DNA Fingerprinting

Since the early years of the 1900s, fingerprint evidence has served as one of the most common methods of crime-scene investigation and has been used to solve more crimes than any other procedure. Although there appears to be little doubt about the uniqueness of fingerprints (apart from identical twins), questions have been raised about the ability of examiners to accurately interpret them, particularly those unintentionally left at a crime scene.

An example of DNA fingerprinting, in which ten different individuals are tested for six loci (locations of a DNA sequence on a chromosome).

In 1984, Alec Jeffreys, a genetics professor at the University of Leicester, unexpectedly found similarities and differences between the DNA of a technician’s family members while examining Xray films of DNA. Three years later, his DNA fingerprinting methods were commercialized and are now not only used in criminal investigations but also for paternity determination, for identification of victims of catastrophic events (such as 9/11), to match organ donors, and to determine livestock pedigree. In 1992, DNA evidence was used to establish that Nazi doctor Josef Mengele was buried in Brazil under a fictitious name.

Forensic laboratories can use blood (the most accurate), semen, saliva, hair, or skin, since identical DNA is present in every cell in the body. Of the total DNA, 99.9 percent is identical among us and only 0.1 percent unique. There is an estimated one in 30 billion chance that two persons, who are not monozygotic twins, will have identical DNA fingerprints. DNA fingerprinting—also called genetic fingerprinting, DNA profiling, and DNA typing—is based on analyzing minisatellites, which are certain DNA sequences that do not contribute to gene function and are recurrently repeated within the gene. DNA is extracted from cell samples, purified, and placed on a gel subject to an electric current (electrophoresis).

United States courts have generally accepted the reliability of DNA analysis, and the results can be included as evidence. Questions have been raised about its accuracy, the cost of testing, and the ability of technicians to avoid sample contamination or to accurately analyze or interpret the findings. Ethical issues have been raised as to whether samples taken without consent violate US Constitutional protections of privacy; in 1985, the US Supreme Court decided it did not.