Re: Why is DNA profiling/'fingerprinting' used to find evidence to exclude ?

Hello Amie,

Basically, it comes down to what you consider a match or an inclusion, and 
this question involves math and statistics.  In the case of matching an 
individual to a source of blood at a crime scene, forensic scientists 
obtain a DNA profile from the bloodstain and the suspect's blood standard 
and compare them.  If they are different, he or she is excluded as a 
source of the stain.  If they match, then the question is, how often does 
this profile occur in the general population.  Studies have been conducted 
on DNA profiles obtained from hundreds of people from different population 
groups to obtain frequency charts.  A good discussion of this material can 
be found at:  http://www.fbi.gov/programs/lab/fsc/backissu/july2000/source.htm

Your fictional case is a little different than the forensic case described 
above.  Paternity testing ivolves analysis of the gentic relations of 
child, mother, and putative father.  Mutation is a factor to be taken into 
account that is not an issue in forensic testing.  According to the 
"Standards for Parentage Testing Laboratories", "an opinion of non-
paternity shall ordinarily not be rendered on tha basis of a single 
indirect exclusion or on the basis of an exclusion at a single DNA 
locus".  DNA loci used for comparison are chosen because they are highly 
polymorphic; however, they are highly polymorphic because they are 
mutable.  In cases brought to establish paternity for child support, 
inheritance, custody, and other purposes, the law gives the claims of the 
parties roughly equal weight and uses a civil, rather than the criminal, 
standard of proof.

In paternity cases, DNA profiles of the mother, child and putative father 
are obtained and compared.  If the putative father is not excluded, a 
"paternity index" is calculated which is a likelihood ratio - the 
probability of the mother-child-father profile combination if the putative 
father is the true father divided by the probability of this combination 
if a randomly selected man is the father.  

In your fictional case, if one of the three suspected men truly was the 
father, two of the three men would immediately be cleared using DNA 
typing.  The other man's DNA would include him as being the father.  
Typical results of paternity typing are on the order of 0.9901 to 0.9999.  
Depending on the DNA expert witness, they may testify that this individual 
is the father to the exclusion of all other individuals.

I hope that this information helps and good luck with your project.

Dale L. Laux
Forensic Scientist