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Animal Hair, DNA, and Snowball the Cat

14 Jun

Earlier today I responded to a question posted on DorothyL about whether the breed of a dog could be determined from the dog’s hair. The answer is yes. More than that, if a follicle is attached or if the dog’s saliva is on the hair from his bathing, then it possible to extract the dog’s DNA. If a suspect dog is located, then DNA taken from the dog can be compared to that from the hair. If it matches, then the hair came not just from a dog of that breed but from this particular dog.

There is a famous forensic case known as Snowball the Cat, where this exact scenario helped convict a killer. I won’t go into the details here but check out this link to this very cool and milestone case:

Snowball The Cat

This case highlights the difference between CLASS evidence and INDIVIDUALIZING evidence. The finding of a dog hair of a certain breed or a white cat hair at a crime scene would serve to eliminate all dogs of a different breed or all cats of a different color as being the origin of the hair. That is, all dogs of the same breed or all white cats would be part of the CLASS that could have left the hair at a crime scene, but dogs of a different breed and cats of the different color are excluded. However, if DNA is obtained from the crime scene hair, either from an attached follicle or from saliva residue, and this is matched to a particular dog or cat, then that hair came from that animal to the exclusion of all others in that class. This would be INDIVIDUALIZING evidence in that it was not just any white cat but THIS white cat that left the hair.

This also brings up another interesting use of forensic evidence. It not only identifies but also LINKS a person to another person, place, or object. Let’s say that a woman is killed in her home and she has a cat very similar to Snowball. Let’s further say that a suspect is identified and his clothing has several white cat hairs on it. Microscopic examination could show that the physical characteristics of the hair on the suspect and that of the cat were consistent and therefore the victim’s cat could have been the origin of that hair (CLASS EVIDENCE). But if the DNA matched, the white hair on the suspect came from the victim’s cat and from no other (INDIVIDUALIZING EVIDENCE). This would link the suspect to the victim’s cat. Still doesn’t make him guilty of murder.

If the suspect said that he did not know the victim, had never been to the victim’s house, had never been near the victim’s cat, and had absolutely no connection to the victim whatsoever, then he would have some explaining to do. How did hair from the victim’s cat get on his clothing? He would be LINKED to the crime scene, the victim, her cat, or all three.

But what if he knew the victim, had been in the victim’s home, or had handled the victim’s cat? Then he would have a legitimate reason for the hair to be on his clothing. Anyone who owns a cat knows that cat hair will grab anything and go anywhere with you.

Let’s take it a step further. The suspect has no connection to the victim and no logical reason for the hair to be on his clothing but:

He dates the victim’s sister, or next door neighbor who feeds and pets the cat sometimes.

A friend of his dated the victim or her sister or neighbor any one of which had been in the house and handled the cat.

He works at a car wash and cleaned the interior of the victim’s car.

He uses the same vet or dates the victim’s vet.

See where I’m going? The hair could have been transferred to him indirectly in any of these ways. Many other ways too. This is the stuff that makes good stories and keeps prosecutors and defense attorneys up late.

 
6 Comments

Posted by on June 14, 2009 in Uncategorized

 

6 responses to “Animal Hair, DNA, and Snowball the Cat

  1. Tanya

    June 15, 2009 at 8:05 am

    Dear Doug,
    Funny that your blog mentions cat DNA. In my romantic suspense completed manuscript, one of my murder suspects unknowingly leaves a strand of cat hair behind at the crime scene (found by CSU’s “combing of a bloody mattress). That and other evidence placing him at the crime scene does him in. I got the idea by the article “Feline Forensics” by Judy Holmes. According to her article, a map of the cat genome is being studied at National Cancer Institute’s Laboratory of Genomic Diversity in Frederick, Maryland. The article goes on to describe cat and human having commonality in gene order and organization (cats 19 pairs chromosomes, humans 23). Anyhow, back to my story,when detectives go to the suspects home, they know about the cat DNA which had been sent to Maryland, they see a Siamese cat in his window. Ah ha! More evidence.

    Tanya

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  2. Jonathan E. Quist

    June 15, 2009 at 9:31 am

    Okay, this is a stretch – but if the found hair belongs to an animal which had social grooming contact with another (be it casual tail-sniffing on a walk, or an established relationship), then it could possibly be matched to the wrong animal, right?

    Turning this into a plausible plot device is left as an exercise for the reader…

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    • D.P. Lyle, MD

      June 15, 2009 at 9:46 am

      Not sure I understand what you’re saying. Do you mean that the victim’s cat picked up a hair from another cat and that it was this hair that was deposited on the suspect’s clothing and found by the police? If so, there would then be no connection to the victim’s cat and the hair would simply be random evidence that had no linkage to the crime. And since cat hair is everywhere and fairly commonly found on clothing, the police wouldn’t really be interested in the hair if it didn’t connect to the crime scene. I doubt they would canvas the neighborhood to see which cat the hair belonged to so it would probably never be connected to the cat that it came from. If it didn’t belong to the victim’s cat it would not be considered evidence and they wouldn’t likely spend the time and money to track it down. But if they did, the hair would suggest that the suspect had at some time been in the neighborhood of the crime or at least had some contact with someone who had been in the area and he would need to explain that.

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  3. Cathy (Lee) Carper

    June 15, 2009 at 1:41 pm

    D.P.,

    I’ll be looking for your post on DL. I get my DL posts in digest form so sometimes it takes awhile before I can read responses. I responded to that gal’s question too (regarding the dog hair) – I didn’t get into the scientific specifics, but the show I watched on it recently, did. Was quite fascinating. I knew of course about the DNA part, but was unaware that you could distinguish between breeds based on the hair without necessarily using DNA.

    By the way, I’m enjoying your blog. I’m thrilled I finally feel like I have an expert in which I can ask specific forensic questions – have not found the medical examiner’s office here in Vermont to be helpful in the slightest (can’t even get them to return a call). I realize they’re busy, but gosh, you’d think they could at least try to be somewhat helpful!
    Cathy

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