewed in light of new DNA evidence where possible. Many legislators credited the moratorium on executions to both the work of the Innocence Project and to a landmark study that asserted that as many as 5% of all life-sentence and death-row inmates may have been wrongly or inconclusively convicted. Several legal groups called for widespread review of capital cases in which convictions were secured before DNA analysis technology was available, where it was not used, or where trace evidence too small to be analyzed previously could now be tested. A handful of states did place a moratorium of executions pending review, some have since lifted their bans.

Furthermore, a landmark U.S. case guaranteed convicted individuals the right to see DNA analysis information and to test all DNA samples connected to their case. In 2001 Harvey v. Horan established that convicted persons could order DNA testing on evidence in the government's possession to try to prove their innocence.

While DNA fingerprinting is a key tool for forensic scientists and law enforcement, privacy advocates are concerned with the increasing amount of personal biometric information being stored by government agencies. The United States maintains a large index of individual DNA sequences, most from persons convicted of a crime. Law enforcement agencies assert that CODIS helps solve crimes, proving especially useful in criminal investigations where only the smallest amounts of trace evidence are available. Most CODIS DNA samples were collected voluntarily, although some were obtained after a court order compelled suspects to give a sample. Several other nations maintain DNA databases similar to CODIS. Privacy and civil rights advocates in Europe are also concerned with the collection of DNA samples and long-term storage of DNA profiles in databases.

Primary Source Connection

The following article was written by Warren Richey, a staff writer for the Christian Science Monitor. Founded in 1908, the Christian Science Monitor is an international newspaper based in Boston, Massachusetts. The article describes how states are utilizing DNA technology to both aid in convicting criminals and to exonerate persons imprisoned for crimes they did not commit.


Even prosecutors are embracing the technology as a protection against wrongful imprisonment

MIAMI —When Alan Crotzer emerged from a Tampa courthouse last week a free man, he became the fifth person in Florida and the 173rd nationwide to be cleared from a wrongful conviction by DNA testing.

But his exoneration after serving 24 years of a 130-year sentence for a crime he didn't commit is significant for another reason. It marks a possible turning point in how Florida prosecutors and lawmakers approach DNA testing. Prosecutors had fought earlier attempts to use DNA to prove someone's innocence, keeping one man behind bars three years after tests proved he couldn't have committed the crime that sent him to prison on a life sentence. In Mr. Crotzer's case, by contrast, they ultimately filed the legal motion that set him free, once defense lawyers convinced them he was innocent.

“The state has jumped onboard and finally started to realize that we can't just use DNA to convict, we have to also use DNA to free,” says Jenny Greenberg, director of the Florida Innocence Initiative and one of Crotzer's lawyers.

Legal analysts say it is part of an evolution taking place across the country in which prosecutors and lawmakers who were once suspicious of the DNA challenges are now increasingly embracing the new technology as a backstop in the criminal justice system.

“There have been some very positive results and legislators have seen the benefits of proving an innocent person innocent,” says Blake Harrison, who follows the issue at the National Conference of State Legislatures. “If anything, there is public pressure to expand the use of these types of post-conviction reevaluations because of the obvious public benefit to making sure that you have got the right person.”

On Thursday, Arthur Mumphrey of Texas was freed after nearly 20 years in prison after new tests showed his DNA didn't match that found at a 1986 rape scene.

Of the 33 states that have passed laws establishing procedures for DNA testing, 12 have sunset provisions, which offer only limited time for DNA tests in cases where inmates have no more options for appeal. Such provisions were enacted in part out of concern five years ago that state courts might be flooded with appeals calling for expensive tests. But the flood hasn't happened, analysts say. In the meantime, the steady flow of exonerations has resulted in widespread public and political support for DNA testing.

Opponents of sunset provisions say states should never enact a deadline that would prevent innocent people from proving their innocence. Innocence Project lawyers say it often takes months or years to locate evidence, investigate a past crime, and prepare the necessary legal arguments needed to convince a judge that a DNA test should be administered.

When Florida passed its DNA law in 2001, legislators gave potential appellants until 2003 to file a request for testing. That deadline was extended to 2005, and then further extended to July 2006. Now a powerful state senator—and former prosecutor—is pushing for a new DNA law that removes any deadline. In addition, it calls for preservation of evidence for the entire length of someone's incarceration, thus preserving the possibility of future appeals as testing technologies improve.

State officials worry about the possible cost of evidence preservation. But proponents say the bill received unanimous support in its first state Senate committee hearing last week.

“What is driving it is the general public's horror at the difficulties that innocent people have had here in Florida to prove their innocence,” says Ms. Greenberg. “Most people in Florida are appalled that innocent people might be in prison and they want something done about it.”

The same trend is apparent across the country, says Kathy Swedlow, codirector of the Innocence Project at Thomas Cooley Law School in Lansing, Mich. Ms. Swedlow has studied post-conviction DNA laws nationwide—including laws with sunset provisions. She says many of the laws have expired at least once and been extended for a couple years as lawmakers study the issue. “The fear I had in early 2001 was that the statutes would expire and it would be over,” she says. “That is not what we are seeing.”

In Michigan, the deadline has been extended from January 2006 to January 2009. Louisiana moved its deadline from August 2005 to August 2007. And New Mexico has pushed its deadline from July 2002 to July 2006.

As more cases work their way through the system, lawmakers and prosecutors are becoming more supportive of the idea that DNA testing can help establish the truth about particular crimes, she says. “These statutes are a win-win. They help us identify finally if people are innocent,” she says. “In some instances they help us learn that even with the crude technologies of 20 years ago, that police got the right guy.”

Authorities in Virginia took the extraordinary step of conducting DNA testing to determine whether Roger Coleman had been wrongfully executed in 1992 for rape and murder. Death penalty opponents investigated the case and suggested he was innocent. But DNA test results showed he was, in fact, the killer.

Warren Richey

richey, warren. “dna tests gain ground as legal defense.” christian science monitor

(january 31, 2006).

See Also Biology: Genetics; Biology: Genetics, DNA, and the Genetic Code; Biology: Genetics: The Human Genome.



Lee, Henry C., and Frank Tirnady. Blood Evidence: How DNA Is Revolutionizing the Way We Solve Crimes. New York: Basic Books, 2003.

Platt, Richard. Crime Scene: The Ultimate Guide to Forensic Science. New York: DK Books, 2006.


Richey, Warren. “DNA tests gain ground as legal defense.” Christian Science Monitor (January 31, 2006).

Adrienne Wilmoth Lerner

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