Dna Typing

DNA testing is to justice what the telescope is for the stars; not a lesson in biochemistry, not a display of the wonders of magnifying glass, but a way to see things as they really are.

(Barry Scheck and Peter Neufeld, Actual Innocence)

In the darkness of the early morning hours of 26 August 1999, a young University of Virginia student awoke to find a gun pointed at her head. The assailant forced her and a male friend spending the night to roll over on their stomachs. Terrorized, they obeyed their attacker. After robbing the man of some cash, the intruder put a pillow over the man's head and raped the female student. The female was blindfolded with her own shirt and led around the house while the intruder searched for other items to steal.

Throughout the entire ordeal, the intruder kept his gun to the back of the male student's head, daring him to look at him and telling him if he tried he would blow his head off. The assailant forced the young woman to take a shower in the hope that any evidence of the crime would be washed away. After helping himself to a can of beer, the attacker left before dawn taking with him the cash, the confidence, and the sense of safety of his victims. Even though the assailant had tried to be careful and clean up after the sexual assault, he had left behind enough of his personal body fluids to link him to this violent crime.

The police investigating the crime collected some saliva from the beer can. In addition, evidence technicians found some small traces of semen on the bed sheets that could not be seen with the naked eye. These samples were submitted to the Virginia Department of Forensic Sciences in Richmond along with control samples from other occupants of the residence where the crime occurred. The DNA profiles from the beer can and the bed sheets matched each other, but no suspect had been developed yet. Because of intense darkness and then the blindfold, the only description police had from the victims was that the suspect was black, medium height, and felt heavy set.

A suspect list was developed by the Charlottesville Police Department that contained over 40 individuals, some from the sex offender registry and some with extensive criminal histories who were stopped late at night in the area of the home invasion. Unfortunately, no further leads were available leaving the victims as well as other University of Virginia students and their parents suspicious and fearful. The police were at the end of their rope and considered asking many of the people on the suspect list to voluntarily donate blood samples for purposes of a DNA comparison. The top suspects were systematically eliminated by DNA evidence leaving the police frustrated.

Then on 5 October, six long weeks after the crime had been committed, the lead detective on the case, Lieutenant J.E. 'Chip' Harding of the Charlottesville Police Department, received a call that he describes as being 'one of the most exciting phone calls in my 22 years of law enforcement.' A match had been obtained from the crime scene samples to a convicted offender sample submitted to the Virginia DNA Database several years before. The DNA sample for Montaret D. Davis of Norfolk, Virginia was among 8000 samples added to the Virginia DNA Database at the beginning of October 1999. (Since 1989, a Virginia state law has required all felons and juveniles 14 and older convicted of serious crimes to provide blood samples for DNA testing.)

A quick check for the whereabouts of Mr. Davis found him in the Albemarle-Charlottesville Regional Jail. Ironically, because of a parole violation, he had been court ordered weeks before to report to jail on what turned out to be the same day as the rape. Amazingly enough he had turned himself in at 6 p.m. just 14 hours after committing the sexual assault! Unless he would have bragged about his crime, it is doubtful that Mr. Davis would ever have made it on the suspect list without the power of DNA testing and an expanding DNA database. At his jury trial in April 2000, Mr. Davis was found guilty of rape, forcible sodomy, and abduction among other charges and sentenced to a 90-year prison term.

DNA typing, since it was introduced in the mid-1980s, has revolutionized forensic science and the ability of law enforcement to match perpetrators with crime scenes. Thousands of cases have been closed and innocent suspects freed with guilty ones punished because of the power of a silent biological witness at the crime scene. This book will explore the science behind DNA typing and the biology, technology, and genetics that make DNA typing the most useful investigative tool to law enforcement since the development of fingerprinting over 100 years ago.


'DNA fingerprinting' or DNA typing (profiling) as it is now known, was first described in 1985 by an English geneticist named Alec Jeffreys. Dr. Jeffreys found that certain regions of DNA contained DNA sequences that were repeated over and over again next to each other. He also discovered that the number of repeated sections present in a sample could differ from individual to individual. By developing a technique to examine the length variation of these DNA repeat sequences, Dr. Jeffreys created the ability to perform human identity tests.

These DNA repeat regions became known as VNTRs, which stands for variable number of tandem repeats. The technique used by Dr. Jeffreys to examine the VNTRs was called restriction fragment length polymorphism (RFLP) because it involved the use of a restriction enzyme to cut the regions of DNA surrounding the VNTRs. This RFLP method was first used to help in an English immigration case and shortly thereafter to solve a double homicide case (see D.N.A. Box 1.1). Since that time, human identity testing using DNA typing methods has been widespread. The past 15 years have seen tremendous growth in the use of DNA evidence in crime scene investigations as well as paternity testing. Today over 150 public forensic laboratories and several dozen private paternity testing laboratories conduct hundreds of thousands of DNA tests annually in the United States. In addition, most countries in Europe and Asia

The first use of DNA testing in a forensic setting came in 1986. Two young girls, Lynda Mann and Dawn Ashworth, were sexually assaulted and then left brutally murdered in 1983 and 1986. Both murders occurred near the village of Narborough in Leicestershire, England with similar features leading the police to suspect that the same man had committed the crimes. A local man confessed to killing one of the girls and his blood was compared to semen recovered from the crime scenes. The man did not match evidence from either crime! Thus, the first use of DNA was to demonstrate innocence of someone who might otherwise have been convicted.

A mass screen to collect blood for DNA testing from all adult men in three local villages was conducted in a thorough search for the killer. Over 4000 men were tested without a match. About a year later a woman at a bar overheard someone bragging about how he had given a blood sample for a friend named Colin Pitchfork. The police interviewed Mr. Pitchfork, collected a blood sample from him, and found that his DNA profile matched semen from both murder scenes. He was subsequently convicted and sentenced to life in prison.

The story behind the first application of forensic DNA typing or genetic fingerprinting, as it was then called, has been well told in Joseph Wambaugh's The Blooding. The DNA typing methods used were Alec Jeffrey's multi-locus RFLP probes, which he first described in 1985. Since it was first used almost 20 years ago, DNA testing has progressed to become a sensitive and effective tool to aid in bringing the guilty to justice and in exonerating the innocent.


Joseph Wambaugh (1989) The Blooding. New York: Bantam Books;


First use of forensic DNA testing have forensic DNA programs. The number of laboratories around the world conducting DNA testing will continue to grow as the technique gains in popularity within the law enforcement community.

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