I have a DNA match with Robertson-21387 but why not with his father?

+4 votes
I have a DNA match with Robertson-21387.  Why don't I also have a DNA match with his father, Robertson-21500?
WikiTree profile: Alexander Robertson
in Genealogy Help by Sheryl Breault G2G Crew (800 points)
retagged by Peter Roberts
Hi Samantha,

Thank You. I feel better.  Thought I was on the wrong track. Hopefully, DNA research will exceed this .39% limit.

Sheryl,  Just for the record, WikiTree is NOT saying that you have a DNA match with Robertson-21387. Please read the information just above where your DNA test information is displayed on his profile. It says "It may be possible to confirm family relationships with Duncan by comparing test results..." "It is likely that these autosomal DNA test-takers will share some percentage of DNA with Duncan." WikiTree is not doing any DNA matching. Your DNA test information only appears on Duncan's profile because WikiTree can tell that your profile is connected to his. You will need to compare your DNA (on Ancestry or MyHeritage) with that of other test takers who show that they are also descended from him, to determine if you actually do match one another.
I'll add that even if you do match the other test takers, it requires more analysis to determine who the common ancestor(s) are.  You can't assume the common ancestor will be Alexander Robinson.  For one thing, all your trees should be well documented, or at the least show plausible lines of connection.  You also need to rule out other possible sources of the shared DNA.

Adding to Kay's and Julie's comment, we also have to keep in mind that when it comes to the "dilution" of our autosomal DNA, we aren't testing the ancestor. At least, not usually.

So it's the number of intervening births that count, the number of chances for the DNA of two parents to recombine and comingle when they produce an offspring. That 0.39% number comes from a simple calculation called the Coefficient of Relationship. Theoretically, all your 6g-grandparents will contribute about that amount of DNA to yours. But research by population geneticist Graham Coop at the University of California Davis has shown that by your 6g-grandparents you have about a 17% chance of inheriting no DNA from any given one of them. By your 8g-grandparents, only about half of them will have contributed to your own DNA.

And it's 8 birth events from you to that 6g-grandparent. But since we test living or recently deceased individuals, that same 8 birth-event count takes you only as far as your 3rd cousins (who would be expected to share about 0.78% DNA with you, by the way...which is what you'd expect from a 5g-grandparent; but never mind; forget that part; WikiTree just counts generations for the cutoff number, not DNA dispersion). smiley

Two recent, really handy articles that can give you an idea of the probabilities of you sharing measurable DNA with cousins are by Amy Williams at Cornell University: "How often do two relatives share DNA?" and "How often do two half relatives share DNA?"

1 Answer

+12 votes
Wikitree doesn't accept raw data so can't do DNA matching. It shows the expected average DNA you would have with any ancestor at that position and it only shows it down to an expected average of 0.39% which is what shows for you on Robertson-21387. So Robertson-21500 is too distant an ancestor for Wikitree to show the expected average DNA.

Please see https://www.wikitree.com/g2g/1137594/did-you-see-the-new-dna-percentages?show=1137594#q1137594 for an explanation of the percentages.
by Samantha Thomson G2G6 Pilot (117k points)

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