Genomic Coordinates

When to use

Any time a coordinate crosses a boundary: between two file formats, between two tools, between two assemblies, or between the genome and a transcript.

The rule

A coordinate is three facts, not one: the number, the convention it is written in, and the assembly it was measured against. Carry all three or the number is not interpretable.

Coordinate errors are the quietest class of bug in genomics. An off-by-one BED file parses, sorts, and intersects without complaint. A GRCh37 VCF joined against a GRCh38 annotation returns rows. A right-shifted indel simply fails to match its entry in ClinVar, and the result is a variant reported as novel. Nothing raises an error; the answer is just wrong, and it is wrong in a direction that looks plausible.

So: convert with the table, not from memory, and verify against the reference whenever a reference is available.

The two conversions

1-based inclusive  ->  0-based half-open :  start - 1,  end
0-based half-open  ->  1-based inclusive :  start + 1,  end

The end coordinate never moves. If a conversion changed both numbers, it is wrong.

Which format is which

Both "MAF" formats exist, they mean different things, and they disagree. UCSC serves 0-based files through a 1-based browser box. references/format-conventions.md has the full table with per-format detail.

cd skills/genomic-coordinates/scripts

python3 convert_coords.py --list                          # the table
python3 convert_coords.py --from bed --to gff chr1 999 1000
python3 convert_coords.py --from ucsc --to bed "chr7:5,530,601-5,530,625"
python3 convert_coords.py --from granges --to pyranges --input regions.tsv

contig  input                 output           length  status  detail
chr7    chr7:5530601-5530625  5530600-5530625  25      ok

Zero-length BED features (chromStart == chromEnd, a legal insertion point) are reported as rather than converted to . Exit code is 1 when any interval is degenerate or invalid.