Archiver > GENEALOGY-DNA > 2010-11 > 1288910728

From: Jim Bartlett <>
Subject: Re: [DNA] Where does the 7cM block come from?
Date: Thu, 04 Nov 2010 17:45:28 -0500 (CDT)


Thanks for your explanations - clearly you are on the frontier, and I hope you get some data from others to help sort out what's really happening.

Of my 90 FF matches, 8 are "Close and Immediate", but only one of those is one of the 10 common ancestors I've determined. That one is a double 3rd cousin (brothers married sisters); has 126cM total including 5 large blocks: Ch 14: 26.18cM; Ch 15: 16.14 and 12.44cM; Ch 17: 18.23cM and Ch20: 8.89cM.

None of my other matches are "Close and Immediate" [6C-1 is 6th Cousin once removed]:
6C-1 has Ch 11: 11.34cM
6C-1 has Ch 5: 8.35cM (alternatively DNA may be from a different 7C-1 ancestor)
7C has Ch1: 8.07cM
8C-1 has Ch21: 10.58cM
8C has Ch10: 8.54cM (alternatively DNA may be from a different 9C ancestor)
8C has Ch2: 8.52cM (alternatively DNA may be from a different 12C ancestor)
9C has Ch1: 7.81cM (this DNA may come down one of 2 ways from the 9C ancestor)
9C has Ch9: 8.03cM 
10C has Ch 15: 8.18cM (alternatively DNA may be from a different 11C ancestor)

[the 9C with 7.81cM is ancestor of my Mom and Dad, making them 8th cousins - just found this out!]

Each of the above FF Matches and I agree on the paper trails; all but the first (Close) one are from Colonial Virginia ancestors.

You can see that I am finding paper-trail ancestors from the "bottom of the barrel" - 9 confirmed out of 90 matches...

So from my perspective, I'm wishing I had the ability to lower the bar a little (look at matches down to 7.0cM), and get some more names and emails to work with!

Four of my eight "Close" matches refuse to reply to my emails - go figure.

Jim Bartlett

On Nov 4, 2010, Ann Turner <> wrote:

There are several issues here.

Yes, 7.7 cM was selected for the threshold in the hopes that it would
identify regions that are Identical by Descent (IBD). Selecting a threshold
is a balancing act between false positives and false negatives. A false
positive would be a segment that wasn't inherited through the pedigree. We
could call that Identical by State, or IBS. A false negative would be a
small segment that was inherited through the pedigree, but it was so chopped
up that it doesn't meet the threshold.

However, reasonably large segments can make "quite a trip" down through the
generations. Each time it survives one more generation in the 50-50 lottery,
it's as if you've rebooted the clock. See the section called "Inheritance
Patterns" in It's certainly quite
possible for a single large segment to come from a single ancestor. We are
seeing what did survive, regardless of how improbable it might have seemed
in the beginning.

I may need to rethink what constitutes a large segment for Family Finder,
however. In a previous thread, I showed an example of how a smaller segments
within a child's match did not occur in either parent. One such paradox was
resolved when I looked at the child's two haplotypes. It wasn't really a
segment at all.

More recently, I looked at how many of the *individuals* in the child's
whole match list did not show up in either parent. It was 41/108, quite a
high percentage. All of them were in the "speculative" category, but some of
the longest blocks were sufficiently long that I don't think "fuzzy
boundaries" could account for them. That's when a few alleles from the other
parent *appear* to continue the long consecutive run of half-identical SNPs.
I often invoke that explanation for similar cases at 23andMe where the
child's match is close to the threshold, but the percentage of such cases is
quite a bit lower there.

This is just one instance, and it might not be a typical case. I'd be
interested in hearing some more stats from father/mother/child trios about
how many of a child's matches do not occur in either parent, and the lengths
of the longest blocks. I used an Excel spreadsheet with the COUNTIF function
-- I can send a template to anyone who's interested.

The big question is what it means if you succeed in tracing a common
ancestor for your DNA matches in the speculative category.

Ann Turner

On Wed, Nov 3, 2010 at 1:38 PM, Jim Bartlett <>wrote:

> Anne, et al,
can I assume that this
> "large" block was present in the common ancestor, and was thus included in
> the DNA passed down through every generation to me and my FF match? Easy to
> visualize for a third cousin; quite a trip for a tenth cousin. Or was this
> large block probaby from the "general gene pool" in Anne's email below?
> Or is this just the case if I can find a close 3rd to 5th cousin, and that
> same "large" block is just a happenstance, a false positive, in the case of
> a 10th cousin?
> And then back my question of several days ago - would that large 7.7cM
> block from say a 5th cousin, be more likely to come from a 3.85cM block from
> each of the parents (making up the common ancestor couple) that just happen
> to fit together in this way; or is it more likely that this 7.7cM block came
> from one or the other of the two parents that were common to me and the FF
> match?
> Jim Bartlett
> On Nov 1, 2010, *Ann Turner* <> wrote:
> In another thread, I wrote:
> On Fri, Oct 29, 2010 at 6:47 AM, Ann Turner <> wrote:
> > I suspect many if not most of the segments at the 5th cousin level come
> > from the general gene pool, not the one ancestral couple that has been
> > identified.

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