What is the science behind DNA tests?

The whole science behind it involves a process called agarose gel electrophoresis, which involves DNA being put on a buffer. First, a comb is placed on the left side of a box, on the negative side. Then, buffer is placed into the box until it cools and becomes solid, similar to frozen gel. Once cool, the agarose is poured onto the buffer so there is just a slight amount of agarose above the level of buffer. The comb is then taken out, leaving holes where the comb was. Then, the DNA is put into the holes made by the comb.

There is an electric power supply that is attached to the box that contains buffer which the DNA was put upon, and when switched on, the electric power supply puts a charge on the agarose, making the DNA go all the way to the right (towards the positive side, since DNA is negatively charged). This will separate the DNA into RFLP's (Restriction Fragment Length Polymorphism). The RFLP's are not all the same length- the bigger RFLP's will move slower, and thus not move too far from the starting point, which was on the negative side on the left. The smaller RFLP's will move fast and far, spreading out the RFLP's by size. After about 2 hours, the electric power supply is turned off, leaving the RFLP's spread out by size.

If you compare one human's DNA to any other human's DNA, there will be little difference. This is the same for every human. However, there is a slight difference in everyone's DNA. When the RFLP's are created after agarose gel electrophoresis, there will be some RFLP's that are different from others. When agarose gel electrophoresis is done for the DNA specimin found, the entire section of RFLP's should match up. If the RFLP's don't match up, than the person was not the culprit.