If you calculate information density, it turns out that 20-base pairs is VERY unique. The human genome is ~3 billion base pairs. Each base pair is one of four

If you calculate information density, it turns out that 20-base pairs is VERY unique. The human genome is ~3 billion base pairs. Each base pair is one of four different bases. So 15 base pairs at 4^15 carries 1 billion bits of information. So long as the information is evenly distributed (which it is not), 15 random base pairs has a likelihood of occurring ~3/genome. So 20 base pairs, at 4^20 is an obscenely localized search space. But again, biology is mushy, so it's more of a time-dependent lossy grep than a true exclusive search. There are of course a lot of caveats to this kind of back-of-the-envelop calculation, but on the whole it's mostly accurate. 20 base pairs contains a LOT of information, and is generally sufficient to be unique in a particular genome.