A Relationship Outlier

Blaine Bettinger runs the Shared Centimorgan Project in which he's collected thousands of data points for amount of DNA shared between people with known relationships.  He can give a range of DNA which has been observed to be shared for people with specific known relationships.  If you have a new DNA match, you can see how much DNA you share with that person and use Blaine's chart to identify potential relationships that are possible with that person.

Shared cM Project.  From http://thegeneticgenealogist.com/wp-content/uploads/2016/06/Shared-cM-Project-Version-2-UPDATED-1.pdf.  Used with permission.

Until now, all of my tested cousins have fallen within the ranges Blaine gives (generally on the higher end due to endogamy)--until now.

When I was in Michigan over July 4th visiting the cousins we originally learned about via DNA, I brought along some DNA kits to get some more data on Diamond DNA.  And the results are starting to come in.  One person I tested was my cousin Dave (father to the Dave who I initially matched back in 2013--who was the key to reconnecting with this branch of my family).  This Dave is a second cousin to my father, so based on Blaine's chart, he should share somewhere between 43-504cM with my father.

Dave's FTDNA match to my father--631cM of shared DNA

Dave wasn't in that range at all.  FTDNA said that he shared 631cM with my father.  I uploaded Dave's kit to GedMatch and looked at how much DNA Dave shared with my father (using a threshold of 7cM):

DNA shared by Dave & my father

GedMatch has the pair sharing even more DNA--642.8cM, including huge segments of 93.9, 66.2, 62.1, 35.2, 32.3, & 29.1cM.

Now, Dave is only 50% Ashkenazi, so there is likely no DNA from his father contributing to shared DNA with my 100% Ashkenazi father.  But how about Dave's sister Beth?  How does she match my father?

DNA shared by Beth & my father

Beth shares only 313.7cM with my father--less than half of what her brother Dave does and well within Blaine's range for second cousins.  (And yes, DNA shows that Dave & Beth are full siblings.)  It seems like through the randomness of DNA recombination, Dave & my father simply happened to get a lot of the same Diamond DNA.

I put together a McGuire Method diagram (a great way to visualize DNA that Lauren McGuire invented--you an read more about this method here) for tested descendants of my great-great grandparents Hillel and Hinda Diamond.  You can see how much DNA each tested person in this chart shares with all his or her tested relatives.

McGuire Method Diagram - check out Dave's match to my father in the yellow box!

It's quite interesting how much more DNA Dave and my father share than do Dave and my father's siblings/first cousins or my father and Dave's sister Beth.  What's also intriguing is that Uncle Leibish's grandson Dan shares a lot of DNA with descendants of my great grandfather (my father, his siblings and his first cousins) but nowhere near as much with descendants of my great grandfather's sister Jenny.

Look at the variations of amounts of shared DNA between second cousins in this chart alone.  This shows why testing multiple people--even siblings--can be very informative.  You never know which will be your Dave.

Note:  I'm on Twitter.  Follow me (@larasgenealogy).

Want to get future blog posts emailed to you automatically?
Enter your email address: