How does a paternity test work?

A long, long time ago, before Watson was a crick in anyone’s neck, at the beginning of the 20^th Century, the question of paternity had no scientific answer. The “presumption of paternity” rule, accepted in the legal system of England 500 years ago, stated that unless a husband could prove he was “sterile, impotent, or beyond the four seas bordering the kingdom” at the time of conception, then he was legally the father of his wife’s child. Today, DNA analysis allows us to determine paternity with almost 100 percent certainty, so easily that Maury Povich can deliver the news to dozens of potential fathers every day in rapid-fire, syndicated succession. How did we come so far?

Blood typing was the first scientific technique utilized to help identify biological fathers, despite its limitations. If you were paying attention in junior high you might remember a chart describing the possible combinations between the four blood types: A, B, AB and O. They are classified by the presence or absence of A proteins, B proteins, and Rh factors (which determine positive or negative status, but aren’t as important for deciphering paternity) on the surface of red blood cells. It’s relatively simple. For example, if the presumed father is type A and the mother is type O, then the child couldn’t be type B. (Admittedly, this isn’t the most effective application of blood typing.) Determining ABO blood types based on the A and B proteins on the surface of red blood cells can reliably prove that a man is not the father, but can’t reliably prove that he is the father.

In the 1970s, Human Leukocyte Antigen (HLA) testing emerged as a much better option. This is another blood test looking at proteins, this time in white blood cells. It requires a much larger amount of blood because there are fewer white blood cells in a sample, but it works much better. While there are only four possible blood types, there is huge variability in HLA types. The HLA molecule is highly polymorphic, meaning there are many different alleles, or versions of the genes encoding the molecule – many hundreds, in fact. So HLA testing can rule out a lot of dudes, but you still can’t narrow them down to one.

Things changed in the 1980s after disco died and DNA testing hit the scene. DNA can indicate paternity with 99.99 percent accuracy. In the early years of DNA testing, technicians needed a blood sample from all three parties for a simple matching game called Restriction Fragment Length Polymorphism.  The technicians would isolate one long string of DNA from the father, one from the mother, and one from the child. To each sample they stirred in some enzymes, which chewed up the DNA into a bunch of uneven fragments. Any fragment from the child’s DNA should be the same length and size of a fragment of either mother or father’s DNA. If roughly half of the fragments match the man in question then he is the father! (Applause.)

In the 1990s, paternity tests got even easier – and accessible to daytime talk show hosts – thanks to Polymerase Chain Reaction (PCR), which requires a quick cheek swab from mother, child and presumed father. PCR makes billions of copies of the DNA, so only a tiny original sample is needed. Lab technicians look at 16 specific fragments. Eight should match the father and the other eight the mother. This method only takes a few days. If you send away for one of those $78 mail-order DNA tests, this is what the lab is using.

But if you want evidence that will hold up in court, Bieber lovers, make sure you use a DNA testing lab accredited by the international not-for-profit association AABB, a professional standards group that works with the Food and Drug Administration to improve the safety of blood transfusions and cellular therapies. The Maury show uses DNA Diagnostic Center, a fully accredited testing lab, for all of their paternity testing, so going on the show is really not a bad option if you want a reliable, legally binding result. Just saying, Justin.