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Genetics in forensics
Fingerprints are the unique identification of an individual but physical fingerprinting is fraught with many errors due to the subjective nature of matching. Fingerprints do lie sometimes and here is a story about an able detective officer who had to be treated for depression after being accused of leaving a fingerprint at the scene of a murder and lying about it. Police detective Shirley McKie from Scotland denied ever being in the house and later went on trial for perjury because she stated this under oath. It turned out the fingerprint was not hers, experts had made a big mistake. Nevertheless it took a heavy toll on her life due to the mental agony and depression she had to go through as a result of this incident.
DNA fingerprinting is much more reliable and is based on identifying individuals by scanning 13 DNA regions known as loci, and the chances of one pattern matching another individual are extremely rare.
This fact can be used to build the DNA profile of a person by using their hair, blood and body tissues. The DNA profile of such items found from the crime scene can be matched against a pre-built database of citizens or suspected criminals in order to try and find the perpetrator of the crime.
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How does it Work?
The basic principle behind genetic forensics is quite simple - it matches the DNA samples found at the crime scene with the DNA sample of the suspect. So whenever a crime has been committed, DNA samples are collected from blood, hair, skin and any other biological material which has been left at the scene unknowingly. These can then be matched against a DNA database of known persons, or if someone is arrested on suspicion, this matching can be used to determine whether he or she was at the scene of the crime.
The DNA from the crime scene is matched with that of the suspect to look out for specific DNA markers. Small DNA probes are created from one sample for this purpose and they bind to the complimentary DNA sequence in the other sample. Some of the techniques of sample comparison are as follows;
- PCR Analysis - it stands for polymerase chain reaction and the process consists of making several thousands of copies of the sample DNA, thereby allowing even highly degraded samples of DNA to be analyzed effectively.
- Y-Chromosome Analysis - the Y-chromosome is the male sex chromosome and hence its analysis can help to identify male relationships where multiple males are suspected of a crime.
- STR Analysis - it stands for short tandem repeat analysis and it is used to identify loci within a DNA. The American investigation agency FBI uses 13 loci to identify DNA samples and the chances of them being same for two individuals are extremely remote.
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It's not fool-proof
DNA fingerprinting does have its own problems - laboratory errors and sample contaminations have been known and there have been problems in some courtrooms with juries and legal teams not understanding what is meant by probability matching. That is the chance that someone else with a similar DNA profile could've carried out the crime that the accused is on trial for.
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