Identical By Descent segment
An Identical By Descent (IBD) segment is a segment of DNA that is found to be identical (neglecting rare mutations and testing errors) in two people who are related to each other, and was passed down to both of them from a common ancestor. Two matching half-identical regions (HIRs) (see below) or full-identical regions (FIRs) which meet minimum threshold conditions can be considered Identical By Descent.
Being Identical By Descent is contrasted to being Identical By State (IBS), that is, similar due to random chance. These terms are more relevant to the results of SNP microarray testing than to results of DNA sequencing, because SNP testing provides so much less information per centimorgan (cM) of DNA. SNP test results have an additional complexity since they report on both copies of the chromosome, but the results are not phased (that is, it is unknown which nucleotide is on which copy of the chromosome). Thus if one person's SNP result is (CC), this could be at least "Half-Identical" to either (CC) or (CT) in a second person. A homozygous mismatch such as (CC) vs. (TT) would be required before one could say the results are *not* identical.
Consecutive SNP results for a short segment of DNA may be half-identical in two individuals when in actuality the DNA sequences are not identical. Therefore we must resort to the use of statistics to determine whether two half-identical (apparently matching) segments are likely to actually be identical (neglecting rare errors and mutations). A long consecutive string of half-identical SNP results (typically about 5 cM / 700 SNPs, depending on the test's error rate and other factors) is required before one can say that two matching DNA segments are probably Identical By Descent, and not just similar due to random chance (IBS). Thresholds for length and number of mismatches (errors or mutations) are set by each testing company; these criteria must be met before the company will report that two individuals very likely inherited their matching segments from a common ancestor. For threshold details see Family Finder versus Relative Finder - Thresholds for relationship matches.
Ranges of total centimorgans of IBD segments expected, based on family relationship
- Parent/child: 3539-3748 centimorgans (cMs)
- 1st cousins: 548-1034 cMs
- 1st cousins once removed: 248-638 cMs
- 2nd cousins: 101-378 cMs
- 2nd cousins once removed: 43-191 cMs
- 3rd cousins: 43-ca 150 cMs
- 3rd cousins once removed: 11.5-99 cMs
- 4th and more distant cousins: 5-ca 50 cMs
- Parent/child: 23-29
- 1st cousins: 17-32
- 1st cousins once removed: 12-23
- 2nd cousins: 10-18
- 2nd cousins once removed: 4-12
- 3rd cousins: 2-6?
- 3rd cousins once removed: 1-4
- 4th and more distant cousins: 0-2
- Wheaton K. atDNA matches. Lesson 9 in the series "Beginners' Guide to Genetic Genealogy", Wheaton Surname Resources website, 2013.
- Cooper K. The ABCs of DNA - IBD vs IBS vs mIBC Kitty Cooper's DNA Genealogy blog, 30 January 2012.
- Turner A. "Satiable Curiosity: Identity Crisis: Identical by State or Identical by Descent?" Journal of Genetic Genealogy Fall 2011, Volume 7.