SNPs are single-letter differences
A SNP is a single-letter change in DNA, part of the natural genetic variation within a population.
Image courtesy of Lauren Solomon, the Broad Institute

SNP (pronounced "snip") is an abbreviation for "single nucleotide polymorphism," a single-letter change in DNA.

DNA, the large spiral-shaped molecule in the nucleus of almost every cell in the human body, is composed of 3 billion pairs of chemical "letters" - A (adenine), C (cytosine), G (guanine), and T (thymine) - known as nucleotides. Taken together, those letters carry the traits inherited from our mothers and fathers.

The genome sequences of any two people are 99.9% identical. About one in 1,000 letters of human DNA can vary in the form of a SNP - either an A or a C at one location, for example. If the human genome were a string of 3 billion colored beads, a SNP would be like substituting a blue bead for a red one. An example of a SNP is the alteration of the DNA segment AAG to ATG, where the second "A" in the first snippet is replaced with a "T". (The word polymorphism, which stems from Greek, means having many shapes.)

SNPs are not considered abnormal; they are simply part of the natural genetic variation within a population that creates diversity, whether the SNPs influence eye color or susceptibility to heart disease. SNPs are part of the reason it is said that "no two people are alike," but they occur in a variety of non-human organisms, too.

Not all single-letter changes in DNA are SNPs. To be classified as a SNP, at least one percent of the general population must have that change. Some single-letter changes in DNA have an effect (i.e., substitution of one amino acid for another in the encoded protein), and some have no effect. Researchers find it is valuable to study SNPs because they may be landmarks for pinpointing a disease-causing gene.

Want to learn more?

Get "the skinny" on how one SNP could be making pigs meatier by linking here.

You can also learn how researchers use SNPs to map diseases and find patterns of variation in dog breeds here.

Access a fact sheet about SNPs, and other information about the Human Genome Project sponsored by the US government.

Explore a treasure trove of information about SNPs and other genetic concepts, from the University of Utah.