Common ancestors between two matches

[Pirlino]

I have two cousins in the relationship range of 3rd-5th cousin (cousin A: 51 Shared Cm and longest block 14// cousin B: 42 Shared Cm 14 longest block). Same chromosome, same position ... I'm pretty sure I've managed to find their common ancestors in a couple that married at the end of the 17th century. Unfortunately I can't find a line in my personal family tree that would lead back to this couple.

My question is, if I would want to work my way to recent times and maybe find a possible candidate ancestor (maybe a NPE event) should I look at all the children this specific couple had and work my way down to recent times. Or...

[MMaddi]

Depending on when you were born and the average generation length in your tree, a 5th cousin would probably involve common ancestors who were having children in the late 18th century or even early 19th century. You've found common ancestors for these estimated 3rd-5th cousins at the end of the 17th century. But those common ancestors aren't found in your tree.

Since these common ancestors lived in a more distant time than normally expected for 5th cousins, the estimate of 3rd-5th cousin may be too optimistic. This is not uncommon. You and these matches may have inherited more than the expected amount of DNA for distant cousins and are being estimated as more closely related than you are in reality.

Also, if the lines where you found the common ancestors for the two matches come from an endogamous population with lots of intermarriage, that also skews the relationship estimate. Long periods of intermarriage in a relatively small population cause more than expected DNA to be shared among distant cousins. Examples of endogamous communities are Ashkenazi Jews, Colonial Americans and French Canadians. Does the line where these two matches share the common ancestors involve any of those ancestries? If so, do you have any lines in your tree that have the same endogamous ancestry as the matches have?

If you can isolate in which line your common ancestors with these two matches may be found, that narrows down your search. Now that you've found common ancestors for the two matches, do those common ancestors match an ancestral location found in your tree in that time period or one or two generations previous to those common ancestors? If so, try looking further back in your tree and the matches' trees for the common ancestors for you and the two matches.

[Pirlino]

I was born in the 90's. My grandfather's line is a huge brick wall, so it could be the match is on that side. Although on my grandmother's side I've got a couple who married in the same village at the end of the 17th century where this common couple lived and had children. The country I'm talking about is Belgium, don't know if you could call it an endogamous population.

[MMaddi]

I was born in the 90's. My grandfather's line is a huge brick wall, so it could be the match is on that side. Although on my grandmother's side I've got a couple who married in the same village at the end of the 17th century where this common couple lived and had children. The country I'm talking about is Belgium, don't know if you could call it an endogamous population.

I was born in 1953 and most of my 4x great-grandparents were having children in the late 1700s/early 1800s. You're more than a generation younger than me, so I would expect that your 4x great-grandparents were having their children in the first half of the 19th century, perhaps a little later.

Belgium is not a particularly endogamous society to my knowledge. It's between the British Isles, Germany and France and I think it would be subject to many migrations/invasions because of that. That would tend to limit the closed type of society that might lead to endogamy.

Given the estimated relationship by FTDNA, I would look to your brick wall grandfather's line. You probably know where he was born. Contact the two matches and see if any of their ancestors were living in the same city or area as where your grandfather was born around the time he was born. That might give you some clues about his unknown parents, if the common ancestors for this match are in his line.

[MMaddi]

I was born in the 90's. My grandfather's line is a huge brick wall, so it could be the match is on that side. Although on my grandmother's side I've got a couple who married in the same village at the end of the 17th century where this common couple lived and had children. The country I'm talking about is Belgium, don't know if you could call it an endogamous population.

I meant to add something to my previous post, but the 15 minute editing period ended. Here are some additional thoughts I have about what you posted, which I've quoted.

However, the fact that you have paper trail ancestors in your tree who married in the same Belgian village where your two matches have common ancestors should not be overlooked. Maybe it's the case that there was some sort of endogamy going on in this village, perhaps due to a religious denomination that practiced marriage within the community, that would account for there being more distant common ancestors than the shared DNA would seem to suggest.

So, don't entirely ignore that possibility.

[Pirlino]

You're right about my 4th great-grandparents having children in the early 19th century. I think back in those times in rural Belgium people didn't travel that much around so it was highly probable someone married there 3rd cousin or something (It's noticeable in the parish registers when the priest talks about 'dispensation', needed when there was some kind of blood tie)

But purely theoretically speaking, if this couple would match my grandfather's side and I would trace back all the possible descendants of this couple, would I 'stumble' on my grandfather?

[MMaddi]

You're right about my 4th great-grandparents having children in the early 19th century. I think back in those times in rural Belgium people didn't travel that much around so it was highly probable someone married there 3rd cousin or something (It's noticeable in the parish registers when the priest talks about 'dispensation', needed when there was some kind of blood tie)

Truly endogamous communities are created when there's a small founding population and extensive intermarriage over at least a few centuries. I'm not sure that the Belgian town you're describing would meet those criteria.

I think it can sometimes occur in isolated small towns to a certain degree. My maternal grandfather was born in a mountainside town in southern Italy that only has 1,000 people today and probably never had many more people than that. Because of the centuries of intermarriage in the community, I've had several DNA matches who also have ancestors from that town. A researcher had access to the church records for that town, going back to about 1600, and has put together a master tree for the town. So, I've been able to find in the paper trail that these DNA matches are related to me in several different ways, in one case 7 different ways. That's how endogamy in larger communities, like Ashkenazi Jews, works, although on a larger scale.

If the Belgian community has something like that going on, it may be where you could find the common ancestors with these matches.

But purely theoretically speaking, if this couple would match my grandfather's side and I would trace back all the possible descendants of this couple, would I 'stumble' on my grandfather?

Potentially, yes. But it's very difficult to do that without finding a truly closer cousin related to you through the brick wall grandfather. By closer cousin, I mean a 1st or 2nd cousin, who would give you a strong resolution to research that line. By "strong resolution," I mean a lot of shared segments, which you could then triangulate with less close matches. That allows you to compare trees which is helpful in narrowing down in which line the common ancestors are likely to be found.

You could get a series of small clues through multiple 3rd-5th cousin matches that allow you to break down a brick wall. Think of a jigsaw puzzle, which you can solve when you get enough of the small pieces to see the larger picture.

[Pirlino]

Truly endogamous communities are created when there's a small founding population and extensive intermarriage over at least a few centuries. I'm not sure that the Belgian town you're describing would meet those criteria.

I think it can sometimes occur in isolated small towns to a certain degree. My maternal grandfather was born in a mountainside town in southern Italy that only has 1,000 people today and probably never had many more people than that. Because of the centuries of intermarriage in the community, I've had several DNA matches who also have ancestors from that town. A researcher had access to the church records for that town, going back to about 1600, and has put together a master tree for the town. So, I've been able to find in the paper trail that these DNA matches are related to me in several different ways, in one case 7 different ways. That's how endogamy in larger communities, like Ashkenazi Jews, works, although on a larger scale.

If the Belgian community has something like that going on, it may be where you could find the common ancestors with these matches.

I think you're right MMaddi, about the unlikeliness of this village creating an endogamous community although I don't exlcude the possibility of sharing more common ancestors with these matches, maybe further back in time.

Potentially, yes. But it's very difficult to do that without finding a truly closer cousin related to you through the brick wall grandfather. By closer cousin, I mean a 1st or 2nd cousin, who would give you a strong resolution to research that line. By "strong resolution," I mean a lot of shared segments, which you could then triangulate with less close matches. That allows you to compare trees which is helpful in narrowing down in which line the common ancestors are likely to be found.

You could get a series of small clues through multiple 3rd-5th cousin matches that allow you to break down a brick wall. Think of a jigsaw puzzle, which you can solve when you get enough of the small pieces to see the larger picture.

You're definitely right about the need for a closer cousin, unfortunately these two matches are the only matches who match each other and me, on the same chromosome and on the same location of the chromosome. I also think I don't really have that much matches in general (245) and certainly a lot of them are not even close to being 'a close match'. Add up the fact that most people on FTDNA don't even have a family tree going back to the 18th century, it surely doesn't make things easier.

But this thinking process is what I like about using DNA for genealogy