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J Man
08-24-2012, 04:10 PM
I know I have asked about this before but it still interests me and I don't feel like I have ever really gotten much of a clear answer to it. I have been looking at the new Relative Finder features on 23andme and Finland pops out on the map as the country where I have the most ''cousins'' listed when it comes to Europe. This is interesting as I am only 25% Finnish by known ancestry. On Ancestry Finder when I lower the bar right down to 5 cM Finland is still on top for me as well when I exclude North America of course. I keep getting new Finnish ''cousins'' on Family Finder over at FTDNA as well. The thing is the vast majority of these Finnish matches are in the 5-7 cM range but the interesting thing is that I have so many of them and quite a few of them are multiple chromosome matches. By that I mean I will share two separate segments on different chromosomes with the same person and this situation is only found much really with my Finnish matches. Also these multiple chromosome matches are almost all small in the 5-7 cM range. So my questions are what do these matches mean?

1.) Are they even true IBD-half segments or are they IBS?

2.) Why do I only get these multiple small segment 5-7 cM chromosome matches with Finns only?

3.) Why do my Finnish matches tend to drown out matches from the other sides of my family when I am only 25% Finnish?

4.) Do these matches have any meaning really for my overall genetic ancestry?

ilmari
08-24-2012, 11:06 PM
4.) Probably because more Finns have tested [in general].

AJL
08-24-2012, 11:31 PM
Yes, I think it's probably that if you have Finnish ancestry you are almost guaranteed to match other Finns on most of (the Finnish parts of) your chromosomes because many have tested, and because there was fairly prolonged population isolation.

For the small segments, I would say they are probably real for the most part, however it may become more difficult to determine how far back they are because you may find any number of potential ways that DNA block could have been inherited far back. This affects even colonial Americans, and of course groups like Ashkenazim, Québécois, and Anabaptists. At a certain point these segments are in a way somewhere between genealogical and population identifiers.

For perspective, you and I are related through your mother and my mother: as we've discussed, probably an Irish or Scottish link and quite possibly through my mother's mother's mother's father. My maternal second cousin who also descends from this line does not match you or your family, but shares with me English, Irish, German, Welsh, and Scottish ancestors. He has tested with Family Finder and has a preponderance of of colonial US matches from his father's side, and even some very distant Scandinavian matches from his Icelandic grandparent, but his only mutual match with me out of my nearly 700 and his 125+ matches is one Irish guy. On the other hand, I have nearly 300 (!) mutual matches with my second cousin once removed, who is entirely Ashkenazi.

I think this speaks to the effects of database composition as well as how DNA can amplify itself quickly in a small group of successful but isolated reproducers.

J Man
08-26-2012, 11:22 PM
Thank you for the answer and thoughts both of you. :)

I believe like you mention AJ that these numerous small 5-7 cM segments that I share with Finns from all over Finland probably relate back to the population bottleneck (s) that Finns have experienced. A member of these other forums that I have been posting on gave a great answer on this topic.

''It's pretty simple actually, and has to do with two things: a) the threshold of IBD detection at 23andMe is 5 cM, and b) the Finnish genetic bottleneck.

If 23andMe went down to 0.5 cM, Finland would drop on the list for many people.

The reason for this is that most Europeans have many more genetic ancestors than Finns do, so the signals from these ancestors are chopped up into small bits, which are usually well under 5cM.

However, as already mentioned, Finns have very few genetic ancestors in comparison, so the segments from these people in Finnish genomes are larger and more numerous, and in fact, even after they're broken up, they're often reconstituted again from smaller segments from both parents.

In other words, having lots of "Finnish" segments doesn't mean significant Finnish ancestry, but rather a lack of diversity in the very distant Finnish "genetic cousins" we have. Indeed, if you do have recent Finnish ancestry, then you'll know about it, because your AF/RF lists at 23andMe will be horribly skewed. It's similar to having Ashkenazim ancestry.''

R. Walker
09-06-2012, 03:41 AM
Well, and then there's the case of our 100% Northern Italian cousin. Switzerland is high on her list. But it is due to me and my brother who are her closest relatives. She has only a few other Italian matches, and after three years only has 77 total RF matches. She has no Swiss, but being related to us, and our many Swiss segments, she gets an artificially high result.

Another thing is endogamy. (sorry about the spelling) Our Swiss ancestors (and Italian ones apparently) lived in the same areas for hundreds and hundreds of years. We (bro and I) have lots of small links with people with Andbaptist ancestry. We are probably related back to the 1500s or 1600s, before they left Switzerland and traveled around Europe before coming to the US. I expect they were relatives of our grandfather's family from Canton Bern. Grandpa didn't come to the US until 1870 when he was a teen. So these are probably ancient, small IBD links. Again, we'll never find them.