By David N. Leff

Science Editor

Animal rights advocates have yet to demand that mammals be allowed to present evidence in a court of law.

Yet something of that sort decided a case of homicide last year in bucolic Prince Edward Island, one of Canada's easternmost provinces.

The mammal in question was a large, white, short-haired cat named Snowball.

The murder victim was a 32-year-old woman who had disappeared from her home in Richmond, P.E.I., on Oct. 3, 1994. Her abandoned car was recovered soon after, and blood in it matched that of the victim.

Three weeks later, a team of soldiers on maneuver came upon a plastic bag in some woods about five miles from the victim's home. It contained a blood-spattered leather jacket and sneakers. The blood matched that of the victim by DNA testing. In the jacket's lining were 27 white hairs. The forensic laboratory in Ottawa identified them as coming from a domestic cat.

Then, on May 6, 1995, the woman's stabbed body was discovered in a shallow grave in a wooded area.

Police arrested and charged her estranged common-law husband, a man with a criminal record, recently released from prison. Presumably, his motive was jealous rage that his one-time consort had taken up with another live-in boyfriend. But there was no hard evidence.

The Royal Canadian Mounted Police learned that the National Institute of Health's National Cancer Institute (NCI), in Frederick, Md., was constructing a linkage map of the feline genome. This work was done at the NCI's Laboratory of Genomic Diversity, headed by molecular geneticist Stephen O'Brien.

As O'Brien recounted to BioWorld Today, "I was minding my own business one day in early 1995, when I took a phone call from a Constable Roger Savoy, an inspector for the Mounties in Prince Edward Island.

"The inspector told me that the suspect was living at home, and his parents had a big white cat named Snowball. He said to me: 'Dr. O'Brien, can you tell us whether or not the hair that came from the jacket, which was linked to the scene of the crime because of the blood, might have come from this cat, Snowball?' "

O'Brien answered Savoy: "I want you to get a subpoena and make a visit to the household of your suspect. Take along a veterinarian to draw some blood from the cat.'

"So he did that," O'Brien continued. "A few weeks later, Savoy showed up at Dulles Airport with two canisters wrapped in evidence tape. One contained the hairs from the jacket, the other, the blood from the suspect's cat."

Repeat Sequences Set Pattern For Forensic Identity

O'Brien and his laboratory staff cut the hairs up into roots vs. shafts, extracted DNA, and then looked for a category of gene markers called microsatellites, or short tandem dinucleotide repeats.

"These repeat sequences," O'Brien explained, "are useful in detecting gene linkage. They have a mutation rate that is about 1,000 times greater than traditional genes. That means that nearly everybody is different from everybody else, because although there's 100,000 of these genomic loci, or addresses, for the single dinucleotide repeats, each one has up to 20 different forms in the population.

"This means if you type one locus out of a single individual and compare him to a guy he's riding the bus with, they're likely to be very different at that locus. And if you start adding many loci, then the likelihood of a match is virtually zero. It becomes a very powerful tool for forensics.

"We had a bunch of these same kinds of markers from the cat," O'Brien said, "because we were building a feline map.

"We were able to amplify and resolve the genotype of 10 loci in the hairs. And when we went to the blood, we used exactly the same markers and achieved a perfect match. Seven of the 10 loci in the hair were heterozygous, which meant they had two different forms, and three were homozygous, only one form. That's a total of 17 different alleles, or variants, which matched perfectly the hairs in the jacket and Snowball."

But that neat concordance left O'Brien with one wide open question:

"What's the chance that we're wrong?" he said. "What's the chances that there's more than one cat with the same genotype as Snowball wandering around Prince Edward Island? It could be that there was a lot of inbreeding in the cats, and that they were all the same genetically.

"To get the frequency of the composite genotype in the population, we needed a data base. So I called up the Mounties and said, 'Gentlemen, you need to go out and collect us some cats.'

"So they collected 19 cats from the island, sent down blood samples, and we ran a population survey. Our results showed that there was an abundant amount of genetic diversity on the island. Plenty of allelic variation, and most of the alleles that we had seen in Snowball, 95 percent of them, were also seen in that population survey.

"We made the computation, and the frequency of the match came out to be on the order of about one in 45 million. That meant that the chance that the hair on the jacket would match a random cat was so small as to be nearly impossible."

O'Brien and his two associates went up to testify at the trial in P.E.I.

"At the end of the trial," he said, "the gentleman who was being charged was convicted of second-degree murder. I think most of the other evidence was rather circumstantial, and the DNA evidence was very persuasive on the part of the jury. That's what I was told.

"The precedent that's important," O'Brien observed, "is the introduction of animal genetic individualization into a forensic or a homicide case. Something like one household in three has a cat; one in four or five has a dog. And criminals live in households.

"So when they go to crime scenes," he concluded, "they can leave not only their own hairs but the hairs of pets. It won't always work, but in this particular case it did."

O'Brien described his forensic work-up on this case in today's issue of Nature, dated April 24, 1997, in a report titled: "Pet cat hair implicates murder suspect." *