DNA and a fingernail clipping

[Kandy Williams]

Hey CrimeSpacers! I need some DNA info for a book I'm working on. When it comes to finding a fingernail clipping, how detailed can the DNA profile be? Can that kind of evidence reveal a person's gender, race and age? Anyone know what kind of limitations such evidence could have? Meaning, will it provide enough DNA to exclusively identify a person? Thanks in advance to anyone with some solid answers!!

[Lee Martin]

Hello,

My name is Lee Martin and I am a retired Forensic Crime Scene Investigator and the author of "Just Another Case". I saw you question on Crime Space.

In referernce to your question, the answer is yes, individual identification can be found from DNA testing of fingernail clippings. There was a recent case in 2007 that is an example of that. There was a murder in 1983 of a 23 year old girl, but at the time, there was not enough evidence to charge anyone, but through recent DNA advances, a suspect was found. The case was in Augusta Maine. It was the Judith Flagg Case. The Maine State Police Crime Lab scientist was Catherine Macmillan and testified in court to her findings. The defendant/suspect was Thomas Mitchell, Jr.

Hope that helps ya
Lee Martin

[Bill Cameron]

Standard DNA testing uses 13 specific gene markers and builds up a profile based on the number of repeating sequences at each location. DNA profiling can determine gender, but race or other distinguishing characteristics cannot be determined using the markers in a standard DNA profile.

At each marker, there are a certain number of possible allele frequencies. At any one marker, a significant part of the population may share a certain frequencies, but as you add markers to the profile, the statistical likelihood of individuals sharing the exact same frequencies goes down. With a profile of 13 markers, the statistic uniqueness approaches 1-in-trillions, though research continues to determine just how unique individual profiles may be.

DNA is great at ruling out suspects, of course. If the sample in evidence has a value at marker one of 13, and your marker one is something else, it's not you. There is some debate, owing to the statistical nature of the profile, as to how sure you can be that you've found the right person when all the markers match. In theory, even though the odds are very low, two different people CAN have the same profile.

I'm probably not explaining this as well as it can be. Try Lee Lofland's POLICE PROCEDURE & INVESTIGATION for excellent DNA info, as well FORENSIC FOR DUMMIES by Doug Lyle. The Wikipedia entry on CODIS is thorough.

[C. M. Albrecht]

Not only all the details you suggest, but they can tell whether the person was neat, clean, a smoker and maybe even if he was the nervous type that bites his nails. A doctor may even be able to tell you something about the person's general health.

[Lee Lofland]

Couldn't have said it better myself, Bill.