<P> 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 . </P> <P> A personal genome sequence is a (nearly) complete sequence of the chemical base pairs that make up the DNA of a single person . Because medical treatments have different effects on different people due to genetic variations such as single - nucleotide polymorphisms (SNPs), the analysis of personal genomes may lead to personalized medical treatment based on individual genotypes . </P> <P> The first personal genome sequence to be determined was that of Craig Venter in 2007 . Personal genomes had not been sequenced in the public Human Genome Project to protect the identity of volunteers who provided DNA samples . That sequence was derived from the DNA of several volunteers from a diverse population . However, early in the Venter - led Celera Genomics genome sequencing effort the decision was made to switch from sequencing a composite sample to using DNA from a single individual, later revealed to have been Venter himself . Thus the Celera human genome sequence released in 2000 was largely that of one man . Subsequent replacement of the early composite - derived data and determination of the diploid sequence, representing both sets of chromosomes, rather than a haploid sequence originally reported, allowed the release of the first personal genome . In April 2008, that of James Watson was also completed . Since then hundreds of personal genome sequences have been released, including those of Desmond Tutu, and of a Paleo - Eskimo . In November 2013, a Spanish family made their personal genomics data publicly available under a Creative Commons public domain license . The work was led by Manuel Corpas and the data obtained by direct - to - consumer genetic testing with 23andMe and the Beijing Genomics Institute). This is believed to be the first such public genomics dataset for a whole family . </P> <P> The sequencing of individual genomes further unveiled levels of genetic complexity that had not been appreciated before . Personal genomics helped reveal the significant level of diversity in the human genome attributed not only to SNPs but structural variations as well . However, the application of such knowledge to the treatment of disease and in the medical field is only in its very beginnings . Exome sequencing has become increasingly popular as a tool to aid in diagnosis of genetic disease because the exome contributes only 1% of the genomic sequence but accounts for roughly 85% of mutations that contribute significantly to disease . </P>

Approximately how many letters make up the human genome