Phasing Your Ancestors’ DNA?

Actually, it’s pretty easy to phase some of your Ancestors’ DNA. If you’ve ever formed a Triangulated Group (TG), you’ve already done it!

I’ve posted before that TGs are phased data. Here. We don’t really care what the SNP values are (A or C or G or T), just that they “the ones” on one chromosome. We know this because all of our Matches in a TG have the same SNP values. That’s how we get the Shared DNA Segments and a TG. We are all in agreement about the segment represented by our matching SNPs.

One of the outcomes of Triangulation is to determine an Ancestral Line that passed down the TG segment to you. This is usually accomplished by determining multiple Match-cousins in the TG with Common Ancestors (CAs) to you on one Ancestral Line. This pretty much confirms the DNA came down through those Ancestors.

So, given a TG with a confirmed Ancestral line, each of your Ancestors in that line, had to have the TG DNA segment in their DNA. They had to have that same phased DNA on one of their Chromosomes, in order to pass it down to you and a Match.

If you are just dying to know the ACGTs of this phased DNA, you’d need to collaborate with some of your Matches in the TG (it doesn’t necessarily need to be the ones with the CAs – they ALL should have the same phased SNPs). From your and your Match’s raw DNA data, compare all of the SNP pairs from the start to the end of the TG, and, for each position, list the single SNP that is the same.

Most of us won’t do this, but we can be content knowing that the data is phased in the TG and is identified by the Matches with specific shared DNA segments. We know the Chromosome, the side, the start position and the end position – the phased data is locked in.

 But that’s not all…

Each of our Matches in a TG almost certainly got a different DNA segment down their Ancestral line – often starting sooner (on the Chromosome) or ending later. Their TG would be different – usually adding some Matches to their TG and not including some of the Matches I had in my TG – and many of the same Matches will still be included. In effect their TG is “offset” some from mine. And their TG is also phased data. And each such Match TG may add more to the phased data of the Most Distant Common Ancestor, and often some of the intermediate Ancestors, depending on where the Match ties into your line.

And that’s not all, either…

Most of our Ancestors are linked to multiple TGs. I have 372 TGs that cover my DNA. That means, on average, about 1/4, or 93 of my TGs come through each of my grandparents. Put another way, my TGs would “cover” about 1/4 of the DNA of each of my grandparents – with my data alone, I could determine phased data for 1/4 of each of my grandparents. Even each 3xG grandparent would average 10 TGs.

And there is more…

My siblings and cousins have DNA that I don’t have, from each of my Ancestors. Their TGs could document more phased data in my Ancestors

There is a limit…

Generally, a parent, or any Ancestor, does not pass down all their DNA to their children – some is lost, forever.

So, quite a journey…  And like most journeys, we need to take it a step at a time. The first step is to Triangulate your own genome. And then work on linking each TG back to a CA (and thus to the Ancestral Line down to yourself).

[14A] Segment-ology: Phasing Your Ancestors’ DNA? by Jim Bartlett 20220420

5 thoughts on “Phasing Your Ancestors’ DNA?

  1. Thanks Jim, another excellent simple explanation. I have a couple of problem maternal 2GGP’s and I’m on the verge of a discovery using similar methods. I have been identifying all the TG’s for my genome and my Mum and her 3 siblings. Like you I have a master sheet which I find easy to access (I use GDAT). My preference for DNA Painter is to paint ancestors. I’ve visually phased my Mum and her siblings so I’m painting my 4 maternal G/GP’s. Then chasing down relevant segment areas as priorities if I haven’t identified TG’s in those areas to split them between my MGGP’s parents = 2 x 2GGP’s. This is exposing the 2 problem G/GF’s – a lot of work but after many years I’m finally getting somewhere! I agree with you about the small segments we lost at AncestryDNA, luckily I preserved mine and am constantly coming across some of those old ones that provide a key clue for some of my clusters.

    Liked by 1 person

    • Ps – plus add other cousins known TGs to the same ancestor DNAP map building up the genome for the ancestor as you are suggesting. At best I think I can build 25-37% for each depending on how recombination plays out.

      Liked by 1 person

    • Thanks for your feedback – there is a lot we can do with our data… and those smaller segments will help find Common Ancestors beyond your Target 2GGPs. Recently one of my saved 7cM Matches finally posted a Tree, and she has the mtDNA of one of my toughest Brick Walls – she was surprised when I offered share the cost of the mtDNA test (we had only just met by message). I assured her it was OK.

      Liked by 1 person

  2. I am intrigued by the SideView technology recently announced by AncestryDNA. I have been consistently identifying my relation to each DNA match as I identify them and now it is paying off. The phased ethnicity is the first application coming from SideView. Do you have any insight into what is coming next from SideView? Will Ancestry identify these triangulation groups for us automagically (without identifying the actual segments as usual)?
    Where segment data is available, I plot it on DNA Painter. I have ~ 88% / 11939 segments painted. I am at a point where I have a deep felt need to complete my Visual Phasing so I can fine tune my guesses on my DNA matches plotted on my chromosome map. My English ethnicity is evenly split between my paternal and maternal and I have likely misplaced many of those segments.
    I am hoping to pick up more insights at the East Coast Genetic Genealogy Conference this weekend. I’ll see you there!


    • Greg, I have no insights as to what AncestryDNA is doing. A few “improvements” I didn’t like. I found a lot of good info in the 6-7cM segments. They used to find Matches out to 8C (Circles), now it’s out to 6C with Thrulines. Messaging took a huge hit – it’s almost back now, but it appears they have lost hundreds of my messages (I used to message most Matches). Although they clearly have the capacity, I doubt they will share TGs with us.
      DNA Painter and TGs are virtually the same time – I was pretty much done with TGs (99.9%) when Painter came along. I can take my spreadsheet values of TGs and Paint them (and have), but it’s just two ways to display the same info. My spreadsheet also works as an inventory of all IBD Matches, and a repository of notes, emails, Tree URLs, competing MRCAs, GEDmatch IDs, etc. So my spreadsheet is my go-to tool. But I recommend DNA Painter to those who prefer a visual method. My spreadsheet does one thing – it tracks every bp in my genome (I started with 45 chromsomes, and my TGs fill up that space).
      See you at ECGGC… Jim


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