Archiver > GENEALOGY-DNA > 2010-03 > 1269760263

From: John Carr <>
Subject: Re: [DNA] 23andme cousin results
Date: Sun, 28 Mar 2010 00:15:14 -0700
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In-Reply-To: <>

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?


On Mar 27, 2010, at 9:28 PM, David Faux wrote:

> John,
> In summary this exercise has been wildly successful. There is something
> almost "magical" seeing a segment of DNA on an autosome shared with someone
> selected because they are a known cousin and so what I am looking at came,
> without a doubt, from say Lt. John Young (I also have some of the artifacts
> he carried in the Revolution - there is a parallel - but more personal).
> David K. Faux.

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