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Shared Segment

A Shared or Matching Segment is a DNA segment that is considered the same (or a match) in two different individuals. It could be of any length on a given chromosome. As you often have two copies of each chromosome (except Y), it could be from either copy. (And in fact, which copy is often not known or determinable.)

In Genetic Genealogy, the test returns SNP values and not actual full-sequence, base-pairs. The idea is if you have a sequence of matching SNP’s, then you likely have a sequence of underlying, matching base-pairs within the segment. The key is having frequent enough SNP’s in a chromosome to capture an accurate length of matching segments as cross-over events tend to break up the sequences in arbitrary places.

Most matching algorithms in Genetic Genealogy will use a minimum of 500 to 700 SNP’s that match, in sequence, to define a matching segment. They also define how many un-matching SNPs can be in a row before you consider the segment unmatched. This is needed because SNP values are sometimes in-determinable in a given person’s test (blank or no-call); or possibly not properly tested (incorrect value). Often 10’s to 100’s of SNP’s have to mismatch before the segment is considered no longer a match and the matching segment is ended.

Most matching segments are reported by half identical analysis tools. The data returned from test companies is reported as un-ordered pairs of values of each SNP; one value from each chromosome of the pair for the autosomes (and possibly X in a biological female). So testing companies aggressively report longer matching segments than may actually exist. This because the matching algorithms use either value of the un-ordered pair of values returned from testing and work to always make a match. This is why it is best to do matching with phased results, if possible. For siblings that you wish to compare to each other, you can only get accurate (as opposed to nonsense) matching segment values from phased comparison matching or by doing full identical analysis.