DNA from Fingerprints
By Julie Heinig, Ph.D.
March 25, 2007
DNA technology is increasingly used as one of the most effective forensic tools to exonerate or convict suspects in criminal cases. Advances in forensic DNA technology in recent years has allowed analysts to extract DNA from evidence without any visible stains. For example, it is possible to obtain a DNA profile from a baseball cap left at the scene of a crime and match it to a suspect who swore he was never there. Not only can we get DNA from a bloodstain on a knife and match it to a victim, but we can also swab the handle and get the DNA profile from the suspect who committed the crime. Due to good collection techniques and PCR (amplification of DNA) commercial technology, the likelihood of getting DNA from the inside of a glove or the handle of the weapon is much better now than it ever used to be.
The question often asked is, “Can we get DNA from a single fingerprint?”
To answer this question, consider this: when people touch things, they may leave behind DNA from cells sloughed off upon contact. How many cells are sloughed off depends on various factors, including how much they sweat. A number of scientific investigators have observed that DNA can be obtained from a wide array of fingerprints, but not all fingerprints give DNA profiles. The amount of DNA associated with a fingerprint will vary from person to person and can vary within the same person.
Low Copy Number DNA Profiling
STR DNA, which is used for human identification, is extracted from nucleated cells. The ability to obtain reliable DNA profiles from fingerprints depends on the number of nucleated cells collected from the fingerprints. The more cells there are in a stain or on a surface, the more DNA there will be. If there are very few cells available, it is referred to as "low copy number DNA profiling." In theory, all you need is one nucleated cell to obtain DNA. However, using PCR commercial technology, one nanogram of DNA is generally needed to obtain a DNA profile, which is equivalent to about 150 nucleated cells.
In the UK, forensic scientists have been performing DNA typing from fingerprints for several years. This type of analysis is not common in the U.S. and remains in the research and development stages. According to Dr. Lawrence Koblinsky, a professor of forensic science at John Jay College of Criminal Justice in New York, low copy number DNA profiling, used in the UK on DNA fingerprints, typically uses only 5 to 20 cells. Koblinsky observed that if ordinary PCR techniques are used with only 20 cells, a DNA profile will not be obtained. If the PCR procedure is modified, however, a DNA profile can be produced (Law Enforcement Technology, 2005). The UK uses a low copy number model, described by Gill et al. in “Forensic Science International”, 2000, 112, 17-40. Koblinksy stated that “touch DNA” will not be admissible in U.S. courts until it meets the acceptable standards for new technology in courtrooms. Standards for this type of analysis have to be set by the Scientific Working Group for DNA Analysis Methods (SWGDAM).
Different surfaces also can result in different yields of DNA. It has been reported by Dr. Robert Bever from Bode Technology Group that from their validation studies, in general, it was easier to obtain DNA from non-porous surfaces (plastic and glass) than from porous ones (i.e. paper).
Research at DDC
Scientists from DNA Diagnostics Center (DDC) have performed preliminary studies on the ability to obtain DNA profiles from fingerprints. Quite the opposite has been observed from that of the Bode group in terms of extracting DNA from paper. Instead of swabbing the fingerprint off the surface of the paper, DDC scientists cut the fingerprint section from the paper, which was used for DNA extraction. DDC has found that it is easier to obtain DNA from a cut section of paper rather than a swabbed section of paper.
Many factors affect the ability of obtaining DNA profile from fingerprints. The length of time the finger was held on the surface is an important factor. It was observed that if a person touched the surface for a minimum of 60 seconds, then DNA could be obtained. DDC also found that DNA can be obtained from chemically processed prints. It was found that black powder, fluorescent powder, magnetic powder, and cyanoacrylate (superglue) did not interfere with DNA typing.
The careful collection of the fingerprint from the evidence is very important. Other groups suggest that the collection of cells associated with a fingerprint should be done with a moist swab followed by a dry swab. DDC scientists were able to get partial DNA profiles from swabbing fingerprints on a glass mirror, a plastic lid, and a metal lid.
Obtaining DNA from a fingerprint is in the preliminary stages of investigation. However, as the techniques of collection, extraction, and amplification improve, obtaining DNA fingerprints will become more common. Fingerprint identification is certainly valuable and far cheaper than DNA analysis. However, if insufficient ridge detail is available to make a fingerprint match in a criminal investigation, DNA identification of that fingerprint may prove to be invaluable.