DNA testing

SIGNIFICANCE: A new and highly accurate way to identify people and match evidence with victims and suspects, DNA testing is revolutionizing investigative techniques in law enforcement and forcing major reconsiderations of the possibilities of convicting the innocent, particularly in capital cases.

DNA stands for deoxyribonucleic acid, the basic building blocks of biological genes. Because every human being has a unique pattern of genetic material, every human being has a pattern of DNA molecules that is like that of no other human being. For this reason, DNA taken from samples of human body tissues and fluids can be used to determine very accurately from what individual person those tissues and fluids come.

Criminal justice agencies have long sought better methods of linking suspects with particular crime scenes, especially when eyewitnesses to crimes are not available. Early methods of establishing such links included fingerprint analysis, blood-typing, and analyses of such trace evidence as cloth fibers and hair. Fingerprint evidence has the potential of establishing positive identifications; however, good fingerprint evidence is often difficult to obtain at crime scenes. Materials such as cloth fibers, human hairs, and blood samples can be helpful but can rarely be used to establish definitive links to any one suspect. In contrast, good DNA evidence not only is generally more readily obtainable, it is more reliable because no two human beings have exactly the same DNA patterns.

Since the mid-1980s, DNA testing has been used as a valuable tool for solving crimes. Criminalists can, for example, tell whether a single human hair or tiny blood spatter found on a victim comes from the victim or from someone else. Moreover, if the police have a suspect in the case, criminalists can determine whether the hair or blood comes from that suspect. The certainty with which tiny quantities of human tissue and fluids can be matched to criminal suspects makes it easier for prosecutors to secure convictions and lessens the chances of false convictions.

History of DNA Testing

DNA was first discovered in 1868, but it was not until 1953 that scientists discovered its physical structure. For this discovery, the American scientist James Watson and the British scientists Francis Crick and Maurice Hugh Frederick Wilkins shared a Nobel Prize in 1962.

Most DNA is identical in all humans, and much of it even matches the DNA of other animals. However, in 1983, British biochemist Alec Jeffreys discovered that certain portions of the DNA sequence vary from individual to individual. These sequences were soon dubbed “DNA fingerprints,” because they—like the patterns of ridges on human fingertips—were thought to be unique to individual human beings. Jeffreys and his colleagues created methods of probing DNA samples to examine the DNA fingerprints, a process that became known as DNA profiling.

Criminal justice agencies quickly realized the potential of DNA profiling as a tool for solving crimes. In 1985, Jeffreys used his techniques to help British government authorities determine whether a Ghanaian boy who wanted to come to England from West Africa to be reunited with his mother was indeed the woman’s son. Jeffreys’s DNA analysis proved that he was. One year later, Jeffreys helped police in Leicestershire, England, solve cases involving the rape and murder of two fifteen-year-old girls. Using traces of semen found on the victims, Jeffreys determined that both girls had been attacked by the same man. Moreover, those samples established that a suspect whom police were holding in custody could not have been the girls’ assailant. After DNA samples were collected from many local men, Jeffreys was eventually able to connect a local resident to the murders.

DNA Testing Techniques

DNA is present in every part of the human body that contains nucleic cells. These include body tissues and such fluids as blood, sweat, semen, and urine. DNA cannot be collected from strands of hair that are cut, but it can be extracted from the roots and follicles of hairs that are pulled from a body or shed naturally. Material in the follicles contains DNA, as do flakes of dandruff and other minute fragments of skin that are naturally shed.

DNA typing was originally done through a process known as RFLP—which is short for “restriction fragment length polymorphisms.” The process used enzymes in a fashion much like chemical scissors to cut out portions of a DNA sample. Those portions were then subjected to various electrical and chemical processes that allowed their structures to be seen. Scientists then compared the patterns observed in those portions with patterns in samples taken from materials found at crime scenes.

During criminal investigations, portions from several different sites on a DNA strand are compared. If one particular person is the source of all the samples, the patterns will match on all sites. The chance of samples from two different persons matching on all sites is minuscule.

During the mid-1990s, a new testing technique was invented: PCR, or polymerase chain reaction. Using the new PCR technique, extremely minute amounts of DNA are chemically replicated until there is a quantity large enough to analyze. Because less than one-billionth part of one gram of DNA material is sufficient for PCR analysis, it can be used to analyze DNA taken from samples as minute as the dried saliva found on cigarette butts and envelope flaps.

As of 2024, the latest form of DNA testing technology in use was called forensic investigative genetic genealogy (FIGG). Using FIGG, law enforcement officials can search public and private databases with the DNA of people who have made voluntary contributions. Using this method, law enforcement has worked private DNA companies to solve many cases, included old murder cases such as those of the Golden State Killer and the Long Island Serial Killer. Newer DNA tests are able to link the DNA from extended family members to the DNA in human remains, which makes it much easier for law enforcement to identify the remains and give them to family members.

In the criminal justice system, thirteen different sites along the DNA strand are used for analysis. The chance of the DNA of any two individuals being identical in all thirteen sites is less than one in 575 trillion. Since the latter figure is roughly 40 million times greater than the number of all the human beings who have lived, it is clear that DNA testing offers an exceptionally powerful method of identifying and exonerating criminal suspects.

A final type of DNA testing that is sometimes used is mitochondrial DNA analysis, or mtDNA. Mitochondrial DNA is found outside cell nuclei, and exists in greater quantities than nuclear DNA. MtDNA testing can be done on samples in which nuclear DNA is too degraded to analyze, or on samples in which nuclear DNA is absent altogether, such as hair shafts. However, mtDNA analysis is costly and more difficult to do, and few labs are able to perform it. Moreover, all human beings have mtDNA that is identical to that of their mothers. Thus, because mtDNA patterns are not unique to each individual, mtDNA analysis is limited in its usefulness as a tool for identifying suspects.

CODIS

The Federal Bureau of Investigation (FBI) has created a computer database program called CODIS, an acronym for “Combined DNA Index System.” Data from DNA evidence from unsolved crimes and from certain convicted offenders are entered into CODIS by local, state, and federal law-enforcement agencies. DNA patterns from different unsolved crimes and from known offenders can now be easily compared. In this regard, CODIS is similar to AFIS, the FBI’s Automated Fingerprint Identification System . By early 2005, CODIS had produced nearly 20,000 DNA matches, or “hits,” that helped solve many cold cases.

The CODIS system is not without critics. Some people have argued that mandated collection of DNA samples invades citizens’ privacy. A major part of this objection is the fact that DNA, unlike fingerprints, contains information about its owners’ physical, behavioral, and health characteristics. Critics are concerned about the ways such personal information might ultimately be used by the government. Some people also are concerned that in some states, such as California, DNA information is collected not only from convicted felons but also from some adults and juveniles who are simply charged with certain crimes.

The Challenges of DNA Testing

In addition to legal and ethical disputes, DNA testing also faces practical challenges. The first of these is the great care that must be used in collecting, storing, and analyzing of DNA evidence. With improper handling, DNA evidence can be contaminated easily. There are methods of detecting contamination during analysis; however, contamination makes analyses more difficult and less certain. There have also been cases in which lab employees have engaged in sloppy and improper techniques of handling and analyzing DNA that have resulted in incorrect data.

A second major challenge of DNA analysis is the time and expertise that it requires. Steadily improving technology has simplified and speeded up testing processes. Nonetheless, the demand for DNA analyses far outstrips the equipment and trained personnel of the crime labs that perform the analyses. Most crime labs have long backlogs in their work, and law-enforcement agencies may have to wait several months before they receive the results of the analyses they request. To complicate matters, the DNA evidence awaiting testing is often stored, not at crime labs, but rather at the law-enforcement agencies, which typically lack the facilities for proper storage. This fact increases the chance of evidence becoming contaminated or degraded.

Other Uses for DNA Testing

In addition to matching suspects to crime scenes, DNA testing has found other important uses in the legal system. As often as it is used to identify offenders, it is used at least equally often to prove that suspects have not committed crimes. In 1992, the Innocence Project was created at Yeshiva University’s Benjamin N. Cardozo Law School in New York City. By 2023, the Innocence Project had used DNA testing to prove the innocence of 575 people who had been convicted of serious crimes. Under new laws, states must now allow certain convicted prisoners access to DNA testing if there is any chance that it might help prove their innocence.

DNA testing is also widely used to establish paternity; that is, to determine whether two particular people are blood relatives. An instance of paternity testing that captured world headlines occurred after Asia’s devastating tsunami in late 2004. In Sri Lanka, it was reported that nine different families claimed the same four-month-old infant who was found in the debris of the tsunami. DNA testing was used to identify his actual parents, with whom he was reunited. During large-scale disasters such as the Asian tsunami and the September 11, 2001, destruction of New York City’s World Trade Center, DNA testing has been used to identify human remains. DNA evidence can also be used to identify human remains in missing persons cases or whenever human remains are discovered.

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