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From: Jim Bartlett <>
Subject: [AUTOSOMAL-DNA] Our atDNA matches are our cousins
Date: Tue, 15 May 2012 17:45:43 -0500 (CDT)


Ann,

FTDNA has posted the probabilities of a specific relative passing their
filter and showing up as a match with us.

Parents and 1st cousins - virtually 100 percent
2nd cousins about 99 percent
3rd cousins about 90 percent
4th cousins about 50 percent
declining percentages beyond that

And so some people literally throw in the towel at 5th cousin and beyond.
But that's wrong! These are percentages of a specific person that we might
select for testing. And it's true that if we get a 5th cousin to test,
he/she probably won't show up as a match.

BUT, we have many, many 5th cousins; and when you multiply the large number
of 5th cousins times the small percentage that a specific one will match,
the odds are higher that you'll get a match with a 5th cousin that with a
4th cousin. You'll actually get more 6th cousins than 5th - I think it
probably levels off at about 7th or 8th cousins.

Think about how many matches you have, and the fact that you and your match
would probably recognize a 3rd cousinship. So those matches must be more
distant.

Look at it another way. Look at the number of shared cM for a close match
that you do have. How many of your matches share that amount of atDNA with
you - very few. Most of our matches are in the 8-12cM range. Well at that
range they are probably 7th cousins or so.

If you believe the 1-in-a-million type of statement, then you must be
thinking that virtually none of the matches reported to your are related -
that's silly. Most of them are related, we just can find the relationship.

IMO

Jim Bartlett

On 05/15/12, Ann Turner<> wrote:

[1]http://genealogy.about.com/b/2012/05/15/autosomal-dna-testing.htm?nl=1
I want to post something sometime in that very interesting long thread
about GEDCOM files, but this article mentions finding close connections at
all three autosomal testing services. Quoting Kathy Johnston from that
thread
"5. Even if I go back 4 generations, am I likely to randomly match any true
3rd cousins? NO!! because you need a database of over a million to find
that one in a million 3rd cousin. Even if you have more than 300 3rd
cousins alive today, there are over 300 million people living in the U.S.
Does FTDNA test that many people? No."
I have got to think more about the statistics here. Obviously we are going
to hear a disproportionate number of success stories, just because they're
so much fun. But it strikes me that we are hearing too many stories for the
one-in-a-million number to hold true. I have a 2nd cousin once removed at
23andMe who tested because his daughter worked at Illumina and got a free
kit. I tested a known cousin at FTDNA, and when she got her results back,
she recognized the name of a 3rd cousin once removed from another side of
her family.
I do think FTDNA overcalls the 3rd cousin bin, relying too much on the
length of the longest segments. Confirmed 3rd cousins tend to have multiple
segments over 5 cM in length. But why are we seeing so many stories about
confirmed close cousins? Just thinking out loud here -- maybe this is
related to the "birthday paradox":
[2]http://en.wikipedia.org/wiki/Birthday_problem
Ann Turner

References

1. http://genealogy.about.com/b/2012/05/15/autosomal-dna-testing.htm?nl=1
2. http://en.wikipedia.org/wiki/Birthday_problem


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