Single nucleotide polymorphism

A single nucleotide , or SNP (pronounced snip), is a variation occurring when a single  -, , , or  - in the  (or other shared sequence) differs between members of a species (or between paired chromosomes in an individual). For example, two sequenced DNA fragments from different individuals, AAGCCTA to AAGCTTA, contain a difference in a single nucleotide. In this case we say that there are two s : C and T. Almost all common SNPs have only two alleles.

Within a population, SNPs can be assigned a - the ratio of chromosomes in the population carrying the less common variant to those with the more common variant. It is important to note that there are variations between human populations, so a SNP allele that is common in one geographical or ethnic group may be much rarer in another. In the past, single nucleotide polymorphisms with a minor allele frequency of ≥ 1% (or 0.5%, etc.) were given the title "SNP," an unwieldy definition. With the advent of modern and a better understanding of evolution, this definition is no longer necessary.

Single nucleotide polymorphisms may fall within coding sequences of genes,, or in the s between genes. SNPs within a coding sequence will not necessarily change the sequence of the  that is produced, due to. A SNP in which both forms lead to the same polypeptide sequence is termed synonymous (sometimes called a ) - if a different polypeptide sequence is produced they are non-synonymous. SNPs that are not in protein-coding regions may still have consequences for, binding, or the sequence of.

Variations in the DNA sequences of humans can affect how humans develop s and respond to s, s,, s, and other agents. However, their greatest importance in biomedical research is for comparing regions of the genome between (such as with matched cohorts with and without a disease).

The study of single nucleotide polymorphisms is also important in crop and livestock breeding programs (see ). See for details on the various methods used to identify SNPs.