The DNA fingerprinting technique developed by Henry Erlich made it possible for scientists to identify an individual from the DNA in a single hair.
All individuals, with the exception of twins and other clones, are genetically unique. Theoretically it is therefore possible to use these genetic differences, in the form of DNA sequences, to identify individuals or link samples of blood, hair, and other features to a single individual. In practice, individuals of the same species typically share the vast majority of their DNA sequences; in humans, for example, well over 99 percent of all the DNA is identical. For individual identification, this poses a problem: Most of the sequences that might be examined are identical (or nearly so) among randomly selected individuals. The solution to this problem is to focus only on the small regions of the DNA which are known to vary widely among individuals. These regions, termed hypervariable, are typically based on repeat sequences in the DNA.
Imagine a simple DNA base sequence, such as AAC (adenine-adenine-cytosine), which is repeated at a particular place (or locus) on a human chromosome. One chromosome may have eleven of these AAC repeats, while another might have twelve or thirteen, and so on. If one could count the number of repeats on each chromosome, it would be possible to specify a diploid genotype for this chromosomal locus: An individual might have one chromosome with twelve repeats, and the other with fifteen. If there are many different chromosomal variants in the population, most individuals will have different genotypes. This is the conceptual basis for most DNA fingerprinting.
DNA fingerprint data allow researchers or investigators to exclude certain individuals: If, for instance, a blood sample does not match an individual, that individual is excluded from further consideration. However, if a sample and an individual match, this is not proof that the sample came from that individual; other individuals might have the same genoytpe. If a second locus is examined, it becomes less likely that two individuals will share the same genotype. In practice, investigators use enough independent loci that it is extremely unlikely that two individuals will have the same genotypes over all of the loci, making it possible to identify individuals within a degree of probability expressed as a percentage, and very high percentages are possible.
Alec Jeffreys, at the University of Leicester in England, produced the first DNA fingerprints in the mid-1980’s. His method examined a twelve-base sequence that was repeated one right after another, at many different loci in the human genome. Once collected from an individual, the DNA was cut using restriction enzymes to create DNA fragments that contained the repeat sequences. If the twelve-base sequence was represented by more repeats, the fragment containing it was that much longer. Jeffreys used agarose gel electrophoresis to separate his fragments by size, and he then used a specialized staining technique to view only the fragments containing the twelve-base repeat. For two samples from the same individual, each fragment, appearing as a band on the gel, should match. This method was used successfully in a highly publicized rape and murder case in England, both to exonerate one suspect and to incriminate the perpetrator.
While very successful, this method had certain drawbacks. First, a relatively large quantity of DNA was required for each sample, and results were most reliable when each sample compared was run on the same gel. This meant that small samples, such as individual hairs or tiny blood stains, could not be used, and also that it was difficult to store DNA fingerprints for use in future investigations.
The type of sequence Jeffreys exploited is now included in the category of variable number tandem repeats
VNTRs are often subcategorized based on the length of the repeated sequence. Minisatellites, like the Jeffreys repeat, include repeat units ranging from about twelve to several hundred bases in length. The total length of the tandemly repeated sequences may be several hundred to several thousand bases. Many different examples have since been discovered, and they occur in virtually all eukaryotes. In fact, the Jeffreys repeat first discovered in humans was found to occur in a wide variety of other species.
Shorter repeat sequences, typically one to six bases in length, were subsequently termed microsatellites. In humans, AC (adenine-cytosine) and AT (adenine-thymine) repeats are most common; an estimate for the number of AC repeat loci derived from the Human Genome Project
In 1988, Henry Erlich used a technique newly developed by Kary B. Mullis—the polymerase chain reaction,
Erlich and his colleagues used the amplified DNA from a single hair to analyze a histocompatibility gene. Histocompatibility genes code for the tissue-type markers that must be matched in organ transplants because they stimulate attacks from the immune systems of individuals with different tissue types. Histocompatibility sequences are highly variable from one person to the next, which is why they are so useful for DNA fingerprinting, since the probability that two unrelated individuals will have the same tissue type is extremely low. DNA fingerprints that are as unique as 1 in 10,000 or even 1 in 100,000 can be obtained by analyzing these sequences. Adding other sequences to the analysis can generate DNA fingerprints that have nearly a zero probability of matching with another person, except for an identical twin. Differences in the histocompatibility sequence chosen by Erlich for typing were identified by matching DNA probes constructed for each histocompatibility sequence.
DNA sequences have the property of self-recognition, and Erlich used this property by preparing samples of the known variants of the histocompatibility sequences. Each variant form can recognize matching forms identical to itself in an unknown sample and bind to them but will not bind to any of the other forms. Erlich took the samples of amplified DNA from the hair cells and applied each probe to each unknown sample. The probes—representing the different variants of the histocompatibility sequence—stick only to their own form and have a stain attached to them so that the high concentrations of probe molecules that stick to a matching sample can be located visually. Erlich was able to identify the differences in histocompatibility sequences from the amplified hair-cell DNA samples by determining which probes remained attached to each sample.
Erlich and his colleagues showed that the results obtained from single hairs were confirmed by results obtained from blood samples taken from the same people who donated the hair. The technique was also successfully used on seven-month-old single hair samples. One of the first forensic applications of the PCR-DNA fingerprinting technique took place in Pennsylvania. In a homicide case, a one-year-old body was exhumed to be examined for evidence, a previous autopsy having been deemed suspicious. The prosecution had accused the defendants of tampering with the body by switching some of its internal organs with those of another body to conceal the cause of death. The PCR-DNA fingerprinting technique was used to show that the child’s embalmed organs, exhumed with the body, did in fact match the victim’s tissue type, and the defendants were acquitted of the tampering charge.
DNA fingerprinting has been refined to the point where an individual can be identified from the DNA in a single hair, which means that one hair, or even microscopic samples of dried blood, skin, or other body fluids found at the scene of a crime, can be analyzed to determine whose body it came from with nearly 100 percent accuracy, with the exception of twins. Properly used, DNA fingerprinting can be so precise that the margin of error in making a match between biological evidence and a suspect’s DNA is less than one in ten thousand, with tests based on tissue-type genes from a single hair, and less than one in a billion with more extensive testing.
Erlich’s method of DNA typing from a single hair was a dramatic refinement of DNA fingerprinting. Hair is one of the most common types of biological evidence left behind at crime scenes, so Erlich’s improvement over traditional methods of analyzing hair color, shape, and protein composition can be widely applied. Erlich’s technique also allows substitution of hair samples for blood or skin when DNA fingerprints are taken; the technique can be automated, making it easier to apply DNA fingerprinting to large populations along with traditional fingerprinting.
The DNA fingerprinting technique developed by Erlich and his colleagues at Cetus Corporation in Emeryville, California, was made commercially available in early 1990 in the form of a DNA-typing kit, allowing more widespread application of the polymerase chain reaction to DNA fingerprinting. Initially, the main disadvantage to DNA fingerprinting was the practical difficulty of transferring a new, highly technical procedure from the research laboratory to routine application in the field. While DNA fingerprinting is virtually 100 percent accurate in theory, and works in the hands of highly trained scientists, methods for reliable and economical mass application had to be developed and proved before DNA fingerprinting became routine.
Evidence based on DNA fingerprinting was introduced for the first time in several dozen court cases in the late 1980’s and played a key role in many of them. Since then it has become widely applied—even to cold cases—and has been introduced into evidence in criminal cases. As juries have become more accustomed to the use of this evidence and educated about its accuracy, they have learned to take it very seriously in their deliberations. Moreover, DNA evidence applied to long-running cases and even cold cases has unmasked guilty persons years after their crimes were committed. Perhaps more important, in several instances DNA evidence has revealed several imprisoned individuals wrongly convicted of crimes and finally set free after years of incarceration.
The technique has also been widely applied to paternity testing, to cases (such as wildlife poaching) involving identification of animals, in immigration cases to prove relatedness, and to identify the remains of casualties resulting from military combat and large disasters. The technique’s ability to identify paternity has led to its use by those who study breeding systems and other questions of individual identification in wild species of all kinds: plants, insects, fungi, and vertebrates. Researchers now know, for example, that among the majority of birds which appear monogamous, between 10 and 15 percent of all progeny are fathered by males other than the recognized mate. DNA fingerprinting also has many applications to agriculture, helping farmers identify appropriate plant species.
The fact that DNA can provide so much information about an individual, in addition to its usefulness in establishing personal identity, raises ethical questions about the use of DNA fingerprinting that have not been encountered with the use of traditional fingerprinting. Laws regarding the collection and use of DNA data must be carefully considered: For example, should such data be routinely collected from anyone arrested for any sort of offense, or restricted to those charged with certain crimes or who have advanced to a certain stage in the criminal justice system? However, the fact that such absolute certainty can exonerate criminal suspects who are innocent as well as help convict those who are guilty makes the responsible use of DNA fingerprinting the most important advance in forensic science since the advent of traditional fingerprinting.
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Appenzeller, Tim. “Democratizing the DNA Sequence.” Science 247 (March, 1990): 1030-1032. Provides nontechnical explanation of the PCR technique and its impact on biology and biological applications. Useful background article for fully understanding Erlich’s DNA fingerprinting technique. -
Fridell, Ron. DNA Fingerprinting: The Ultimate Identity. New York: Scholastic, 2001. Presents good basic background on the history of DNA fingerprinting and discussion of the technology’s uses. Includes photographs. -
Kirby, Lorne T. DNA Fingerprinting: An Introduction. New York: Stockton Press, 1990. One of the first books on the topic of DNA fingerprinting written for a general audience. Intended for scientists unfamiliar with the techniques and for lawyers and criminologists who want an authoritative, comprehensive, and practical introduction. Covers topics from basic genetic theory and techniques used to legal and ethical issues and case studies. Includes glossary and indexes. -
Lewin, Roger. “DNA Typing on the Witness Stand.” Science 244 (June, 1989): 1033-1935. Nontechnical article describes the legal issues surrounding the initial applications of DNA fingerprinting as evidence in criminal trials. Focuses on the Castro murder case in New York City, in which expert scientific witnesses from the prosecution and defense met outside the courtroom during the trial, independent of legal counsel, to assess the scientific validity of DNA fingerprinting evidence introduced by the prosecution. Also discusses the status of DNA fingerprinting with respect to the standards for admissibility of scientific evidence in court. -
Rudin, Norah, and Keith Inman. An Introduction to Forensic DNA Analysis. 2d ed. Boca Raton, Fla.: CRC Press, 2002. Addresses forensic DNA analysis from both scientific and legal perspectives, emphasizing the limitations and advantages of particular techniques. Intended for professionals working in the legal system, law enforcement, and forensic science.
Cohen and Boyer Develop Recombinant DNA Technology
Berg, Gilbert, and Sanger Develop Techniques for Genetic Engineering
Geneticists Create Giant Mice
Mullis Develops the Polymerase Chain Reaction
Murray and Szostak Create the First Artificial Chromosome
Jeffreys Discovers the Technique of Genetic Fingerprinting