MadSci Network: Genetics

Re: What genetic techniques are involved in DNA fingerprinting?

Area: Genetics
Posted By: Sharon Shriver, Instructor (faculty; Ph.D.), University of Pittsburgh
Date: Tue Jul 29 12:07:43 1997
Area of science: Genetics
ID: 869020931.Ge
	Biochemical and molecular analysis of evidence in criminal cases has 
been around for a long time, and new techniques are being developed at a 
fast rate.  Some of the earliest tests used biochemical methods to 
determine whether the suspect’s blood type (A, B, AB, or O) matched samples 
from the crime scene.  Recently, even these tests have been converted into 
a “genetic” system where the blood type can be determined by analyzing DNA 
from evidence such as bloodstains, vaginal swabs, cigarette butts, hair 
roots, and even 10-year old stamps.  DNA typing, as this is known, can 
generate results within hours and can provide information from very small 
specimens and even from samples with partially degraded DNA.
	Because there are only a limited number of blood types, and therefore 
many indivividuals who will have the same blood type as an evidentiary 
sample, this analysis is only useful as a first-step screening or for cases 
where there are a limited number of suspects (such as paternity testing).  
Other molecular techniques are much more useful and take advantage of the 
DNA variation which occurs at thousands of sites in the human genome and 
which defines an absolute uniqueness for each individual.  In most cases, 
this variation is detected in VNTR (variable number of tandem repeat) 
regions in the human genome.  In each of these regions a particular short 
(2 to 30 base) DNA sequence is repeated many times.  In the past, these 
regions were characterized, or typed, using RFLP (restriction fragment 
length polymorphism) analysis.  In this method, DNA from evidence is 
isolated and treated with enzymes which cut the DNA at a few specific 
sites.  The resulting DNA fragments are separated by electrophoresis, which 
allows them to be visualized.  Differences in the length of a VNTR located 
between two enzyme sites results in longer or shorter fragments, and this 
DNA “profile” or “fingerprint” can then be compared to the suspect’s DNA 
fingerprint.  This technique, known as Southern blotting, is pretty labor-
intensive and time-consuming, and therefore not of much use if there are 
many suspects. It also requires fairly large amounts of DNA, which are 
often not available.
	Most of the recently developed techniques utilize the polymerase chain 
reaction, or PCR.  In this method an enzyme is used to “amplify” regions of 
DNA by making many copies, which can then be detected by electrophoresis.  
DNA fingerprinting using PCR is more sensitive and can generate data from 
tiny, degraded samples (such as in the ABO genotyping described in the 
first paragraph); it can resolve samples which are mixtures of evidence 
from several individuals; and it is very specific, i.e. it has a high power 
to discriminate among individuals.  This method also utilizes the variation 
present in regions of repeated DNA.  PCR is used to amplify regions 
containing VNTR loci, and the length of the resulting “PCR products”, which 
vary depending on the number of repeats, are compared.  By looking at a 
large number of VNTRs, the likelihood that an individual will match a crime 
sample at all the loci can be a millionfold higher than other techniques.  
This technique is very sensitive, and recently forensic scientists were 
able to generate genetic profiles from DNA isolated from actual 
fingerprints (fingerprint fingerprinting!) as well as items that had been 
handled briefly (such as telephones) and hands swabbed after a handshake.
	There are some drawbacks to such a sensitive test, related to possible 
technical errors.  Because the sample is amplified by PCR, it is possible 
that it is not the sample that gets amplified, but contamination.  Thus the 
technician may accidentally contaminate the evidence with DNA from the 
suspect, or both the evidence and the suspect samples might be contaminated 
with the technician’s DNA.  Extensive precautions are always taken to avoid 
contamination, and experimental controls are included to identify false 
results.  Ideally, different samples are handled by different technicians 
in different labs, providing independent confirmation of the results.  

	Some references that might prove useful are:

van Oorschot, R.A.H., and Jones, M.K. (1997) DNA fingerprints from 
fingerprints. Nature 387:767

Hochmeister, M.N. (1995) DNA technology in forensic applications. Molecular 
Aspects of Medicine 16:315

Pena, S.D., Prado, V.F., and Epplen, J.T. (1995) DNA diagnosis of human 
genetic individuality. Journal of Molecular Medicine 73:555.

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