One of the findings of the Human Genome Project is that the DNA of any two people, all 3.1 billion molecules of it, is more than 99.9 percent identical, but that 0.1 percent accounts for all the genetic differences between people. In literal terms, that means that one person might have blue eyes rather than green, or a susceptibility to lung cancer, or perfect pitch, because the sequence of their DNA — a long chain of adenine (A), guanine (G), cytosine (C) and thymine (T) molecules — differs from another person's. Rather than having an A-T pair of molecules at a certain spot on the DNA chain, a person might have a G-C pair. On the other hand, that difference might not have any effect at all on a person's health or appearance. These differences in DNA sequence are called single nucleotide polymorphisms, or SNPs.

SNPs do not occur randomly. There isn't an equal chance that any one of the 3.1 billion base pairs in your genome will be different from someone else's. SNPs are mutations that occurred once in history and then were passed on to future generations. So if your ancestor developed a SNP 5,000 years ago, you, along with many of your other very distant relatives would inherit that SNP, but those not descended from that ancestor would lack it. Perhaps in 15 percent of the population the 1,253,334,078th base pair along the genome at the very end of Chromosome 16 is a T-A, not a C-G like it is in the other 85 percent of the population. And most SNPs that we care about are like this, they are common, to a greater or lesser degree, throughout large parts of the population. This makes sense, since very few attributes, like eye color or a disease, occur only in one person.