## Description

This track shows alternate sequence indicated by GRC (Genome Reference Consortium) to the GRCh37 (hg19) reference sequence in an effort to provide the best possible reference assembly for the human genome. GRC does this by both generating multiple representations (Haplotypes/alternate loci) for regions that are too complex to be represented by a single path and by releasing regional fixes to short sections of DNA known as Patches. This allows users who are interested in a specific locus to get an improved representation without affecting users who need chromosome coordinate stability.

Download the GRC Patch Release track data sets from the Genome Browser downloads server.

### Haplotypes

Items in the Haplotype track show sequence variation present in the population for certain sections of the genome. Items in this track were either designated as alternate loci or novel patches by GRC. To view the alternate sequence, click on the "Fetch alternate sequence" link that appears on the sequence details pages.

### Patches

Items in the Patch track indicate areas of the sequence that have been corrected by GRC and will be updated to new sequence in the next full human assembly release. To view the new, corrected sequence, click on the "Fetch alternate sequence" link that appears on the sequence details pages.

### Alignment

The Alignment track is the PSL representation of the Chain track. This PSL representation allows the side-by-side alignment to be viewed. It was created from the Chain track data using the chainToPsl kent source utility. This track follows the display conventions for PSL alignment tracks.

### Chain Track

The chain track shows the alignment of human genome alternate sequence to the human $date genome sequence using a gap scoring system that allows longer gaps than traditional affine gap scoring systems. It can also tolerate gaps in both human and the alternate sequence simultaneously.

The chain track displays boxes joined together by either single or double lines. The boxes represent aligning regions. Single lines indicate gaps that are largely due to a deletion in the human assembly or an insertion in the alternate sequence. Double lines represent more complex gaps that involve substantial sequence in both sequences.

In the "pack" and "full" display modes, the individual feature names indicate the chromosome, strand, and location (in thousands) of the match for each matching alignment. By default, the chains to chromosome-based assemblies are colored based on which chromosome they map to in the aligning organism. To turn off the coloring, check the "off" button next to: Color track based on chromosome (click "Chains" text in controls to view options).

To display only the chains of one chromosome in the aligning sequence, enter the name of that chromosome (e.g. chr4) in box next to: Filter by chromosome (click "Chains" text in controls to view options).

### Net Track

The net track shows the best human/alternate chain for every part of the alternate sequence. It is useful for finding orthologous regions and for studying genome rearrangement. The human sequence used in this annotation is from the $date assembly.

In full display mode, the top-level (level 1) chains are the largest, highest-scoring chains that span this region. In many cases gaps exist in the top-level chain. When possible, these are filled in by other chains that are displayed at level 2. The gaps in level 2 chains may be filled by level 3 chains and so forth.

In the graphical display, the boxes represent ungapped alignments; the lines represent gaps. Click on a box to view detailed information about the chain as a whole; click on a line to display information about the gap. The detailed information is useful in determining the cause of the gap or, for lower level chains, the genomic rearrangement.

Individual items in the display are categorized as one of four types (other than gap):

- Top - the best, longest match. Displayed on level 1.
- Syn - line-ups on the same chromosome as the gap in the level above it.
- Inv - a line-up on the same chromosome as the gap above it, but in the opposite orientation.
- NonSyn - a match to a chromosome different from the gap in the level above.

## Methods

### Haplotype/Patch tracks

The locations of the Patches and Haplotypes were established by NCBI and specified in the files at:
ftp://ftp.ncbi.nih.gov/genbank/genomes/Eukaryotes/vertebrates_mammals/Homo_sapiens/ GRCh37/ALT_REF_LOCI_*/placed_scaffolds/alt_locus_scaf*primary.pos

And the patch release site:
ftp://ftp.ncbi.nih.gov/genbank/genomes/Eukaryotes/vertebrates_mammals/Homo_sapiens/ GRCh37.p*/PATCHES/alt_scaffolds/alt_scaffold_placement.txt

### Chain track

The lastz alignments between the human sequence and the alternate sequence were restricted to only the region that the haplotype overlaps the human sequence or the region of the human sequence that is replaced by the updated patch sequence. The resulting alignments were converted into axt format using the lavToAxt program. The axt alignments were fed into axtChain, which organizes all alignments between the single human region of the alignment and a single region from the alternate sequence into a group and creates a kd-tree out of the gapless subsections (blocks) of the alignments. A dynamic program was then run over the kd-trees to find the maximally scoring chains of these blocks. $matrix

Chains scoring below a minimum score of '$chainMinScore' were discarded; the remaining chains are displayed in this track. The linear gap matrix used with axtChain:
$chainLinearGap

### Net track

Chains were derived from lastz alignments, using the methods described in the chain track description above, and sorted with the highest-scoring chains in the genome ranked first. The program chainNet was then used to place the chains one at a time, trimming them as necessary to fit into sections not already covered by a higher-scoring chain. During this process, a natural hierarchy emerged in which a chain that filled a gap in a higher-scoring chain was placed underneath that chain. The program netSyntenic was used to fill in information about the relationship between higher- and lower-level chains, such as whether a lower-level chain was syntenic or inverted relative to the higher-level chain. The program netClass was then used to fill in how much of the gaps and chains contained Ns (sequencing gaps) in one or both species and how much was filled with transposons inserted before and after the two sequences diverged.

## Credits

Lastz (previously known as blastz) was developed at Pennsylvania State University by Minmei Hou, Scott Schwartz, Zheng Zhang, and Webb Miller with advice from Ross Hardison.

Lineage-specific repeats were identified by Arian Smit and his RepeatMasker program.

The axtChain program was developed at the University of California at Santa Cruz by Jim Kent with advice from Webb Miller and David Haussler.

The browser display and database storage of the chains and nets were created by Robert Baertsch and Jim Kent.

The chainNet, netSyntenic, and netClass programs were developed at the University of California Santa Cruz by Jim Kent.

## References

Chiaromonte F, Yap VB, Miller W. Scoring pairwise genomic sequence alignments. Pac Symp Biocomput. 2002;:115-26.

Kent WJ, Baertsch R, Hinrichs A, Miller W, Haussler D. Evolution's cauldron: Duplication, deletion, and rearrangement in the mouse and human genomes. Proc Natl Acad Sci U S A. 2003 Sep 30;100(20):11484-9.

Schwartz S, Kent WJ, Smit A, Zhang Z, Baertsch R, Hardison RC, Haussler D, Miller W. Human-Mouse Alignments with BLASTZ. Genome Res. 2003 Jan;13(1):103-7.