Forensic scientists in the US are applying DNA fingerprinting methods to the cannabis plant. They say the technique, which is being used to create a database of DNA profiles of different marijuana plants, will help them to trace the source of any sample.
"It links everybody together: the user, the distributor, the grower," says the database's creator, Heather Miller Coyle of the Connecticut State Forensic Science Laboratory in Meriden. "That's the real intent of it, to show it's not just one guy with a little bag of marijuana, but it's a group of people."
A method for spotting the tiniest traces of marijuana, based on detecting DNA unique to cannabis chloroplasts, has already been developed in the UK (New Scientist print edition, 7 August 1999). But the profiling method, based on the same principles as DNA fingerprinting of people, can distinguish between closely related cannabis plants (Croatian Medical Journal, vol 44, p 315).
In a case awaiting trial in Connecticut, prosecutors plan to use cannabis DNA profiles to show that two apparently separate cannabis growing operations were actually linked. The two operations, in different parts of the state appeared separate until analysis of the plants revealed that some had identical DNA fingerprints, showing that the growers were sharing material. "From the investigative point of view that was phenomenal," says Timothy Palmbach, director of scientific services at the laboratory.
The big difference between human and plant DNA fingerprinting is that in people, each fingerprint is almost certainly unique to one person. So if a crime scene sample matches a person's profile, there is little doubt that it came from that individual.
In plants, by contrast, identical clones are easily created by taking cuttings, a method growers often use to perpetuate potent strains of dope. So showing two samples have matching DNA profiles does not by itself prove they come from the same grower, let alone the same plant. But Palmbach says that growers tend not to give away cuttings of their best plants, so linking samples in this way is an important lead for investigators and will still be useful in tracing samples.
"What growers have done to get more potent plants has played right into our hands," says Palmbach. And if several matching profiles are found in separate samples, the chances are high that they are somehow linked.
Coyle is establishing a database of DNA profiles from hundreds of marijuana samples seized in Connecticut. "We want to track how many varieties are out there, what the trends in distribution are, the probability that a plant can be related to another," says Palmbach. The database is being extended to include samples from all over the US and the rest of the world. "We invite anyone to send us samples," says Coyle.
Exactly how law enforcement agencies will apply the method remains to be seen. If a link can be established between a user and a grower or dealer, casual users might find themselves in deeper trouble than they bargained for. "If you're buying marijuana from somebody with terrorist ties, it could be traced back to that person," warns Gary Shutler of the Washington State Patrol's crime laboratory division.
On the other hand, he says, where medical uses of marijuana are legal, the technology could help characterise strains with the desired medicinal properties. Several US states have voted to legalise the medical use of cannabis, though these efforts are being fought by federal authorities.
The technology will not help police investigating the production or sale of highly processed or synthetic drugs such as cocaine and ecstasy. Nor does the team think it would work with hashish, which is made from resin exuded by cannabis plants, as not enough cellular material can be recovered. If the cannabis profiling technique does prove to be an effective tool in investigations and in the courtroom, dealers may switch to selling hashish.