A Single Nucleotide Polymorphism, or SNP (pronounced "snip"), is a small genetic change that occurs within DNA and can be used as a genetic marker.
DNA is specified by four nucleotide "letters" A, C, T, and G. SNP variation occurs when a single nucleotide (e.g. A) replaces one of the other three nucleotide letters (e.g. C, G, or T). An example of a SNP is the alteration of the genetic code AACCTTA to ATCCTTA, where the second "A" in the first sequence is replaced with a "T".
SNPs occur at a rate of about 1/10,000,000 bases per generation, but only 3-5% of the genome is important to form the make-up of an individual. Most SNPs actually occur in regions that are not important in determining phenotype, but which are close to causative mutations and therefore useful indicators of gene status.
Through comparing the genetic code of a variety of breeds from around the world, including Texel, Romney, Merino and Polled Dorset, many SNPs were identified. The best SNPs were included in developing the "SNP Chip", which contains 50,000 individual SNPs covering the sheep genome.