SNP
Meaning of SNP: Millions of these single letter differences dot every genome, and only a small share change how a gene or its protein behaves.
Definition of SNP
SNP stands for single nucleotide polymorphism, a difference between people at one position in the DNA sequence, where a single building block is swapped for another. SNPs are the most common form of genetic variation between individuals, and the majority of them sit outside the stretches of the genome that carry instructions for building protein.
What does a single SNP actually change?
That depends entirely on where it falls. DNA is read in three letter groups, each specifying one amino acid in a protein, so a swapped letter can leave the instruction unchanged, alter one amino acid, or cut the instruction short. A SNP outside a gene may still matter if it lands in a switch region that controls how much protein is made or in which tissue. Most SNPs do neither and produce no observable difference at all.
| Where the SNP falls | Usual consequence |
|---|---|
| Coding region, same amino acid | Protein unchanged |
| Coding region, different amino acid | Protein altered, effect ranges from none to substantial |
| Control region near a gene | Amount or timing of protein production shifts |
| Non-coding stretch with no known role | Usually no effect, but useful as a marker |
How does a SNP differ from a variant or a mutation?
The polymorphism in the name traditionally implied that the alternative nucleotide is common enough to be found across a population rather than confined to one family. A single letter change that is rare is now more often called a single nucleotide variant, or SNV. Mutation is the older word for the same physical change, and it has largely given way to variant in reporting, because mutation implies harm that most single letter changes do not carry.
How are SNPs used in research?
Researchers compare SNP patterns across large groups of people with and without a condition, looking for positions where one version turns up more often in one group. A SNP flagged this way frequently is not the cause of anything. It sits close to the real culprit on the same chromosome and is inherited alongside it, acting as a signpost to a region worth investigating. The same approach is applied to medicines, where SNPs in the genes for drug metabolizing enzymes help explain why one person clears a drug quickly and another slowly, information that genetic testing can supply before treatment begins.
Also known as
Used in a sentence
Genetic testing identified several SNPs that are being studied for their possible association with the condition.
Good to know
A single SNP found on genetic testing seldom settles anything on its own, since most SNPs have no known functional effect and many serve only as markers for a nearby stretch of DNA.