How do you tell through DNA if 2nd cousins match with both common great grandparents (husband and wife) or just one gg.?

Question

Jane and Anne have just found that they are related through DNA and they are 2^nd cousins. When comparing their relationship on Wikitree it states that William is their common great grandfather. What about Mary, William’s wife? Can she be considered a common great grandmother too?

Gedmatch shows Jane and Anne share a Total of 167.3cM the largest being 41.3 cM  (suggesting a distance of 3.2 gen). On the X-DNA, for both total and largest cM’s it reads 5.8 cM.

Comments

Here is another great tool for you--you can put in the amount of shared cMs and it will show you all the possible relationships.  167cMs is perfectly in the range for 2nd cousins.

https://dnapainter.com/tools/sharedcm

Answer 1

If there is a tree that can be viewed and one grandparent had a second spouse and you have others to compare you might be able to narrow it down to one grandparent who shares the DNA.

With all of the charts I like to go with the averages and it seems to work well for me. The DNA Painter tool is awesome. I like that I can send a link to show people the possible relationships.

You are possibly correct in your belief that it could be the amount is due to a different spouse. But then every time I make that sort of assumption, the DNA acts like DNA and some just got squeezed out. You might be able to see if there is another person to triangulate with that would validate your theory.

Answer 2

Check out this neat chart that show how much auDNA you should share with each relative.

https://casestone.com/threlkeld/assets/DNA/Autosomal-DNA-Table-of-Consanguinity.html

The chart says second cousins should share 3.125% or 212.50 (cM) of their auDNA.

 

Comments

Thank you. On looking at that chart, can I assume that if only one of the g .grandparents were genetic, that the result should be the same as say the 2^nd Cousin once removed 106.25 (cM), half of the 212 cM?

167.3 cM is just above half way between the two. Jane and Anne played together when children 70 years ago, and have been excited to find each other after all that time. Their family genealogy is accurate, but I wasn’t sure if the DNA cM’s showed the same

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Fabulous chart Erik?  Is this one of Edison William's creations?

Answer 3

To answer your question, yes, for second cousins, it is considered that the both g grandparents are common ancestors because together they had the children who had children and so on that lead to Jane and Anne. The WikiTree relationship finder only shows one of the two people of the couple--perhaps to make it less messy to follow.

Comments

the relationship finder will show both -- but only one at a time.

use the drop-down selection to chose the alternate relationships.

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Dennis, can you explain which drop-down selection you are referring to?  I haven't seen/noticed this before.

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Emma, once you have found the relationship, look directly below it to the box which says  Explore more.  Choose another common ancestor from this selection box

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Thank you Karen.  I had forgotten about this drop down and didn't realize it shows the other half of the couple in the MRCA.  I also didn't realize it shows alternate relationships at the same generation level.  I ran the relationship finder for one known cousin and I see now that we are related at the same level 10C1R three different ways!

Answer 4

Blaine Bettinger has found full 2nd cousins sharing anything from 46 to 515 cM, and half 2nd cousins sharing anything from 9 to 397 cM.

You have to triangulate the shared segments on one side or the other.  The idea that you can get anything from the numbers is just silly.  They're far too random.

Even if your number is bang on average for a relationship, it doesn't prove that you actually have that relationship.  The averages are 233 and 117, but those numbers are well within each other's spreads.

Over-interpreting random numbers is basically just genealogy by coin-tossing.