used IBD sharing to identify the chromosomal location of a gene responsible for benign recurrent intrahepatic cholestasis in an isolated fishing population. Via IBD mapping, genome-wide significant regions in isolated populations as well as outbred populations were found while standard association tests failed. Using simulated data, Browning and Thompson showed that IBD mapping has higher power than association testing when multiple rare variants within a gene contribute to disease susceptibility. IBD mapping can be seen as a new form of association analysis that increases the power to map genes or genomic regions containing multiple rare disease susceptibility variants. IBD mapping is similar to linkage analysis, but can be performed without a known pedigree on a cohort of unrelated individuals. Long shared segments of IBD, which are broken up by short regions may be indicative for phasing errors. Īnother application of IBD is genotype imputation and haplotype phase inference. Measurement of relatedness can be used in forensic genetics, but can also increase information in genetic linkage mapping and help to decrease bias by undocumented relationships in standard association studies. Therefore, one application of IBD segment detection is to quantify relatedness. As noted above the amount (length and number) of IBD sharing depends on the familial relationships between the tested individuals. Identified IBD segments can be used for a wide range of purposes. For a specific DNA segment, the probability of being IBD decreases as 2 −2 n since in each meiosis the probability of transmitting this segment is 1/2. The expected number of IBD segments decreases with the number of generations since the common ancestor at this locus. 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) Morgans (M). 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. During meiosis segments of IBD are broken up by recombination. Theory Īll individuals in a finite population are related if traced back long enough and will, therefore, share segments of their genomes IBD. A colorblind-friendly version of this image. The origin of IBD segments is depicted via a pedigree. DNA segments that are IBD are IBS per definition, but segments that are not IBD can still be IBS due to the same mutations in different individuals or recombinations that do not alter the segment. An IBS segment is identical by descent ( IBD) in two or more individuals if they have inherited it from a common ancestor without recombination, that is, the segment has the same ancestral origin in these individuals. Identical nucleotide sequence due to inheritance without recombination from a common ancestorĪ DNA segment is identical by state (IBS) in two or more individuals if they have identical nucleotide sequences in this segment.
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