Single nucleotide polymosphisms (SNPs)

A site in a DNA sequence that is variable for at least 2 nucleotides (alleles) - the site becomes the locus. Most are bi-allelic. In non-coding regions. Human genome on average 1 major SNP (present in >5% population) every 300 to 1000 bp. Non-synonymous substitutions will change protein sequence - functional variation – possible selection pressure