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