21. SNPs: Frequently Occurring Genetic Variants
There are over a million single nucleotide polymorphisms (SNPs) in the human genome. SNPs are specific sites within a human genome at which some individuals will have one nucleotide present while other individuals will have a different one. SNPs begin their existence as point mutations, and they eventually become established in a population. This substitution must occur in a significant proportion (more than 1 percent) of a large population for it to be called a SNP. Here is an example: In the DNA sequence TAGC, a SNP occurs when the G base changes to a C, and the sequence becomes TACC. When SNPs occur within a gene, the protein that results usually remains somewhat functional.