DNA Fingerprinting Crime
In the last few years, DNA evidence has started to play a big part in many nations' criminal justice systems. It has been used to prove that suspects were involved in crimes and to free people who were wrongly convicted. What Is DNA Fingerprinting? One of a Kind DNA
Everyone, except identical twins, has a one-of-a-kind DNA genome. The DNA molecule a nucleic acid, is made of monomers called nucleotides, and the specific order in which nucleotides occur in a DNA molecule represents each individual's unique genetic code
old system
the blood of individuals has been known for many years now. Karl Landsteiner discovered the first blood group system--the ABO system--about 100 years ago, and over the next 60 years more than 30 different blood group systems were discovered. The ability to distinguish between individuals is quite limited for each blood group--for example in most American and European populations between 40 percent and 50 percent have the same type O blood.
An additional problem with information from blood groups was that blood samples had to be in good condition, and available in reasonable amounts. Suitable samples are rarely if ever available in criminal cases.
The key to DNA evidence lies in comparing the DNA from the scene of a crime with a suspect's DNA. To do this, investigators have to do three things:
Collect DNA at the crime scene and from the suspect (see How CSI Works)
Analyze the DNA to create a DNA profile
Compare the profiles to each other
Authorities can extract DNA from almost any tissue, including hair, fingernails, bones, teeth and bodily fluids. Sometimes, investigators have DNA evidence but no suspects. In that case, law enforcement officials can compare crime scene DNA to profiles stored in a database
The samples of DNA, or deoxyribonucleic acid, can be taken from all kinds of biological material including blood, semen, hair, skin and saliva.
The technique often works even if the samples are months or years old.
The amount of biological samples available, whether it be blood, semen or hair, may often be old and in poor condition. Fortunately DNA is quite stable and resistant to degradation. More seriously, the amount of sample available may be very limited. However, with the latest procedures available sufficient DNA may be obtained from just a single hair follicle.
When DNA samples are taken from a suspect and crime scene, they are shipped to a lab and each exposed to the same array of restriction enzymes
In crime cases, it may frequently be the situation that no suspect has been identified. In such circumstances, an increasing number of states can resort to a DNA database of individuals who have previously been convicted of a violent crime. By 1998, around 200 "matches" had been made using such databases.
Most would agree that false positive errors are unacceptable, while false negative errors, while unfortunate, are acceptable providing they occur seldom
coclusion
Genetic fingerprinting is widely considered to have great potential in crime detection, possibly the most significant advance since the discovery of fingerprinting in the late 19th century. ''It is an enormous breakthrough in the field of forensic science,'' said John Hayward, an independent consultant, who worked for 18 years as a forensic scientist for the British Government.
acuurcy
while ordinary blood samples can rule out a certain suspect if his type does not match that at the scene, genetic fingerprinting gives a positive identification. Except for identicial twins, the odds against two people having the same genetic pattern are measured in terms of millions, or even billions, to one.
''The identifying patterns are absolutely specific for each individual,'' said Dr. Alec Jeffries
senstivty
The increased sensitivity of PCR-based typing methods using
fluorescence-based detection methods of amplified STR fragments has increasingly changed this
focus to include all types of contact stains from objects which might have been touched by hands,
face or even the ear of a perpetrator, e.g. an earprint from a door where someone might have listened.
Typically, the area of the presumed skin contact is swabbed using a sterile dry or moistened cotton
swab, which is subsequently dried and sent to the laboratory
Volume 48, Issue 5 (September 2003)
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