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IBD vs IBS

Terms
Identity by Descent (IBD) Identity by State (IBS)
Definition

In GWAS and statistical genetics, IBD and IBS usually mean identity-by-descent and identity-by-state - not clinical disease labels or the Iberian 1000 Genomes code - unless the surrounding text is clearly about phenotypes or reference-panel tables.

Topics
Population genetics Cohorts and databases

How they differ

IBD — Identity by Descent IBS — Identity by State
Meaning Two alleles descend from the same ancestral chromosome segment (shared ancestry through a common ancestor, no intervening recombination between them at that locus). Two alleles have the same genotype / nucleotide at a marker, whether or not they came from a common ancestor.
Observable? Not directly observed at one SNP; inferred from patterns of sharing across many markers (e.g. long segments, kinship). Directly counted from genotypes (e.g. 0, 1, or 2 alleles shared IBS at each site).
Typical use Kinship, cryptic relatedness, relationship inference (PI_HAT), family-based association, segment-based phasing/imputation. Allele-sharing metrics, distance matrices (e.g. for MDS), inputs to algorithms that estimate IBD from genome-wide IBS.

Rule of thumb: IBS is “looks the same on the genotype”; IBD is “inherited as one piece from shared ancestry.” Unrelated people can share alleles IBS by chance at common variants; IBD is the quantity relationship methods try to recover from IBS (and other information).

For more detail, see also IBS0, allele sharing, relatedness, relatedness and sample structure, and kinship.

Other meanings (optional)

  • 1000 Genomes population code: In cohort and reference-panel tables, IBS can mean Iberian Population in Spain (IBS)—a sample label in 1000 Genomes Phase 3 (European super-population), not identity-by-state. Always check whether the column is genetic sharing vs. population abbreviation (e.g. next to CEU, GBR). See 1000 Genomes Project.
  • Clinical phenotypes: In medicine and some EHR or phenotype files, IBD often means inflammatory bowel disease (e.g. Crohn’s disease, ulcerative colitis) and IBS means irritable bowel syndrome. Those are not the same concepts as identity-by-descent or identity-by-state. This dictionary does not yet host separate entries for those clinical terms; use the surrounding protocol or data dictionary to see which meaning is intended.

References