What's the longest surviving large segment you've seen?

[sjadelson]

Hello,

I've just received an intriguing match in FF, 163.5cM total, and a 49.5cM longest segment. Two others over 10cM: 18cM and 13cM. Predicted second cousin.

While I am Ashkenazi, and I've seen more than my share of false positives (or perhaps just overly aggressive prediction), I've never personally seen a continuous segment this large outside of parents, even with known 2nd cousins. My question to the group is: Is 49.5cM a reasonable large value suggesting close relation, or have you seen segments really last beyond second cousins without getting recombined into something smaller?

[NYMark]

I have not seen any segments of that length in my results or in those of my fully Ashkenazi half-siblings. It seems like it would be a likely sign of a pretty close relationship, especially given the two other sizable ones.

There is an additional possibility, however. The segment could be a composite of paternal and maternal, something I don't think can be determined without phasing or a parent's results for comparison. I have seen this on a >29 cM segment where a >5 cM block in the middle of the long block matched the person's mother, while the rest did not. The father had not tested, so I couldn't tell whether the 5 cM block was common to both parents.

Hello,

I've just received an intriguing match in FF, 163.5cM total, and a 49.5cM longest segment. Two others over 10cM: 18cM and 13cM. Predicted second cousin.

While I am Ashkenazi, and I've seen more than my share of false positives (or perhaps just overly aggressive prediction), I've never personally seen a continuous segment this large outside of parents, even with known 2nd cousins. My question to the group is: Is 49.5cM a reasonable large value suggesting close relation, or have you seen segments really last beyond second cousins without getting recombined into something smaller?

[sjadelson]

Thanks, NYMark. I know about recombining into apparent longer segments, although even when I've seen that occur between my parents and myself, it's never been on the order of this one.

I'm working on my match to see if there are other family member who can test and help narrow it down (and, I hope, reconfirm) from her side. Unfortunately, testing her parents is not an option.

[MMaddi]

This does seem like a good possibility to be something like a 2rd cousin or an unusually robust genetic connection to a 3rd cousin or 3rd cousin once removed.

My example is a paper trail documented 2nd cousin, once removed. We share a total of 110 cM, with the largest segment being 41.8 cM and three other segments of 10 cM or more. This is at 23andMe, where they only count segments of 5.0 cM or more. If your match is at FTDNA, the total cM may be inflated since Family Finder counts segments under 5.0 cM.

How many segments do they count for this match? My 2nd cousin, once removed and I share 5 segments, with the smallest being 8.3 cM.

[sjadelson]

FWIW, the total of 5+ cM is 8 segments totaling 114.2 cM

[MMaddi]

FWIW, the total of 5+ cM is 8 segments totaling 114.2 cM

And my 2nd cousin, once removed and I share 5 segments of 5+ cM totaling 110 cM. That's very similar, although my segments, other than the largest segment, are larger on average than your segments. So, using my match as a guide, you're probably looking at a 2nd cousin, once removed, with a possible range of 2nd-3rd cousin.

I should add that my sister shares less with this 2nd cousin, once removed than I do. The cousin and my sister share seven 5+ cM segments totaling 82 cM, with the largest being 19.5 cM and three others being 10+ cM. I think I received more of my great-grandparents' (common ancestor with the 2nd cousin, once removed) DNA than my sister did. So, it may be that my shared DNA with the 2nd cousin, once removed is on the high end of the amount of sharing usually seen in this relationship.

[hansonrf]

I have 314 total and 72 longest with a second cousin. Not Ashkenazi.

[nwgen]

My uncle has a match that couldn't possibly be any closer than a 4th cousin. Largest segment 66.5 cM.

[hansonrf]

Uncle 1750 total and 115 longest.

Keep in mind the longest possible is a full chromosome 1 (the longest chromosome) and that is 286 cM avg.

[sjadelson]

My uncle has a match that couldn't possibly be any closer than a 4th cousin. Largest segment 66.5 cM.

Interesting one, nwgen.... What's the total cM and total 5+ cM in this case, if it's not difficult to look up?

[nwgen]

Interesting one, nwgen.... What's the total cM and total 5+ cM in this case, if it's not difficult to look up?

84 cM total. 1 block over 5cM and it is 66.5 cM and 16000 SNPs.

[MMaddi]

84 cM total. 1 block over 5cM and it is 66.5 cM and 16000 SNPs.

This match is not of the same type as the ones sjadelson and I have posted about. There's only one block above 5 cM and it's very large. I think it's safe to say that 2nd and 3rd cousins almost always share more than one segment above 5 cM.

This seems very unusual, especially since the relationship is 4th cousin or more distant. It almost defies the laws of probability involved in DNA recombination.

[nwgen]

This seems very unusual, especially since the relationship is 4th cousin or more distant. It almost defies the laws of probability involved in DNA recombination.

Indeed it is very large and unusual. Ironically, I'm pretty certain this match is along my uncle's paternal line. You are a YDNA match along this same line, though not a close match. Six degrees of separation....

[MMaddi]

Ironically, I'm pretty certain this match is along my uncle's paternal line. You are a YDNA match along this same line, though not a close match. Six degrees of separation....

You're saying that I'm a yDNA match along your uncle's paternal line? This is news to me, since I have only one match at the 12, 25, 37 and 67 marker levels in the FTDNA database. Is your uncle's paternal line Italian?

[Javelin]

84 cM total. 1 block over 5cM and it is 66.5 cM and 16000 SNPs.

I've looked at all my various tested known cousins and there are no cases precisely like this. I even have a set of first cousins three times removed, and they have way more blocks than this though none as long.

This seems most similar among my kits to a set of second cousins twice removed, who share a largest block of 44 cM and 11,700 SNPs with no additional blocks over 5 cM. Thus I am going to guess your match is something like a second cousin twice removed or a third cousin once removed.