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
segment-ology 
I joined Newport Tipperary GEDmatch ancestor project. I thought if any matches it might help me to figure out where my ancesotors came from in Ireland. I thought they might have been born in Newport. I have 16 matches with three clusters. Is my next step to work with the people in each cluster to figure out the MRCA? I know for sure whom my GGG grandparents are (my mother’s relatives). The MRCA’s range from 4.8 to 7.49. However I have no definite proof as to whom their parents or siblings were. Thanks for your help and any guidance or suggestions as to how I should proceed.
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Val, You are on the right track. A hint for you – set aside (or in the backgroud), what you already know, and let your Matches tell you the Common Ancestor for each Cluster. In each Cluster most of the Matches will have the same CA. That CA may, or may not, be what you thought it should be. But work with it. Jim
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My question is with regards to what to do next with 3 clusters from my 16 matches. The GEDmatch says these 16 matches have MRCA’s from 4.8 to 7.49. Do I work with the people in each group of clusters to see where we might all have someone in common?? I know for sure who my GGG grandparents are – born in Ireland. However I have yet to confirm as to who their parents or siblings were. Thanks for any guidance or suggestions as to how I should proceed. I have joined Newport Tipperary ancestor project as I thought this is where my GGG grandparents might have been born and the above matches and clusters are from this group specifically.
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Val, YES! Get your Matches to tell you who their CA is. Jim
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The mother of a DNA match was adopted. A Leeds Chart was created of the match’s maternal side to get two groups, one for her mother’s father and the other for her mother’s mother. How can we determine which of these two groups is the mother’s maternal and/or paternal side?
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I cannot. Build the two Trees, and determine who from one married someone from the other. Jim
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Thanks for this list. I have done both 1) and 2) with several matches and any new matches that turn up on those lines are easier to evaluate. But, I have a situation among multiple 4th cousins that frustrates me for assigning a single MRCA because they are DOUBLE 4C’s. My 2nd G-GPs had siblings who married each other. Is there a way I can use this pile-up of Db’l 4C’s to identify which set of 3rd GP’s contributed the DNA? Is there something I could do with the 4C’s who are not doubles but are on these lines? Or do I need to find a 5C who triangulates with me and a Db’l 4C?
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Barb – I think the only solution to this will be Chromosome Mapping and trying to devine the difference by association with adjacent segments. Maybe one of our blog readers has a method that would work. Jim
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I have two matches who share no dna with each other. Our trees tell which is my 2nd great grandfather & which is my 2nd great grandmother as their ancestor is a sibling of each ( in my tree & their’s.)
Hope this is enough evidence. I am just about to try dna painter for the first time, so can use the individual surnames ( I hope)?
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Joyce – so one of your two 2nd great grandparents had a second partner? I other words your 2nd great grandparents were not both the ancestors of your Matches? Jim
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No. One match rs descended from a sibling of my 2nd grandmother & the other match is descended from a sibling of my 2nd grandfather.according to genealogy. This is from.one ancestral couple
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So for dna painter, I will have to go one generation up for each 2nd gr grandparent & put in those names?
I have just realised! Silly me!
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It is possible to isolate the MRCA individuals in the case of half siblings where they share only one of the parent couples. Where they mesh is probably attributable to the common parent.
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Pat, Yes! that is essentially the Exception – one Ancestor/two partners – the DNA has to come from the commom parent. Jim
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