A reference genome is a digital nucleic acid sequence database, assembled by scientists as a representative example of the set of genes in one idealized individual organism of a species. As they are assembled from the sequencing of DNA from a number of individual donors, reference genomes do not accurately represent the set of genes of any single individual organism. Instead a reference provides a haploid mosaic of different DNA sequences from each donor. There are reference genomes for multiple species of viruses, bacteria, fungus, plants, and animals. For example, the human reference genome, GRCh38, from the Genome Reference Consortium is derived from thirteen anonymous volunteers. As the cost of DNA sequencing falls, and new full genome sequencing technologies emerge, more genome sequences continue to be generated. Reference genomes are typically used as a guide on which new genomes are built, enabling them to be assembled much more quickly and cheaply than the initial Human Genome Project. Most individuals with their entire genome sequenced, such as James D. Watson, had their genome assembled in this manner. For much of a genome, the reference provides a good approximation of the DNA of any single individual. But in regions with high allelic diversity, such as the major histocompatibility complex in humans and the major urinary proteins of mice, the reference genome may differ significantly from other individuals. Comparison between the reference and Watson's genome revealed 3.3 million single nucleotide polymorphism differences, while about 1.4 percent of his DNA could not be matched to the reference genome at all. For regions where there is known to be large scale variation, sets of alternate loci are assembled alongside the reference locus. Reference genomes can be accessed online at several locations, using dedicated browsers such as Ensembl or UCSC Genome Browser.
Properties of reference genomes
Measures of length
The length of a genome can be measured in multiple different ways. A simple way to measure genome length is to count the number of base pairs in the assembly. The golden path is an alternative measure of length that omits redundant regions such as haplotypes and pseudoautosomal regions. It is usually constructed by layering sequencing information over a physical map to combine scaffold information. It is a 'best estimate' of what the genome will look like and typically includes gaps, making it longer than the typical base pair assembly.
The human reference genome GRCh38 was released from the Genome Reference Consortium on 17 December 2013. This build contained around 250 gaps, whereas the first version had roughly 150,000 gaps. The GRCh38 assembly saw the closure or reduction of more than 100 gaps. Nanopore sequencing has seen the closure of 12 gaps in the GRCh38 reference assembly through the use of ultra-long reads. The human reference genome is derived from thirteen anonymous volunteers from Buffalo, New York. Donors were recruited by advertisement in The Buffalo News, on Sunday, March 23, 1997. The first ten male and ten female volunteers were invited to make an appointment with the project's genetic counselors and donate blood from which DNA was extracted. As a result of how the DNA samples were processed, about 80 percent of the reference genome came from eight people and one male, designated RP11, accounts for 66 percent of the total. The ABO blood group system differs among humans, but the human reference genome contains only an O allele, although the others are annotated). There are limitations to the Human Reference Genome due fact that it is "single" distinct sequence. It is specifically named as a "reference" because of this. The main purpose to which it is put, is as an index, or a locator, for genetic features. The 1000 Genomes Project is creating a database to provide information about the variations in genomes across the human population. Recent genome assemblies are as follows:
