Benefits of Triangulating Autosomal DNA Shared Segments

1. Grouping – Triangulation of segments shared with your Matches will put most of them into Triangulated Groups (TGs). If nothing else, this organizes your list of shared segments.

2. Common Ancestor (CA) – all the Matches in a TG will have the same CA*. The shared DNA segment was passed down from the CA to each Match (which is why the segments match!). Each Match pair in a TG will have a Most Recent Common Ancestor (MRCA) between them. Between different Match pairs in a TG there may be different MRCAs – but all the MRCAs descend from the same CA.

3. Team synergy – all the Matches in a TG are cousins to each other; and they all have the same goal: determine the CA. They are automatically a research team. Think of the synergy of such a Team, all focused on the same goal. Each sharing their own insights and info. All working together…
4. Multiple Common Ancestors – You may have more than one Common Ancestor with a Match (it’s not uncommon with Colonial American or Ashkenazi Jewish or other endogamous populations). TGs let you sort this out. TGs help you determine which one provided the shared segment (and is thus proved by matching DNA.) When several Matches in a TG can agree on the same CA, we have genealogy triangulation. This means several widely separated cousins have the same CA, so it’s highly probable that is the correct one; and all in the TG should also have the same one.

5. Multiple Shared Segments – You may have some Matches with multiple shared segments with you. Usually, these shared segments will be from the same CA, but not necessarily. You could be related to a Match different ways, on different segments. Again, the Matches in a TG need to agree upon the correct CA, and in this way you can determine the few Matches that are related differently on multiple shared segments.

6. Eliminate false segments – the shared segments that come from a CA are called IBD (Identical By Descent). Not all of the segments identified by a company as a “matching” or “shared” segments are IBD, some are false positive matches. They were made up by the computer algorithm that searches for matching segments; and they typically include pieces from both of your parents which are not from one Ancestor. By forming TGs, there will be a few segments that overlap, but don’t match either of the two TGs for that chromosome area. These are false positives – often called Identical by Chance (IBC) or Identical by State (IBS). So TGs will identify and eliminate these segments – they are not in TGs.

7. Form “Pointers” – When a TG includes a known close cousin, the cousin provides a “Pointer”. You and the cousin have an MRCA – grandparents, great grandparents or more distant grandparents. Other Matches in the TG are usually more distant, but the CA for all of you in the TG has to be ancestral to the MRCA that you and your close cousin share. So that MRCA forms a Pointer to where the CA has to be.

8. Brick walls – as an example, let’s say you have 3 Matches in a TG: a first cousin, a fourth cousin and a distant Match. You know the MRCA with the first cousin, and quickly extend that back to the MRCA with the fourth cousin. You have a “Pointer”, but you have a Brick Wall beyond that. Working as a Team, your fourth cousin and the distant Match determine an MRCA that is one or two generations beyond (ancestral to) the MRCA you have with your fourth cousin. Very probably this MRCA (previously unknown to you), will be the parent or grandparent of the above MRCA with your fourth cousin – and you have worked through a Brick Wall.

9. Adoptee involvement – TGs are mechanically formed. No genealogy required. Adoptees can do this with several advantages. Their TGs validate your TGs (everyone in a TG should share most of the Matches). Their TGs may add additional Matches to the TG. The adoptee can analyze the Trees and genealogy data provided by the other Matches. An adoptee can serve as an arbiter, facilitator, even Team leader of a TG. The adoptee brings diversity and analysis to the TG Team. This is an excellent way for adoptees to “get involved”.

10. Mapping – TGs define each person’s chromosome map – their personal jigsaw puzzle. Each TG describes a puzzle piece. Once you’ve formed as many TGs as you can with your Match segments, you have a better picture of your map. With sufficient TGs, they will be heel-and-toe on each chromosome, even if you don’t know which side the TGs are on. Assigning TGs to a maternal or paternal side (a maternal or paternal chromosome) involves genealogy. But once TGs are assigned, they lock in the structure of your chromosome map.

11. Expanding TGs – As additional Matches are added to your Match List each week, they can usually be added to the appropriate TG fairly easily. Each new Match-segment will match (or be In Common With) several of the segment in one TG or the other in this chromosome area. If the new segment does not match either TG (usually with a segment size 7-10cM), then segment can be categorized as IBC. Also the TG boundaries will firm up.

12. TG names [extra credit] – now that you have TGs you can give each one a unique name – I recommend start with chromosome number, 01-23, add a letter, A-Z, to indicate about where on the chromosome it is; and then letters M and P, to indicate Maternal or Paternal chromosome (or A and B if you don’t know, yet). Examples: 07GM or 18BP. This can be used as a filing system, as each one will tie to a particular ancestral line for you, and all the info you collect from Matches or research will apply to this TG. Experience indicates you’ll have 400 TGs or so.

13. Crossover points [advanced topic] – the TGs define the recombination crossover points unique to your chromosomes. Using the recombination knowledge we have (roughly 1 crossover per 100cM in each generation), we can check our chromosome maps against this knowledge. So if we already see 4 alternating large blocks from grandparents on Chromosome 1, with an unknown block in the middle – we can be pretty confident that that block is from the grandparent that leaves us with 4 blocks, rather than now having 6 alternating blocks.

14. Phasing not required – No phased data is needed to form Triangulated Groups.

15. Phasing equivalent – Phasing is separating the values of 700,000 SNPs a person (usually you) got from each parent. For instance, a person’s maternal phased atDNA would match his/her mother’s DNA 100%. When you form a Triangulated Group on your maternal side, you know that the TG segment has exactly the same string of SNPs as your mother’s DNA would have for the same location. You don’t know the values of those SNPs, but you don’t really need to know. A TG is basically a phased segment.

16. Multiple Ethnicities – TGs will often pretty clearly show different ethnic groups. This is particularly true if parents are from very different admixtures (African, Ashkenazi Jewish, Melugeon, Native American, TimBukTu, etc.) We might also expect ancestries from England or Scandanavia to group differently; or ancestries from Germanna, Pennsylvania Dutch, or other enclaves, to group together. Multiple Ethnicities may even aid in assigning TGs to their respective sides.

* “All the Matches in a TG will have the same CA…”  Actually some TGs may span large segments (15cM or more). In these cases a close cousin is often involved. These TGs will actually subdivide into smaller TGs, which will have different CAs. Much more on this in later blog posts. In any case, the segments which tightly overlap each other will have a single CA. But be careful with spread out TGs – they may subdivide and have different CAs.

09 Segmentology: Benefits of Triangulation; Jim Bartlett May 2015

A DNA segment is a block, chunk, piece, string of DNA on a chromosome. It is typically determined by a start location and an end location on a chromosome. A segment refers to all the DNA in between and including the start and end locations.

We use the term segment in at least two fundamentally different ways:

1. An ancestral segment is one which is passed down from an ancestor. Ancestral segments are passed to you from your parents, who got them from their ancestors. Each of your chromosomes are made up of ancestral segments – much more on this later.
2. A shared segment is one which both you and a match have. Both you and your match have segments which are identical from start to end. Also sometimes called an HIR (Half-Identical Region). Also sometimes referred to as a matching segment. Note: a shared segment is determined by a computer algorithm – it may or may not come from a common ancestor – much more on this later.

IBD: When a shared segment comes from a common ancestor, we say it is IBD (Identical By Descent). Both you and your match have these identical segments on a chromosome because these segments came from the same ancestor.

Note that IBD shared segments are based on ancestral segments. The ancestral segment that you and/or your match received from an ancestor, may be (and often is) larger than the shared segment. What you see in the DNA match lists, reports, tables, or chromosome browsers, is the overlapping portion of your ancestral segment and your match’s ancestral segment. This overlap is the identical part that is reported as a shared segment.

 IBC: Sometimes a “shared segment” does not come from an ancestor. The computer algorithm creates the apparent shared segment from parts of the DNA which are not all from one ancestor. This can happen in your “segment”, your matches “segment”, or both. Thus, the fact that they appear to be identical to the algorithm is by chance. We refer to these segments as IBC (Identical By Chance) or IBS (Identical By State). In any case these shared segments do not exist on one chromosome for you and/or your match, and they both therefor are not IBD – much more on this later. The “shared segment” or “matching” segment is therefor IBC.

Launching Segment-ology, a blog about segments and autosomal DNA. This will be a series of articles about the various aspects of DNA segments and how they are used in genetic genealogy. My desire is to help genealogists get the most out of their atDNA results.