When working with Matches on one side (Maternal or Paternal), segment Triangulation is a snap. Overlapping segments are all you need! The overlap should be at least 7cM, and more is better.

The basic rules to form Triangulated Groups, were designed to insure your overlapping shared DNA segments were on the same side – in other words on just one of your chromosomes. This means, from your viewpoint, the overlapping segments were both (or all) on your maternal *or* paternal chromosome. It didn’t matter which side it was on for your Match. You can have lots of shared segments on one chromosome, but some may be on your maternal chromosome and the others on your paternal chromosome. It is virtually impossible for Match A’s shared segment on your maternal chromosome to also match Match B’s shared segment on your paternal chromosome. So the requirement is/was to compare Match A and Match B to insure they match each other – and are thus on the same chromosome with you.  *IF* you already know Match A and Match B are on, say, your maternal side, then their shared DNA segments with you would be on your maternal chromosome, and there is no additional need to compare them to each other – they Triangulate.

I am sure, in the grand scheme of genetic genealogy, that an occasional glitch could occur. I’d estimate this as way less than 1% probability.

FTDNA has maternal and paternal buckets which appear to be pretty accurate. If the companies designated a “side” and allowed us to filter Matches based on that side, it would sure speed up segment Triangulation. Just look at a spreadsheet for natural crossover breaks in each chromosome.

In the meantime, if you can designate your Matches as Maternal or Paternal in some way (compare to a parent’s test, ethnicity, shared matches, etc.), you can use that info to filter your Matches and ease the segment Triangulation process. There’s still a lot of work to do, but this should ease the process some.

[10E] Segment-ology: Triangulation on a Side Is a Snap by Jim Bartlett 20230730

A Segment-ology TIDBIT

mtDNA is passed from a female Ancestor down the all-female line to each of us. A Triangulated Group (TG) DNA segment is passed down from an Ancestor to us. The concept of DNA being passed down a specific ancestral line – from an Ancestor to us – is the same. Such is also the case for Y-DNA – it is passed from an Ancestor down the all-male line to a man. In the case of mtDNA, the ancestral path is all females; in the case of Y-DNA, the ancestral path is all males; but in the case of atDNA, the ancestral path can zig-zag between male and female Ancestors. Any of our Ancestors could pass an atDNA segment down to us.

The point is the TG segment is found only on one specific ancestral line (like the mt or Y line). However, it is still a genealogy task to figure out which line. As we “walk the segment back” from our own DNA back up our ancestry, there are only two options at each generation. If we know a TG segment is on our maternal side, the next generation back must be one of the maternal grandparents – and so on.

Just as we use mtDNA or Y-DNA, looking for a someone who shares that same DNA with us, to find our Common Ancestor; so, too, we understand that our atDNA Matches in a TG (thus sharing that same atDNA with us) will have a Common Ancestor with us.  

This is just another way to think about our DNA segments – they are just as focused as the mt or Y on *one* ancestral line.

[22BX] Segment-ology: Triangulated Group Segments Are Like mtDNA TIDBIT by Jim Bartlett 20230728

A Segment-ology TIDBIT

Here is the set up for my BROWN story, without dragging the reader through the whole back story. This line includes most of the descendants in the BROWN Y-DNA Project Group-40.

I’m searching for the children of Wilson BROWN (he probably had 10 children, only two daughters are known). This is my Tree at AncestryDNA. I expected ThruLInes to find some Matches… Nada. I had Matches from Keziah (and her husband Elliott BAKER) on down. I had none from Wilson – not too surprising because no one has any Trees for Wilson (except for daughter Keziah). I expected some 6C Matches from James, because I know they are out there – but… nothing.

So I used my “Search on a Surname” process [here] – I searched for the BROWN surname, and checked each Match’s Tree for likely families. One family that quickly became the standout was the family of Thomas BROWN 1773 married Nancy LITTON. I was getting a lot of “hits” on that family. So, I looked them up at Ancestry – there are 2,668 Trees for that line! Almost everyone who shows his parents, has John/James BROWN b 1731 MD; married 1755 Plymouth, MA; d VA & Sarah LITTLE b 1737 VA; d 1779 VA.

Two key points about Thomas BROWN 1773:

1. I have found over 70 Matches who descend from Thomas BROWN b 1773 (shared DNA segments from 10 to 30cM). These are spread over virtually all of his children.

2. Two descendants of Thomas BROWN 1773 – through different children – have taken a Y-DNA test and are in BROWN Group-40. So, Thomas BROWN 1773 is BROWN Group-40. No one else in Group-40 has claimed descent from his father, John BROWN 1731.

I have concluded that Thomas BROWN 1773 must be a son of Wilson BROWN and so I added him (and his children) to my Tree.  I stand alone in doing so…

I waited over a month for Ancestry’s ThruLines to show me the 70 Matches I had found – nada. Disappointing… Ancestry clearly had Thomas BROWN 1773 locked onto John BROWN 1731. I’ve written at least 10 blog posts about the power and usefulness of ThruLInes – search for links to them in the Segmentology Outline [here]. One post is about ThruLines X-Ray vision looking into Private Trees…

So, I decided: maybe Ancestry is correct! Maybe if I accepted their version, ThruLInes would report some of my DNA Matches as cousins. So, I changed my Ancestor Keziah BROWN from the daughter of Wilson BROWN to the wife of Thomas BROWN 1773 (so the two of them looked like the parents of my ancestor, John Brown BAKER – almost like Thomas had an affair with Keziah.)

The next day Ancestry listed 31 new ThruLines Matches (spread from 6 to 30cM) – all descending as half-cousins from Thomas BROWN 1773 – WOW.  All of these were new to me. 2xWOW! Near the top of the list was a Match with a Tree with only 2 parents, and 3 grandparents – ThruLines built the Tree back to Thomas 1773. I have built a lot of Quick&Dirty Trees in my BROWN searches, but I would not have tried that one. 3xWOW! And another Match had a Private (but searchable) Tree. I’d never have found that one. 4xWOW!

The fact that I adopted the on-line version of BROWN Tree does not detract from my goal: find more DNA Match cousins from Thomas BROWN 1773. And ThruLines delivered.! The Matches share DNA with me (no matter how the Tree is drawn).

I still need to put all of these in a spreadsheet; make sure they are reasonable; figure out the averages and see how they compare to the Shared cM Project. And I’ll wait a few more days – fully expecting another tranche in the next day or two.

BOTTOM LINE:

This method will sure save a LOT of scrolling through all the thousands of 8-9cM Matches for BROWN Matches (it took me over a Month of steady focus to just get through the 10cM BROWN Matches). And it will find cousins with Private Trees and cousins with very small Trees that don’t have BROWN in them!

[22BW] Segment-ology: Getting ThruLines to Work for Me TIDBIT by Jim BARTLETT 20230707