Although the overwhelming majority of the human genome is identical for everyone, it’s been found out that you can tell people apart and whether they are related using their DNA.
Scientists have been looking at regions of the human genome called STRs, which stands for Short Tandem Repeats. These regions can be thought of as repeated strings of genetic letters or bases, a bit like the same word written many times along a line.
You inherit these sequences, along with the rest of your DNA, from your parents. But because the STRs mostly occur in parts of the genome called “Non coding regions”, regions that are not actually used to encode proteins, genetic mutations and duplications are most likely to occur. This means that a person will have a more similar number of repeats, but not the same, to a close relative and will probably have a totally different set of repeats compared to a complete stranger.
These differences can be exploited to genetically distinguish (RECOGNIZE) one person from another using a technique called DNA fingerprinting (also called DNA profiling or genetic fingerprinting), used by forensic scientist in the identification of individuals, given their DNA. It’s used principally for parental testing and criminal investigations. A typical instance of DNA fingerprinting is carried out like this:
The DNA is extracted from a sample, such as a mouth swab or a blood sample, and then purified. In order to isolate the Short Tandem Repeats, a procedure called Polymerase Chain Reaction (PCR) is carried out on the DNA. What PCR does is to copy the targeted part of the DNA (in this case the STRs) and amplify it as many times as necessary, increasing their number in the sample.
The different lengths of STRs are then separated used a process called Electrophoresis. It involves adding the DNA to an agarose gel and applying an electric current through it. Because DNA is negatively charged, it will move through the gel toward the positive electrons. The larger and longer STRs are “heavier”, so they will move through the gel more slowly than shorter ones. So, by running the process for a certain amount of time, the STRs will spread out through the gel. After that, they can be revealed using a dye to label them up, producing a line on the gel, also called band. The pattern of these bands corresponds to the person's DNA fingerprint.