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How often do single nucleotide polymorphisms occur in the human genome?

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Diagram of a DNA double helix showing its twisted ladder structure, with a close-up of the sugar-phosphate backbone, paired nitrogenous bases (adenine, thymine, cytosine, guanine), and labeled nucleotides and hydrogen bonds. — human genome The human genome is made up of approximately three billion base pairs of deoxyribonucleic acid (DNA). The bases of DNA are adenine (A), thymine (T), guanine (G), and cytosine (C).

In the human genome, single nucleotide polymorphisms (SNPs) occur frequently, appearing at a rate of approximately one in every 100 to 300 nucleotides. Other estimates suggest a frequency of roughly one in every 300 bases, which corresponds to an estimated total of 10 million SNPs across the 3 billion nucleotides that make up the full-length human genome.12

These variations, which involve a change in a single nucleotide—adenine, guanine, thymine, or cytosine—account for roughly 90 percent of the genetic variation observed between humans. While most SNPs do not affect cellular function, they serve as critical tools in genetics research for mapping genes and identifying genetic associations with human disease.12