‘Fingerprints’ in DNA Hold Identity Clues

British scientists, using genetic engineering techniques, have found a way to produce DNA “fingerprints” of human blood, hair and semen that appear to be unique for every individual.

The breakthrough technique could have broad application if it can be reproduced by other scientists and refined into a routine procedure.

It could allow, for instance, police investigators to match blood, hair or semen samples obtained at crime sites to specimens obtained from a suspect.

Unlike existing techniques, which can only exonerate a criminal suspect, the use of DNA “fingerprints” has the potential to identify a perpetrator with virtual certainty.

Potentially, the new method also could be used to help establish paternity or maternity in legal disputes, as well as to trace the path of hereditary diseases.

The development of the technique was described by geneticist Alec Jeffreys and his colleagues at the University of Leicester in a series of papers this year in the British journal Nature.

Jeffreys found that DNA--deoxyribonucleic acid, which serves as a blueprint for all living organisms--from humans contains many segments, or regions, that vary in composition and size from individual to individual. Jeffreys calls these segments mini-satellites because they are small and typically are located next to specific genes.

He and his colleagues then developed DNA probes, short strings of synthetic DNA, that can bind to the mini-satellite regions and improve separation of the mini-satellites by a technique called electrophoresis.

That separation produces a “fingerprint” composed of individual bands representing each mini-satellite and probe. A typical probe might produce about 15 bands.

Diminishing Percentage

If DNA from two individuals is compared, the chances are about 20% that any given band will appear in both DNA fingerprints. The chances that two given bands will be present in both samples is 4%. And the chances that all 15 will be present in both are virtually nil: about three in 100 billion--or somewhat greater than the chance that conventional fingerprints from two different individuals will be identical.

Those odds can be made even smaller by using more than one probe. The DNA “fingerprints” can thus be used to show definitively that two samples came from the same individual.

In the most recent paper in Nature, published last week, Jeffreys and Peter Gill and David Werrett of the Home Office Forensic Science Service in Aldermaston, England, reported that the DNA “fingerprinting” technique could be used with blood samples as old as 4 years and with semen samples several weeks old.

DNA “fingerprinting,” they wrote, has several advantages over conventional techniques, which rely on the identification of specific proteins and enzymes in blood and seminal fluid.

Large Sample Needed

The conventional techniques require that a number of different tests be run on an individual sample in order to obtain high accuracy. This requires relatively large samples and is time-consuming. DNA “fingerprinting” can be done in one procedure and on a very small sample.

Conventional tests on semen from rape victims, furthermore, can be compromised by proteins from the woman’s vaginal fluids or by bacterial contamination. Neither problem occurs in DNA “fingerprinting.”

“It is now possible for the forensic scientist to be positive about (the source of a specimen), whereas in the past, it was only possible to be sure of negative associations,” the British researchers wrote.

The DNA “fingerprinting” technique needs to undergo further study before it can be used with confidence. And even if the technique is validated, privacy considerations may limit its use in court.

Potential Demonstrated

The potential of DNA “fingerprinting” was illustrated in an October article in Nature by Jeffreys. In that report, he described the case of a Ghanaian boy, born in England, who emigrated to Ghana to join his father but subsequently attempted to return to England to rejoin his mother, brother and two sisters.

British immigration officials refused a residence permit for the boy, however, fearing that he was either unrelated to the mother or was the son of one of the mother’s sisters.

Conventional blood-typing techniques showed that the mother and the boy were probably related, but could not determine whether the woman was the boy’s mother or aunt.

The case was further complicated because the mother was not sure if her husband was the boy’s biological father.

Using the DNA “fingerprinting” technique, Jeffreys was able to show equivocally not only that the woman was the boy’s mother, but also that the husband was the father.

The boy was subsequently granted residence.