single-nucleotide polymorphism

A single-nucleotide polymorphism, usually pronounced 'snip', is a spot in the genome where people commonly differ by just one DNA letter. At one position you might carry an A while someone else carries a G — a tiny, single-base variation that is nonetheless shared by a substantial fraction of the population.

SNPs are by far the most common type of genetic variation between individuals; the human genome holds tens of millions of them. Most fall in regions where a single-letter change has little or no effect, but some sit in or near genes and can influence traits, disease risk, or how a person responds to a drug.

Because they are abundant and easy to measure, SNPs are workhorses of modern genetics. They serve as landmarks for mapping genes, as markers in association studies that link variants to traits, and as the basis for ancestry and consumer genetic tests. A SNP being associated with a trait, however, does not by itself prove that the SNP causes it — it may simply sit near the true culprit.