The term “single nucleotide polymorphism” contains two defining criteria. First, it refers to a single nucleotide, i.e., an individual base pair, that can differ between individuals. Second, the word polymorphism indicates that a particular nucleotide change of interest is shared by at least 1% of the population.
SNPs occur when one base pair replaces another base pair in a point mutation (see DNA entry for discussion of bases). For example, an A-T pairing may be replaced by a G-C pairing. Such a mutation does not typically harm the organism.