Editing Human genome (section)

=== SNP frequency across the human genome ===

Single-nucleotide polymorphisms (SNPs) do not occur homogeneously across the human genome. In fact, there is enormous diversity in [[Single-nucleotide polymorphism|SNP]] frequency between genes, reflecting different selective pressures on each gene as well as different mutation and recombination rates across the genome. However, studies on SNPs are biased towards coding regions, the data generated from them are unlikely to reflect the overall distribution of SNPs throughout the genome. Therefore, the [[SNP Consortium]] protocol was designed to identify SNPs with no bias towards coding regions and the Consortium's 100,000 SNPs generally reflect sequence diversity across the human chromosomes. The [[SNP Consortium]] aims to expand the number of SNPs identified across the genome to 300 000 by the end of the first quarter of 2001.<ref>{{cite journal |pmid=11005795 | volume=9 | issue=16 | title=Single nucleotide polymorphisms as tools in human genetics | year=2000 | journal=Human Molecular Genetics | pages=2403–2408 | doi=10.1093/hmg/9.16.2403 | vauthors=Gray IC, Campbell DA, Spurr NK| doi-access=free }}</ref> [[File:TSC SNP Distribution.jpg|thumb|TSC SNP distribution along the long arm of chromosome 22 (from https://web.archive.org/web/20130903043223/http://snp.cshl.org/ ). Each column represents a 1 Mb interval; the approximate cytogenetic position is given on the x-axis. Clear peaks and troughs of SNP density can be seen, possibly reflecting different rates of mutation, recombination and selection.]]Changes in '''non-coding sequence''' and synonymous changes in '''coding sequence''' are generally more common than non-synonymous changes, reflecting greater selective pressure reducing diversity at positions dictating amino acid identity. Transitional changes are more common than transversions, with CpG dinucleotides showing the highest mutation rate, presumably due to deamination.{{citation needed|date=March 2023}}