Single nucleotide polymorphism

  A single nucleotide polymorphism, or SNP (pronounced snip), is a alleles : C and T. Almost all common SNPs have only two alleles.

Within a population, SNPs can be assigned a minor allele frequency - 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 greater than or equal to 1% (or 0.5%, etc.) were given the title "SNP," an unwieldy definition. With the advent of modern bioinformatics and a better understanding of evolution, this definition is no longer necessary.

Single nucleotide polymorphisms may fall within coding sequences of genes, non-coding RNA.

Variations in the DNA sequences of humans can affect how humans develop diseases and respond to chemicals, drugs, vaccines, and other agents. However, their greatest importance in biomedical research is for comparing regions of the genome between cohorts (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 genotyping). See SNP genotyping for details on the various methods used to identify SNPs.

See also

• Single Base Extension
• Variome
• TaqMan
• Affymetrix
• International HapMap Project
• tag SNP

References

• Human Genome Project Information — SNP Fact Sheet
• Relation of SNP's with Cancer