Editing Identity by descent (section)

== Theory ==
All individuals in a finite population are related if traced back long enough and will, therefore, share segments of their [[genomes]] IBD. During [[meiosis]] segments of IBD are broken up by recombination. Therefore, the expected length of an IBD segment depends on the number of generations since the [[most recent common ancestor]] at the locus of the segment. The length of IBD segments that result from a common ancestor ''n'' generations in the past (therefore involving 2''n'' meiosis) is exponentially distributed with mean 1/(2''n'') [[Centimorgan|Morgans]] (M).<ref name="Browning.2008">{{Cite journal
| last1 = Browning | first1 = S. R.
| title = Estimation of Pairwise Identity by Descent from Dense Genetic Marker Data in a Population Sample of Haplotypes
| doi = 10.1534/genetics.107.084624
| journal = Genetics
| volume = 178
| issue = 4
| pages = 2123–2132
| year = 2008
| pmid = 18430938
| pmc =2323802
}}</ref> The expected number of IBD segments decreases as the number of generations since the common ancestor at this locus increases. For a specific DNA segment, the probability of being IBD decreases as 2<sup>−2''n''</sup> since in each meiosis the probability of transmitting this segment is 1/2.<ref name="Thompson.2008">{{Cite journal
| last1 = Thompson | first1 = E. A. | author1-link = Elizabeth A. Thompson
| title = The IBD process along four chromosomes
| doi = 10.1016/j.tpb.2007.11.011
| journal = Theoretical Population Biology
| volume = 73
| issue = 3
| pages = 369–373
| year = 2008
| pmid = 18282591
| pmc =2518088
| bibcode = 2008TPBio..73..369T }}</ref>