Animal CSI: Cat and Dog DNA Helps Solve CrimesPublished December 15, 2008
In 1998, police investigating the killing of a Seattle couple and their dog had only minimal evidence against their main suspects until an investigator struck on the idea of contacting Dr. Joy Halverson, a veterinary geneticist who had done extensive work using DNA to verify pedigrees. Halverson, now of QuestGen Forensics in Davis, California, tested the dog's blood against blood found on the suspects' clothing and testified that it matched, helping to convict both suspects. It marked the first time dog DNA was admitted as evidence in the United States. But it's been far from the last. When seven-year-old Danielle van Dam was abducted from her home and murdered, police suspected her neighbor, David Westerfield. Upon searching his house and motor home, they found several dog hairs. Westerfield didn't own a dog, but the girl's family did. By comparing the DNA of the dog hair to that of her family dog, forensic investigators determined it was likely the hairs matched. This evidence helped convict David Westerfield. A dismembered body of a woman was found in several trash bags in a lake. Dog hair found on the tape used to close the bags yielded DNA that matched her ex-husband's dogs. He was convicted. A rape victim noticed her dog urinating on a hubcap of her attacker's pickup during the attack. Police swabbed the tire of their suspect and found a perfect match -- despite the suspect's claim that he had never been near the victim's home or dog. Three construction workers were found murdered in a home. The suspect's shoes had dog droppings on the sole with DNA that matched droppings found in the walkway of the home where the murders occurred. The suspect, who had denied being at the home, was convicted. A man accused of stabbing his dog denied the bloody knife found in his home was involved, but DNA from the blood matched that of the dead dog. DNA scraped from the road surface was used to convict a person accused of dragging a dog behind a car. Two dogs ran off after attacking two people, killing one woman. Police officers killed one dog, which was positively identified as an attacker because it had tissue from one victim in its stomach. But which of the many other loose dogs later trapped by the police was involved? DNA collected from the victims' clothing came from only one of the suspect dogs, the one that also matched the victim's description. A young boy was attacked by one of three Great Danes. Because nobody knew which dog did it, authorities were considering destroying all three. DNA from saliva on the boy's torn clothes matched that of one of the dogs, confirming the other two were not involved and in turn, spared their lives. A dog suspected of attacking a miniature horse ran off. Swabs from the rim of the dog's drinking bowl yielded DNA from the horse, probably from blood that was carried home in and around the dog's mouth. Puppies matching the description of stolen puppies were recovered, and their DNA compared to their parents. The DNA matched, and the dognapper confessed. Blood, hair, saliva, urine, feces and tissue have all been used to recover DNA as evidence in trials in which dogs were witness, victim, suspect or just stay-at-home pet. Cat DNA has also played prominent roles in similar cases. The take-home lesson: If you're planning to commit a crime, you're best off avoiding pet owners, or being a pet owner, because as any pet owner knows, wherever you go, pet hair -- and DNA -- follows. Criminal DNA Codes QuestGen Forensics is a leader in non-human DNA forensics, relying on two types of DNA analyses. The first uses Short Tandem Repeat (STR) markers, which is the same technique adopted by the AKC and other registries for parentage verification. A complete STR is virtually unique for each dog, discriminating even between closely related dogs. Using the standard 10 STR markers, the probability of the sample matching a random dog in the population is greater than one in one million. The problem with STR analysis is that it requires DNA from cell nuclei, most often from blood, saliva and hair roots. With only one copy per cell, it's hard to get an adequate sample in usable condition. In fact, because most shed dog hair does not contain the hair root, STR analysis may not be possible with even mounds of hair. That's when the second type of DNA analysis comes into play. Mitochondrial DNA (mtDNA) can be extracted from the hair shaft, as well as from degraded blood and tissue samples. Unfortunately, mtDNA does not provide the unique identity that STR analysis does because mtDNA is inherited only from the mother. Although mtDNA matches are not as statistically compelling as STR matches, they can be significant and can also definitely exclude a dog as a sample donor.
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