DNA Evidence

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 whether 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. Juries use this statistical result 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 the way in which DNA evidence is used in the criminal justice system. After a Vermont woman was KIDNAPPED and raped in a semi-trailer truck, police identified Randolph Jakobetz, a truck driver, as a suspect in the crime. Officers searched the trailer that Jakobetz had hauled on 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 30 years in prison.

Jakobetz appealed the decision, claiming that DNA profiling was unreliable and that it should not be admitted as evidence. In the first major federal decision on DNA profiling, the U.S. Court of Appeals for the Second Circuit 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 the way in which 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, defense lawyers have used DNA analysis as evidence to reverse the convictions of their clients.