Identical by Descent (IBD)
Definition
A chromosomal segment shared between two individuals because both inherited it from a common ancestor without an intervening recombination event. The amount of IBD shared predicts genetic relatedness distance.
- Abbreviation
- IBD
- Requires
- A shared common ancestor, no intervening recombination
- Contrast
- Identical by state (IBS), which need not share ancestry
- Application
- Estimating relatedness distance in genetic genealogy
Common questions
How is IBD segment length used to estimate how closely two people are related?+
Longer and more numerous shared segments indicate a more recent common ancestor, because recombination breaks chromosomes into shorter pieces with each passing generation, so genealogists use total IBD length and segment count to place a match within an estimated relationship range.
Why does forensic genetic genealogy rely on IBD rather than a direct STR match?+
Standard forensic STR profiles are built for individual identification, not distant relative matching; the much denser SNP data used in genealogy databases lets analysts detect the small shared IBD segments that reveal distant family relationships an STR profile cannot resolve.
Related terms
- DOJ Interim Policy 2019
- US Department of Justice Interim Policy on Forensic Genetic Genealogy DNA Analysis (24 September 2019), limiting IGG to violent crimes and unidentified...
- ForenSeq DNA Signature Prep Kit
- Verogen MPS library-preparation kit typing 27 autosomal STRs, 7 X-STRs, 23 Y-STRs, 94 identity SNPs, and 56 ancestry SNPs in a single...
- GEDmatch PRO
- The law-enforcement tier of GEDmatch, acquired by Verogen in 2020. Operates on an opt-in model; only profiles of users who have explicitly...
- Investigative Genetic Genealogy (IGG)
- A forensic identification technique that uploads a crime-scene DNA profile to a consumer genealogy database, identifies partial matches suggesting distant family relationships,...
- Massively Parallel Sequencing (MPS) / NGS
- High-throughput sequencing technology that generates millions of short reads simultaneously; used in forensics for sequence-level STR typing (Verogen MiSeq FGx, Ion S5)...
- Othram Inc.
- Texas-based forensic genetics company offering the FORCE target-capture SNP profiling service for degraded and low-quantity crime-scene DNA and unidentified-remains casework using IGG.
- Parabon NanoLabs
- A US company that became the primary commercial provider of forensic IGG casework following the Golden State Killer case, offering the Snapshot...
- Precision ID GlobalFiler NGS STR Panel
- Thermo Fisher Scientific MPS library-preparation kit for the Ion S5 platform typing the 20 CODIS core loci and additional markers with sequence-level...