Your DNA segments (that make up the 23 Chromosomes passed down to you from a parent) are not the same as shared DNA segments with a Match (as described by a chromosome browser) aka a Half Identical Region (HIR). All of your DNA is real, down to any size you want to analyze. This is not necessarily so for a shared DNA segment (or HIR)!

From the ISOGG Wiki: A half-identical region (HIR) is a region of two paired chromosomes where at least one of the two alleles from one person’s pair of chromosomes matches at least one of the two alleles from a different person’s pair of chromosomes throughout the entire region. A half-identical region may be either identical by descent (IBD) or identical by state (IBS).

In my words, for genetic genealogy, a computer compares your DNA test to a potential Match’s DNA test. The computer compares the two raw DNA data files – about 600,000 SNPs with two values (alleles) for each SNP. The two values are one from the DNA passed down from the father and one from the mother. The computer is looking for a long string of matching SNPs, which are then reported as a shared DNA segment. This meets the HIR definition above – at least one value is the same at each SNP in the shared segment. The theory is that, although much of our DNA will be the same, there is some variation, and a long enough string of matching SNPs will indicate this segment of DNA is from a Common Ancestor. This also implies that the long string is on one side – on one chromosome from our mother OR our father. A lot of reported genetic data indicates that such an HIR is true when it’s at least 15cM.

But why aren’t all shared DNA segments true? Because the computer algorithm blindly looks at *both* values at each SNP for you and the potential Match. The computer may create a string of your SNPs that agree with your potential Match’s SNPs, but some are from your father and some from your mother. Clearly this “zig-zag” result, using SNPs from both your parents’ DNA, is not a representation of your DNA on one chromosome. It’s not a DNA segment passed down from one of your parents to you. It’s a false segment! Or this might have happened with your potential Match’s data, or with both of you. Bottom line: wherever the “zig-zag” occurred, the shared DNA segment is false.

The good news is that this “zig-zag” result doesn’t occur with long enough segments – over 15cM. And it occurs very infrequently with 14cM shared DNA segments. And there is a rough distribution curve – probably different for each of us – which drops down to about half of our 7cM segments are false. And most shared DNA segments are false below 7cM – which is why they are generally not used. Some of the companies use other, proprietary, algorithms to discard (not report) some of these false Matches. Also, as I’ve blogged before, Triangulated Groups are very good at culling out the false segments.

This also ties into the ISOGG terms: Identical By Descent (IBD) and Identical By State (IBS), noted above. IBD would apply to true shared DNA segments – you and your DNA Match got the shared DNA segment from a Common Ancestor. IBS means the computer found a “match”, but IBS is usually used in genetic genealogy to indicate the false segments. I usually just stick to “true” and “false” shared DNA segments (or HIRs).

Another quirk in this discussion is using the term HIR to refer to a shared DNA segment.  This is proper and OK. But, an HIR only refers to a shared DNA segment between you and one particular Match. We virtually never find exactly the same HIR with two Matches (although it’s possible with Matches who are closely related to each other.) When we look at segment Triangulation, the Triangulated Group is comprised of different HIRs. So HIR should not be used to refer to a TG. A TG represents a segment of your DNA (from a specific Ancestor) – there are many different HIRs in a TG. And each Match in a TG would have a different (but overlapping) segment from the Common Ancestor, with different HIRs. Because the whole process is so random, we just don’t get the same segments from our Common Ancestors that our Matches get.

Bottom Line: A shared DNA segment is also an HIR – formed by a computer by comparing raw DNA test data (about 600,000 SNPs) with two values (alleles) for each SNP. Shared DNA Over 15cM all are true segments (IBD); below 15cM some are false (IBS). A shared DNA segment (aka HIR) is usually unique to a specific Match.

[22DH] Segment-ology: Half Identical Region by Jim Bartlett 20250521