Among the many new tools science has provided for the analysis of forensic evidence is the powerful and controversial analysis of deoxyribonucleic acid, or DNA, the material that makes up the genetic code of all organisms. DNA analysis, also called DNA typing or DNA profiling, examines DNA found in physical evidence such as blood, hair, and semen and determines whether it can be matched to DNA taken from specific individuals. DNA analysis has become a common form of evidence in criminal trials. It is also used in civil litigation, particularly in cases involving the determination of paternity or identity.
History and Process of DNA Analysis
DNA, sometimes called the building block or genetic blueprint of life, was first described by the scientists Francis H. C. Crick and James D. Watson in 1953. Crick and Watson identified the double helix structure of DNA, which resembles a twisted ladder, and established the role of DNA as the material that makes up the genetic code of living organisms. The pattern of the compounds that constitute the DNA of an individual life-form determines the development of that life-form. DNA is the same in every cell throughout an individual's body, whether it is a skin cell, sperm cell, or blood cell. With the exception of identical twins, no two individuals have the same DNA blueprint.
DNA analysis was first proposed in 1985 by the English scientist Alec J. Jeffreys. By the late 1980s, it was being performed by law enforcement agencies, including the Federal Bureau of Investigation (FBI), and by commercial laboratories. It consists of comparing selected segments of DNA molecules from different individuals. Because a DNA molecule is made up of billions of segments, only a small proportion of an individual's entire genetic code is analyzed.
In DNA analysis for a criminal investigation, using highly sophisticated scientific equipment, first a DNA molecule from the suspect is disassembled, and selected segments are isolated and measured. Then the suspect's DNA profile is compared with one derived from a sample of physical evidence to see if the two match. If a conclusive nonmatch occurs, the suspect may be eliminated from consideration. If a match occurs, a statistical analysis is performed to determine the probability that the sample of physical evidence came from another person with the same DNA profile as the suspect's. This statistical result is used by juries in determining whether a suspect is guilty or innocent.
Although DNA analysis is sometimes called DNA fingerprinting, this term is a misnomer. Because the entire DNA structure of billions of compounds cannot be evaluated in the same way that an entire fingerprint can, a "match" resulting from DNA typing represents only a statistical likelihood. Thus, the results of DNA typing are not considered absolute proof of identity. A DNA nonmatch is considered conclusive, however, because any variation in DNA structure means that the DNA samples have been drawn from different sources.
An example from the early 1990s illustrates how DNA evidence is used in the criminal justice system. After a Vermont woman was kidnapped and raped in a semitrailer truck, the police identified Randolph Jakobetz, a truck driver, as a suspect in the crime. Police officers searched the trailer hauled by Jakobetz the night of the crime and found hairs matching those of the victim. After arresting Jakobetz, law enforcement officials sent a sample of his blood to the FBI laboratory in Washington, D.C., for DNA analysis and for comparison with DNA taken from semen found in the victim shortly after the crime.
At Jakobetz's trial, an FBI expert testified that the blood and semen samples were a "match," concluding that there was one chance in 300 million that the semen samples could have come from someone other than Jakobetz. Based on this and other strong evidence, Jakobetz was convicted and sentenced to almost thirty years in prison.
Jakobetz appealed the decision, claiming that DNA profiling was unreliable and should not be admitted as evidence. In the first major federal decision on DNA profiling, the Court of Appeals for the Second District upheld the lower court's decision to admit the DNA evidence (United States v. Jakobetz, 955 F.2d 786 [2d Cir. 1992]). The U.S. Supreme Court later declined to hear an appeal.
The Jakobetz case illustrates how the probabilities generated by DNA analysis can be used as devastating evidence against a criminal suspect. Juries have tended to view the statistical results of this analysis as highly incriminating, which has caused many defense attorneys to challenge the validity of the results and many prosecuting attorneys to defend them. At the same time, DNA analysis has also been used by defense attorneys as evidence to reverse the conviction of their clients.
Legal History of DNA Evidence
In general, DNA evidence has been increasingly accepted by state and federal courts as admissible evidence. The first state appellate court decision to admit DNA evidence was in 1988 (Andrews v. Florida, 533 So. 2d 841 [Fla. Dist. Ct. App.]), and the first major federal court decision to admit it occurred in Jakobetz. By the mid-1990s, most states allowed DNA test results into evidence.
No court has rejected DNA evidence on the grounds that the underlying scientific theory is invalid. However, some courts have excluded it as evidence because of problems with the possible contamination of samples, questions surrounding the significance of its statistical probabilities, and laboratory errors. Several states have passed laws that recognize DNA evidence as admissible in criminal cases, and others have enacted laws that specifically admit DNA evidence to resolve civil paternity cases.
The admissibility of novel scientific evidence such as DNA profiling is governed by two different judicial tests or standards: the Frye, or general acceptance, standard, and the Daubert, or relevancy-reliability, standard. The Frye test, which comes from the 1923 case Frye v. United States 293 F. 1013 (D.C. Cir.), holds that the admissibility of evidence gathered by a specific technique (such as DNA analysis) is determined by whether that technique has been "sufficiently established to have gained general acceptance in the particular field in which it belongs." In Frye, the Court of Appeals for the District of Columbia ruled that a lie detector test using a blood pressure reading was not admissible as evidence. By the 1970s, forty-five states had adopted this common-law standard for the admission of novel scientific evidence.
The U.S. Supreme Court overruled use of the Frye test in federal courts in its 1993 decision Daubert v. Merrell Dow, 509 U.S. 579, 113 S. Ct. 2786, 125 L. Ed. 2d 469. In Daubert, the Court held that the Federal Rules of Evidence, which were created by Congress in 1975, govern the admission of novel scientific evidence in federal courts. It found that Frye provides too stringent a test and is incompatible with the federal rules, which allow the admission of all evidence that has "any tendency to make the existence of any fact that is of consequence to the determination of the action more probable or less probable than it would be without the evidence" (rule 401). The Court found that judges have a responsibility to "ensure that any and all scientific testimony or evidence admitted is not only relevant, but reliable."
In general, courts that have used the Daubert standard have been more likely to admit DNA evidence, although many jurisdictions that have relied on Frye have permitted it as well. Nearly all the cases in which DNA evidence has been ruled inadmissible have been in jurisdictions that have used Frye.
States are free to adopt their own standards for the admission of evidence, and have been increasingly adopting the Daubert standard. By 1995, the number of states using the Frye standard had dropped to twenty-three, while twenty-one had adopted the Daubert standard.
See: forensic science.
