Early DNA paternity tests were conducted using the blood types of each parent and that of the child, but were in no way conclusive. If the father was Type A and the mother Type B, the only way the child might not be his, was if it was Type O. That left a huge margin for error or interpretation.

DNA paternity testing on the other hand, is almost 100% conclusive in its results. It cannot tell the difference between two brothers’ particular DNA, but it can definitely show from which family the DNA originated. As long as the two possible parents in question are from different families, DNA paternity testing will work conclusively.

For example, someone might claim to be a descendent of Thomas Jefferson. Conclusive testing back to his family can be done, but since Thomas Jefferson had at least one brother, the DNA would conclusively prove that someone from Thomas Jefferson’s family did indeed pass their DNA down through a different bloodline than that of the accepted family tree. Jefferson yes; Thomas, who knows?

DNA paternity testing works by isolating the DNA of both parents, and then comparing it with the child’s DNA. The comparison is similar to that of blood testing, but individual DNA is being examined, as opposed to broad blood types.

To isolate the DNA, a sample of saliva is swabbed from the mouth, and the DNA extracted using a restriction enzyme that cuts the DNA into identical and easily manageable lengths. The cut DNA pieces are then sorted according to size by the use of a special gel. The DNA is placed at one end of a slab of gelatin and is then drawn through the gel by an electric current. The gel acts like a sieve, allowing small DNA fragments to move more rapidly than larger ones.  

The result is that after the gel has separated the DNA according to size, a blot is made to trap the DNA in their positions, with small DNA fragments near one end of the blot and large ones near the other end. The blot is now treated with another piece of DNA called a probe that binds to the DNA on the blot. If the patterns are identical, the result is a conclusive match. The technician can then reveal his findings of his DNA paternity testing to the courts or law enforcement agencies that may require the evidence to prosecute a deadbeat dad, or allow a misnamed man to go free.