Gordon Revisited

Several years ago, I helped an adoptee I called “Gordon” identify his biological father.  (Gordon is a pseudonym, as are all other names in this post.)  Gordon already knew that his birth mother was Helene Mills, the daughter of Oscar Mills and Florence Mattieson and the sister of Chris, Tony, and Michael.

Gordon’s case was particularly tricky, because he had long sections of DNA where he’d inherited the exact same genetic information from both parents.  These so-called “runs of homozygosity” (ROH for short) indicated that the only candidates for his father were his own grandfather, Oscar, or an uncle (Chris, Tony, or Michael).

The total centimorgan amount of ROH DNA cannot distinguish between father–daughter incest and brother–sister incest, although sometimes the number of ROH segments, mapping those ROH segments, and subtle differences in shared DNA with 2nd cousins (2C) and beyond can point us in the right direction.  In 2018, Dr Andrew Millard of Durham University, UK, ran some computer simulations and considered all three factors.  Overall, he estimated that it was about 35,000 times more likely that Oscar was Gordon’s father than one of Helene’s brothers.  (Read about that analysis here.)

Dr Millard’s conclusion was convincing, but it was well beyond the reach of the average genetic genealogist.  I’ve since learned how to do these analyses myself; they involve a lot of time and a lot of—shall we say—choice language.

Now, there’s an easier way!

Distant Cousins Give Us Clues

Two of the three analyses that Dr Millard performed would not be possible in most genetic genealogy databases, because they don’t show ROH segments.  The third approach is based solely on shared DNA amounts to 2C and beyond.

Consider the diagram below.  If Gordon’s father was Helene’s brother (left), then Gordon is the great-grandson of Mr Mills & Ms Walton twice and the great-grandson of Mr Mattieson and Ms Kennedy twice.  In that case, we’d expect him to share, on average, the same amount of DNA with his Mills cousins as with his Mattieson ones.

If, on the other hand, Helene’s father Oscar was also Gordon’s father, then Gordon is the both the grandson (through Oscar) and the great-grandson (through Helene) of Mr Mills and Ms Walton, while he’s great-grandson of Mr Mattieson and Ms Kennedy only once (through Helene).  In this case, we’d expect Gordon to share, on average, more DNA with his Mills cousins than his Mattieson ones.

Based solely on DNA matches to 2C and beyond, Dr Millard estimated 85-to-1 odds that Oscar was Gordon’s father.

BanyanDNA Can Analyze Cases Like Gordon’s

Back in 2018 when Dr Millard did his analysis, there were only five 2C-and-beyond DNA matches to consider.  One of them, Debbie, was related through both the Mattieson side and the Mills side.  In the intervening years, another 11 matches have been mapped onto the tree, including another both-sides cousin, William.

I built this expanded tree in BanyanDNA, a forthcoming tool that can analyze complex relationships, like pedigree collapse, double cousins, and yes, even incest.  As you can see, this tree is definitely complex!

To help bring some order to the chaos, I used blue lines for the Mills side, red lines for the Mattieson side, purple lines for the Mills–Mattieson descendants, orange lines for William and Debbie (both sides), and yellow lines for Gordon’s possible parentage.  (That part’s a tangle.  We’re working on improving the presentation.)

The DNA matches are as follows:

• Mills side (blue lines):  Milton 181 cM, Francine 101 cM, Ernesto 267 cM, Leslie 175 cM, Alberto 130 cM, Barry 296 cM, Van 353 cM
• Mattieson side (red lines): Valerie 218 cM, Kirk 112 cM
• Mills–Mattieson descendants (purple lines): Lorenzo 450 cM, Rafael 725 cM, Patty 1749 cM, Nadine 850 cM, Beryl 997 cM
• Both sides (orange lines):  William 206 cM, Debbie 325 cM

I had four hypotheses:

• Hypothesis 1:  Oscar (grandfather) was Gordon’s father.
• Hypothesis 2: Tony (uncle) was Gordon’s father.
• Hypothesis 3: Chris (uncle) was Gordon’s father.
• Hypothesis 4: Michael (uncle) was Gordon’s father.

The gory details of how BanyanDNA works will be explained elsewhere.  For now, suffice to say that it models DNA inheritance down through the tree that you give it.  It does this hundreds or thousands of times (called “trials”) to account for randomness in inheritance, then it performs some statistical magic to compare the actual DNA amounts to the expected ones.

This is what BanyanDNA has to say about Gordon’s father after 1,000 trials.

There’s a 93% chance that his grandfather Oscar was also his father as compared to one of his three uncles.  This is not far off from the prediction that Dr Millard made (85-to-1) based on shared DNA using fewer matches and much more labor-intensive analyses.

In BanyanDNA, you can even expand each hypothesis section to see the match-by-match analysis, including how much DNA the match actually shares, how much they’d be expected to share given the relationship(s) in the tree, the ideal range of shared DNA for those relationships, and the number of standard deviations between actual and expected (smaller is better when it comes to standard deviations).

BanyanDNA is going to be a game-changer for all of us with complex trees, whether that means pedigree collapse, double cousins, incest, or (eventually) endogamy!  It’s also the only tool purposely built to analyze DNA matches through both sides of a union couple—like Oscar Mills and Florence Mattieson—at the same time.

Who’s Behind BanyanDNA?

BanyanDNA is a collaboration among five partners: two talented developers, Jaren Campbell and Carson Wilde; a statistician, Mike Charleston; and two genetic genealogists, Margaret Press and myself.  You can learn more about the team here.

BanyanDNA promises to revolutionize genetic genealogy for complex trees.  The official launch will be on 29 February, 2024, at RootsTech, with a live lecture, a hands-on workshop, and booth #622 in the Expo Hall.  Early access starts on or about February 10.  Anyone can participate in this open beta with the understanding that we will still be ironing out any final glitches, so you may encounter bugs.  Sign up for our mailing list or check out our Facebook page for updates.

We can’t wait to see what you accomplish with BanyanDNA!