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Archiver > GENEALOGY-DNA > 2010-03 > 1269764031


From: "Tim Janzen" <>
Subject: Re: [DNA] 23andme cousin results
Date: Sun, 28 Mar 2010 01:18:11 -0700
In-Reply-To: <253CFBB8-15B8-4B84-81E5-A4F86DEEC8D7@earthlink.net>


Dear John,
You may want to look at the chart I created that is at
http://kquilting.homeserver.com/23andme/ref.html to help gain a broader
perspective on this topic. In my opinion, the simplest thing to look at is
the total number of cMs of Identical by Descent segments (HIRs) in 23andMe's
Advanced Family Inheritance function when attempting to look at the probable
degree of relationship when the genealogical relationship is uncertain. The
percentage of genome in common per 23andMe's Relative Finder function is
also helpful.
This is a summary of the ranges from cousins (who don't appear to
come from endogamous populations) I have seen thus far for the total number
of cMs of Identical by Descent segments (HIRs) in 23andMe's Advanced Family
Inheritance function:
First cousins: 548-1034
Second cousins: 101-378
Third cousins: 43 (only one value thus far)
Third cousins once removed: 11.5-99

I haven't seen any data for situations were the parental ancestries
are completely separated by thousands to tens of thousands of years. I do
know that the number of cMs of Identical by Descent segments for people who
come out of endogamous populations (Jews, Mennonites, etc) are higher at any
given degree of relationship than for people who don't come from endogamous
populations. In my dad's Mennonite relatives I have seen the total number
of cMs of Identical by Descent segments that my dad shares with other
Mennonites at the 5th cousin level of relationship be as high as 115 cMs.
The take home message is that one must be very careful about using
this data to determine the probable degree of relationship, particularly
with endogamous populations. Whenever possible it is best to use data from
multiple relatives rather than only comparing two distant cousins to each
other.
For siblings, the lowest percentage of genome in common per
23andMe's Relative Finder function that I have seen to this point is 45.2
and the highest is 58.02. I would be very surprised to see a percentage
under 35 or one over 65.

Sincerely,
Tim Janzen

-----Original Message-----
From:
[mailto:] On Behalf Of John Carr
Sent: Sunday, March 28, 2010 12:15 AM
To:
Subject: Re: [DNA] 23andme cousin results

David, Tim, others as well,

Are the genome wide SNP shared segments within known cousin relationships a
fraction of a percent as 23andme shows for our so called cousin matches or
are the shared segments usually larger?

Given that the whole genome SNP results for two siblings from the same
mother and father could range between having all segments match to no
matching segments, with the greater frequency of matches within sibling
relationships being closer to 50% than 0% or 100%, it would seem that there
would be a wide variation of matching across the genomes of biological 1st
to 5th cousins. While 100% matching is less likely, the occurrence of 0%
segment matches should have a higher occurrence rate than for siblings.
The relatedness of the DNA donating ancestors would surely affect the
result, a succession of first cousins being more likely to share closer to
100% than when the DNA donors shared ancestors are several generations
removed. Your studies look at situations where there are a lot of first
and second cousin parents, thus the expectation is that you will find a high
percentage matching genome frequencies within the group. I would expect
that actual 4th to 5th cousin rela!
tionships for most people whose ancestors were not first cousins would
still have a higher occurrence rate than a fraction of a percent. Though, as
I stated, there will be some with near 0% matching segments.

So, the question is, what do practical studies show? Are there sufficient
studies of three or four generations within a family to determine the modal
frequency with which SNP segments can be expected to be shared between the
grandparents and the grandchildren?

How does this vary between situations where the parents are close cousins
and situations where the parental ancestries are separated by thousands to
tens of thousands of years?

John



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