Does the DNA Come from the MRCA Father or Mother?


A Segment-ology TIDBIT

Bottom Line: We cannot tell from a single Match; but there are at least three ways to figure it out.

SETUP: One cousin, or even several cousins, who share a Most Recent Common Ancestor (MRCA) with us. The “Common Ancestor” is really a Common Ancestor “Couple” – usually a husband and wife. You descend from one child of this couple, and the matching Cousins descend from one or more of their other children. The question comes up, which parent passed down the DNA segment to us? From this data, we cannot tell which parent passed down the DNA. All we know is that one or the other parent passed it down. I know of three ways to figure this out – maybe you know of additional ways…

1. Grouping *to* the parents’ parents. One DNA Match-cousin back to the MRCA won’t do it. We need a group of Matches – at least one back to the MRCA, and other Matches – often with smaller shared segments – that go back another generation. The group can be formed via Clustering (grouping Shared Matches); or segment Triangulation (or DNA Painter). The goal is to find a Match in the group who is a cousin at least one generation farther back than the MRCA. This will almost always tell you which side of the MRCA the DNA came from. Note: there is a very low chance, this might not work, but finding more than one more distantly related cousin on this line adds insurance. It is best to do this with segment Triangulation which represents a single DNA segment going back, but Clustering works, too. As you find more and more Matches with the MRCA, eventually (with smaller cM segments) they will break into two groups, one for each parent in the MRCA. Each of these groups will be based on a different DNA segment (a different subset of a Triangulated segment, involving smaller shared segments).

Note this is really a subset of Chromosome Mapping, and/or Walking the Ancestors Back.

2. Different partners. If one of the parents had children with a different partner (married or not), and the other Matches descend from this other partner, then you know the shared DNA segment had to come from the Ancestor who had multiple partners – the same DNA could not have come from different partners.

3. Differing ethnicities. If the two parents in an MRCA have very different ethnicities overall (or you can tell the ethnicity is different for this specific shared DNA segment (usually a TG segment), then a review of the Matches’ ethnicities might indicate which one passed down the DNA.

As with many things in genetic genealogy, the DNA may throw you a curve ball. I’m old enough to remember the Mickey Mouse Club on TV – and the “anything can happen day”. As you continue on your genetic genealogy journey, the evidence will mount. It should all point to the same results – which match your unique Ancestors and DNA segments.

Feel free to use this blogpost as a way to answer this recurring question on-line.

Amended slightly to call out the different partners as one of three ways.

If you have a favorite method of figuring this out – please post in the comments.

[22BS] Segment-ology: Does the DNA Come from the MRCA Father or Mother? TIDBIT by Jim Bartlett 20230515

How to define the Most Recent Common Ancestor (MRCA)


There are, generally, two discussions about this.

1: One is the MRCA is really a couple – and both your and your DNA Matches descend from this MRCA (couple). You descend from one child of the couple, and your Matches descend from other children of the couple. We may not know which individual in the couple passed down the DNA we share, but we do agree it’s from one or the other.

2: The other discussion is that the child of the couple that you descend from had to have the DNA segment and is then *the* MRCA (singular). But, at least in my mind, this MRCA (singular) is not the MRCA of the Matches, and things get a little complicated trying to have a discussion among the Match-cousins. The various MRCA (singular)s are not “common” to the various Matches.

In Segment-ology, I use MRCA to mean MRCA (couple). This couple is common to Matches who descend from them.

[22BR] Segment-ology: How to Define the Most Recent Common Ancestor by Jim Bartlett 20230513

How To Detect False Matches Theory


Small-segment Matches (<15cM) with conflicting Shared Matches (no consensus) are probably false.

This Observation/Theory comes from two directions:

1. Science and observation tells us that (A) almost all Shared DNA Matches (Matches) >15cM are true; and (B) to varying degrees, some Matches below 15cM are false. There is a distribution curve which has a small percentage of 14cM Matches as false, down to about half of 7cM Matches are false and larger percentages of even smaller segments are false. It’s hard to sharpen this marshmallow of data, so please grasp the overall concept that as the cMs decrease below 15cM, the percentage of false Matches increases to areas where most Matches are false.  And don’t be confused – you can have a true Common Ancestor with a false Match – it’s just that there is no DNA link (or “proof”). In fact, most of our true 4th cousins (4C) and greater will not be DNA Matches at all. Another point is that there is no such thing as “partly true or false” – the shared DNA segment is all true (from a Common Ancestor) or all false (not from a Common Ancestor). Please don’t go down the “rabbit hole” that *part* of a small segment may be true – we are already down to small segments, even tinier segments aren’t a step in the right direction, unless you have a very, very special case. Not the thrust of this blogpost!

2. My observations of Matches under 20cM shared segments as I search/analyze this huge group (80,000) for Matches with a BROWN Ancestor – also a huge subset (thousands) of my under 20cM Matches.

Background 1: I’ve done the “homework”. Using the Walking-The-Clusters-Back process (starting here), I was able to identify hundreds of Clusters to specific parts of my Ancestry. Almost all to a parent; 98% to a grandparent; roughly 90% to a Great grandparent; etc. out to some that were tagged to 7xG grandparents (8C level). Overall, I have in my AncestryDNA Notes for almost all over-20cM Matches, *some* indication of their line in my Ancestry. Most of these have also been tagged to specific Triangulated Groups (TGs). Part of my analysis of the under-20cM Matches is a check of their over-20cM Shared Matches. Most have some Shared Matches. Sometimes, an under-20cM Match will have many Shared Matches in consensus – most in the same Cluster or TG. Sometimes they will not.

Background 2: My BROWN Project. For this project, I filter my Match List by Maternal (the side for my BROWN line), a cM range (ie: 12 to 13cM); and the BROWN surname. I then look at each Match, and put something in the Notes box…

Background 3; Keeping Track.

[“Good” Matches] I’ll add (ie: impute) the Cluster/TG to the Notes for each concensus Match. Note: some of these will be a consensus on the BROWN line; some will be a consensus on some other line. I also add a Note about the oldest BROWN I’ve found for each of these Matches, if it’s likely to tie to my BROWN line – a judgment call.  

[When I’m finally done ferreting out all of these lines in a consensus BROWN Cluster and/or a good probability of a link to my Tree, I’ll look at the number of Matches in each family group, and really dig into the research – this is sort of like collecting clues in Quick-and-Dirty Trees to see what’s worth pursuing.]

[“Iffy” Matches] Some Matches don’t neatly fall into the Good or Bad category – I look at them and usually build their Tree back far enough to determine if a tie-in to my Tree is possible – judgment call.

[“Bad” Matches] I add a Note to Matches with no Shared Matches: ”SM:0”. And I add a Note to Matches with various, conflicting Shared Matches (often on both sides): “SM – var”.  

Observation 1: Research/Tree building for Matches with “SM:0” and “SM – var” Notes almost always went nowhere. They had a BROWN Ancestor (a search filter), but their BROWN Ancestor was from England or Scotland or New England, etc. Clearly a very low probability of linking to my BROWN line in Colonial VA. For these Matches I added “X BROWN” in the Notes to remind me they’ve been looked at and discarded.

Observation 2: Finding a Match with consensus Shared Matches that had known/suspected BROWN Clusters/TGs was a BINGO! They *almost always* had BROWN Ancestry that linked to mine – whether it was in their Tree or in the Q&D Tree I built out for them. Some of the iffy Matches which had a few Shared Matches with favorable BROWN Clusters/TGs, but not a consensus, turned out to link to my BROWNs. Some did not.

In my BROWN project, I’m done with the 11cM to 19cM DNA Matches and am part way through the 10cM Matches – a lot still to do. I wanted to record my observations that DNA Matches who had a BROWN Ancestor pretty easily fell into probable/possible vs “no way” categories. And my growing belief is that Matches with “SM – var” are probably FALSE Matches – not going to be a genetic cousin on any line.

BOTTOM LINE: Matches under 15cM with various conflicting Shared Matches are probably FALSE. Certainly, to be culled out to focus on Matches who have a clear consensus of Shared Matches on one line. This is *not* a guarantee, nor does it mean all other Matches are true. But given the shear number of BROWN Matches to go through, I’m going to begin using this theory as a “TRIAGE” method. It’s the only way I can get through this BROWN Project.

[22BQ] Segment-ology: How To Detect False Matches Theory by Jim Bartlett 20230506