DNA profiling is a forensic technique used to identify people by the characteristics found in their DNA. It is used to determine if a suspect is guilty in many crimes today. A DNA profile is a small sequence of DNA variations that are different between each individual person. Even though 99.9% of all human DNA is the same, there is still enough differences to distinguish an individual from another (unless they are identical twins). Common ways to sample human DNa is through a buccal swab, collections of blood, saliva, semen, hair of stored medical samples (such as biopsy tissue). Samples from blood relatives can also sometimes be used. The sample can be analyzed through various techniques such as:
RFLP analysis
DNA from these samples is found in the cells. Scientists will cut these cells into small pieces using a restriction enzyme. This generates many DNA fragments of many different sizes as a consequence used to determine the variations in between individuals and then separated by size using gel electrophoresis. The separate fragments are then transferred to a nitrocellulose, a procedure called "southern bolt". The repeat sequences of the fragments very in length in each individual person and then used to match a suspect to the fragments found on the crime scene.
PCR analysis
This is a process in which specific portions of the DNA samples can be amplified. The process mimics the natural biological process of DNA replication but confines it to the specific sequence of interest. This process has the ability to recover information from samples that are too small or degraded to be tested themselves. This process is more complicated than RFLP but very effective in most circumstances.
STR analysis
This method uses highly polymorphic that have short repeated DNA sequences. This method works well to distinguish in between unrelated individuals because unrelated individuals have different numbers of repeat units. Usually, each STR allele will be shared by 5 - 20% of individuals. The power of STR analysis comes from looking at multiple STR loci (locations on a chromosome) simultaneously. The pattern of alleles can identify almost all individuals very accurately. The more STR regions tested on an individual, the higher the accuracy of identifying the individual.
Other DNA facts to consider when profiling:
Genetic relationship: Although almost everyone has a distinct, unique set of genes, extremely rare cases of individuals known as "chimeras" do occur. These individuals have two completely different sets of genes. This happens when two eggs are fertilized at the same time and fuse together, creating one individual instead of twins. There have even been two separate cases of DNA profiling that falsely accused a mother of being unrelated to her children due to having double the genes.
Fake DNA evidence: Although very rare and difficult to execute, "fake" DNA samples can be made using molecular biological techniques. Some criminals have tried to use this method to help prove themselves as innocent as well as taking blood or other bodily fluid from other people and using them for DNA profiling. In 1992 John Schneeberger was accused for rape after leaving traces of semen in his victim's underwear. When police drew (what they thought was) John's blood to be compared against the DNA evidence of the crime scene, they found they did not match. It turns out that Schneeberger had actually surgical placed a Penrose drain into his arm filled with anticoagulants and foreign blood!
Genetic relationship: Although almost everyone has a distinct, unique set of genes, extremely rare cases of individuals known as "chimeras" do occur. These individuals have two completely different sets of genes. This happens when two eggs are fertilized at the same time and fuse together, creating one individual instead of twins. There have even been two separate cases of DNA profiling that falsely accused a mother of being unrelated to her children due to having double the genes.
Fake DNA evidence: Although very rare and difficult to execute, "fake" DNA samples can be made using molecular biological techniques. Some criminals have tried to use this method to help prove themselves as innocent as well as taking blood or other bodily fluid from other people and using them for DNA profiling. In 1992 John Schneeberger was accused for rape after leaving traces of semen in his victim's underwear. When police drew (what they thought was) John's blood to be compared against the DNA evidence of the crime scene, they found they did not match. It turns out that Schneeberger had actually surgical placed a Penrose drain into his arm filled with anticoagulants and foreign blood!
These are only a few of the options available in the amazing science of DNA profiling, to learn more (or for more detailed descriptions of the methods above) please click any of the links below:
http://biotechlearn.org.nz/focus_stories/forensics/dna_profiling
http://grow.cals.wisc.edu/health/how-dna-profiling-works
https://dna-explained.com/2012/10/01/4-kinds-of-dna-for-genetic-genealogy/
http://nij.gov/topics/forensics/evidence/dna/basics/pages/analyzing.aspx
https://en.wikipedia.org/wiki/DNA_profiling
These websites were also used as sources in the making of this blog
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