Breaking Down Brick Walls

There are several methods to break down brick walls, including the bio-parents of an Ancestor. I think of two groups of methods:

1. Blind Luck

2. Use a process

I don’t mean to be flippant when I say “Blind Luck”, but luck plays a large part in some cases. Your list of Matches may include one who shares 2600cM of DNA – that would be a full sibling. Or even a 1750cM share – which would indicate a grandparent, grandchild, half sibling, Aunt/Uncle or Niece/Nephew. Wow. That could get you into your bio family pretty quickly. So always look at your closest Matches – maybe you’ll be lucky and find some really close relatives to work with.

But most of the time we are not that lucky. And sometimes the brick wall is somewhat further back.

I wrote a blogpost: Let the Matches Tell Us the Cluster Common Ancestor. You can review it here.

In this blogpost I want to review a more generic process. Depending on how many of the DNA testing companies we use, we can get from tens of thousands to over a hundred thousand DNA Matches. That’s a lot of data to sift through, so let’s see if we can narrow it down. Here is a handy chart I often use:

The first three columns are almost trivial – they are easy to produce and can be extended upward, if need be. Say you have 10,000 DNA Matches to work with – the 4th column shows you, roughly, what percent of those Matches will come to you through each one of your Ancestors. You already know that each parent gave you half of your DNA – except for special situations, we would expect roughly half of all your DNA Matches to come from each side. And by extrapolation about 1/4 to come from each grandparent, etc.  That’s still a lot of data to work with.

So the next part of this generic process is to look at the cousins you’d expect from each generation – such as 2nd cousins from a Great Grandparent. And from the 5th column we see that 2nd cousin share an average of 229cM – or a range of 41 to 592cM. This means if we set a threshold of 200 or 300cM, and used only Matches over that threshold, they’d mostly be 2nd cousins. At AncestryDNA I only have 4 Matches over 200cM. That’s a start – to look carefully at those 4. At a threshold of 100cM, I have only 28 Matches at AncestryDNA.

My point here is that we need to start with the Brick Wall generation, and go one generation back – to the parents of the brick wall. This is based on the premise that if a lot of folks had identified my Brick Wall Ancestor (BWA), I’d probably have found him or her by now. So there is something about my BWA that has blocked me for many years. I need to find the parents and work down. Very similar to finding a bio-parent.

So I look at my chart, above, and select one generation above my BWA, and note a range of cMs that should include appropriate cousins.

The next step is to group those DNA Matches – by segment Triangulation at most companies and by clustering at AncestryDNA. Depending on the threshold you choose this grouping can be done by hand or through programs at Genetic Affairs, DNAGedcom Client or Shared Clusters (see my review of these programs, here).

It really helps to have a known close cousin in a Cluster or TG – to act as a pointer toward your BWA.  Otherwise, we need to analyze each one of the groups. As a Segmentologist, I’m all for analyzing each of my TGs, but I also pay particular attention to those that point in the direction of my BWAs.

The idea is to find enough Trees in a group to find a Common Ancestor (CA) for that group. This CA is then, very likely, to be an Ancestor of the BWA. The final step is to find the descendants of the CA, keeping mindful of probable birth year and place of the BWA.

This kind of process is often not easy or straightforward. But with enough Matches (test at all four companies and upload to GEDmatch), there should be enough data to significantly narrow down the search.

[19K] Segment-ology: Breaking Down Brick Walls by Jim Bartlett 20210203