Re: What is the accuracy rate for DNA testing to establish identity?

Hi Elliott,

Very good question.  When we testify about DNA, we have to answer 
questions that lawyers ask regarding a DNA match and then the judge 
ultimately decides whether to allow the testimony or not.  Most judges and 
courts have allowed experts to testify regarding a DNA match based on it's 
widespread acceptance in the scientific community.  This is probably the 
single most importnat factor.  The belief that DNA is the genetic material 
that passes on our traits to our children is not disputed.  Neither is the 
fact that no two people share the same DNA except identical twins.  But 
humans, for the most part, share common genes that make us human, 2 arms, 
2 legs, 1 head, etc.  Forensic scientists examine only a small portion of 
the entire DNA of a cell (the genome).  And this small portion of the 
genome is highly polymorphic (many forms)and the randomness of inheritance 
of each of these "pieces" makes a DNA profile of a person unique.

DNA is terrific to use in forensics because it is stable (for years if 
kept dry), highly polymorphic, the same in all the cells of our body, 
never changes, and can be obtained from the smallest amounts of material.
Defense attorneys like DNA because the results are conclusive in 
elimination cases; a scientist can say with absolute certainty that a 
stain did NOT come from someone.  In fact, the very first use of DNA in a 
criminal case in the world showed that the DNA could not have come from 
the person who confessed to the crime.  The police subsequently found out 
who committed the crime.

A DNA match is a little more difficult to explain.  At each genetic "site" 
a person inherits a factor from the mother and one from the father.  The 
occurrence of this "type" may be 1 in 10.  If the frequency of occurrence 
is 1 in 10 at the next site, then the chance of having this profile at 
both sites is 1 in 100, and so on.  We look at 13 loci and in my simple 
example, the frequency of occurence would be 1 in 10,000,000,000,000.  
This number is larger than the population of the earth and so one would 
think that this is identity.


But the accuracy of the test still depends on the scientists doing their 
job correctly.  Mistakes can happen but we take safeguards in all that we 
do to ensure 100% accuracy.

Some good information can be found at Scientific American :  http://www.sciam.com/askexpert/biology/biology3.html

This will lead you to some other useful sites for more information.  

I hope this helps and good luck in your project.

Dale L. Laux
Forensic Scientist