Identical by state

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Identical by state is a term used in genetic genealogy to describe DNA segments which do not signify the sharing of a recent common ancestor and are therefore not identical by descent. IBS segments are spurious segments (pseudosegments) which have the appearance of being a continuous long segment but which are in fact composed of two or more compound segments. IBS segments result in false-positive match predictions with autosomal DNA tests.

For an explanation of compound segments see the Family Tree DNA FAQ I have tested my husband, our child, and myself. Why does our child have matches that neither parent has?.

Phasing is necessary in order to distinguish between IBD and IBS segments.

Further reading