Daily Archives: June 14, 2009

Animal Hair, DNA, and Snowball the Cat

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.

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Posted by on June 14, 2009 in Uncategorized

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