Is a second cousin twice removed as DNA close as a third cousin?

+12 votes
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In the DNA confirmation page, the basic DNA confirmation requires a third cousin or closer relation.

I was hoping second cousin twice removed could use the same basic confirmation, the number of steps between the two being 8 in both cases, 5/3 for second cousin twice removed and 4/4 for third cousins, but the instructions do not state that, the instructions only go as far as second cousin once removed 4/3.

Would a 5/3 relation be more DNA distant than a 4/4 relation for the purposes of DNA confirmation?

It would be nice if we could get the underlying rationale for the DNA Confirmation process documented on the Help page.

Note that this related question does not appear to answer this question in my reading of it.

in Policy and Style by William Foster G2G6 Pilot (120k points)
retagged by Ellen Smith
The page https://dnapainter.com/tools/sharedcmv4 indicates that these two relationships are equivalent from the perspective of autosomal DNA sharing.

1 Answer

+16 votes
 
Best answer
Yes. They are genetically equivalent relationships. The help pages would get too complicated if they listed all relationships, but you can use a 2C2R match like a 3rd cousin match, and in particular you don’t need triangulation to mark the appropriate links on Wikitree as confirmed with DNA
by Barry Smith G2G6 Pilot (285k points)
selected by William Foster
Missy: when you say the mother (2C2R), is she not 1C3R? 15cM is extremely low for that relationship distance and should be viewed as suspect.

Man; I really am no good at all the cousinship stuff. It's like doing math but without equations.  frown

From a glance, I think the MRCA (couple or not...but, yeah, if a singleton small segment I personally don't think it can be assumed that both in the couple donated DNA, but those are the WT policy guidelines) is the 3g-grandparent of Missy's mom, so 4g-grandparent to Missy. The Ancestry match is the 3g-grandchild of the MRCA, so her mother is a 2g-grandchild. That would make Missy's mom the 2C2R of the match's mom, and make Missy and the match 3C2R.

I think. My head hurts.

I scrunched the numbers from last year's Shared cM Project update using the standard deviation figures that Blaine provided us for the first time. That scrunching is in a table here. By the CoR, the theoretical sharing between 2C2Rs  would be 0.7813%, or about 53.13cM. By the Shared cM Project's scrunched numbers, 95% of the time the value will be 0 to 158cM; 68% of the time, between 29cM and 115cM.

Edited: Missy and Barry got that by email, but I'm deleting the rest of the response. Because I was wrong. Sheesh. Not my day!

I'd tried to walk back the generational dates and thought I'd discovered a discrepancy on the match's side. Never mind. But a singleton 15cM segment is pretty low--if any match is detected at all--for 2C2Rs.

I'm walking away from the keyboard now...

Edison, you are too funny! wink Thank you for your help. Btw- I am not very good at the “cousins removed!” I say embarrassingly. blush

If I am understanding everything correctly;

I would feel quite comfortable in using this match back to Erik Isaksson and Carolina Jonasdotter IF we shared on more segments (and preferably more cMs).

Is that correct or am I just as dumb as when we started? laugh

What does it mean that we only share on one segment?

EDIT: We both also match another sibling’s descendant. She is a 3C1R. This is what she and my mother share:

Shared DNA: 19 cM across 1 segments
Unweighted shared DNA: 20 cM
Longest segment: 20 cM

Could we all just happen to share on the same segment? So I should compare them on MyHeritage or GEDMatch?

Thank you for your help!

To clarify the relationships: when you said that the mother shares only 1cM more than her daughter with your mother, you mean that the daughter shares 13cM? And that daughter is then a 3C1R to your mother? That makes more sense.

You can happen to match that other 3C1R on the same segment. Or maybe not. You can compare either on MyHeritage or GEDmatch, but you'll need to get all three tests on the same site to make the comparison. Either MyHeritage or GEDmatch will show you if you triangulate. GEDmatch has somewhat more extensive tools, but probably you would have more shared matches to investigate if you got all the kits on MyHeritage. So there isn't a clear winner for which site to use --- use both, if you can.
Yes on the relationships and dna. Thank you for the tips!
I've just updated the DNA Confirmation Citation maker app to allow for 2C2R (and 1C3R) matches to work with it, as of today.  (https://apps.wikitree.com/apps/clarke11007/DNAconf.php)

- Greg
Thank you, Greg! Are we “allowed” to use 1C3R here? I think I have a couple of those with dna matches and good paper trails. Thank you!

Edited content.
Hi Missy! My understanding now is that a 1C3R match is as solid as 2C2R and 'legit', based on number of centiMorgans, and also staying within the number of steps between matches. If you have the paper trail to back it up, go for it!
A 1C3R is actually similar in number of steps to a 2C1R for autosomal DNA sharing. Here's an example that shows the relationship groupings using the link that Ellen shared in her comment above on the original post and supplying a value of 122 cM:

https://dnapainter.com/tools/sharedcmv4/122

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