I posted one of my blog posts to LinkedIn (1) and someone asked an interesting question that gave me pause.

“What if the two people you are comparing share DNA that you are not testing? How does one know that what is tested allows one to deem common ancestry “impossible,” thereby capable of being eliminated?

I know that direct-to-consumer (DTC) DNA tests test about 600,000 SNPs, and not all of them overlap. You can have a look at the ISOGG Wiki (2) to see the specific number for each company. Is it possible we’re missing SNPs, and potentially missing out on matches? I wondered if perhaps whole genome sequencing (WGS) (which tests all the SNPs) would show us what we might be missing out on.

I decided to bring the question to the experts at Genetic Genealogy Tips and Techniques (3). If you’re not part of the group yet, you should really join!

Blaine Bettinger, The Genetic Genealogist (4), answered my question about whole genome sequencing. He pointed me towards a paper (5) that compared WGS to the current micro-array, and while it did increase relative matching by 5-15%, many of those new matching segments were small, which means they might not be valid.

As I wrote about in my post DNA in a Nutshell (6), it’s like comparing two books, and saying they were descended from the same book because they both had the word “the.” The longer the segment we share, the more certain we can be that it is not just random chance that two people happen to share it.

Jim Bartlett, of Segment-ology (7), reminded me that we are not deeming common ancestry impossible when we say a person is not a match. What we are saying is that using the settings that we are using, if there is a common segment it is likely that the shared segment is not ancestral. Check out Jim’s post on segments here (8). Remember my post Don’t Touch the Settings? (9) Do you really want bananas showing up on your match list? (I don’t, my match list is bananas enough as it is — thanks endogamy!).

That’s the price we pay for a Match list that is reasonable – a balance between most of our close relative and not too many very distant ones. -Jim Bartlett (10)

The fact is, we are missing out on relatives when we do DNA tests. As I pointed out in my post We all have two family trees (11), we know that 10% of third cousins and 50% of fourth cousins don’t share enough DNA to show up on each other’s match lists. Knowing this helps to mitigate problems — we don’t assume someone is not related if they are more than a second cousin and don’t share DNA. It’s why when trying to identify unknown DNA, closer matches are better.

I really like questions like this that get me thinking. We don’t know what we don’t know, so having other people point out things I have never thought of helps me learn and grow. Keep those questions coming!

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1) “Jennifer Wiebe,” LinkedIn (https://www.linkedin.com/in/jennifer-w-64aa52162/ : accessed 17 December 2022).

2) “Autosomal DNA testing comparison chart,” ISOGG Wiki (https://isogg.org/wiki/Autosomal_DNA_testing_comparison_chart : accessed 17 December 2022).

3) “Genetic Genealogy Tips & Techniques,” Facebook (https://www.facebook.com/groups/geneticgenealogytipsandtechniques : accessed 17 December 2022).

4) Blaine Bettinger, The Genetic Genealogist (https://thegeneticgenealogist.com/ : accessed 17 December 2022).

5) “Relationship Estimation from Whole-Genome Sequence Data,” PLOS Genetics (https://journals.plos.org/plosgenetics/article?id=10.1371%2Fjournal.pgen.1004144&fbclid=IwAR1sKrVNW4U3OcAU2sTCCgN_um3Qf2iXgLKwoIHMTEcyYxovzcLm7Tg-Als : accessed 17 December 2022).

The article is: Hong Li et al., “Relationship Estimation from Whole-Genome Sequence Data,” PLOS Genetics (January 2014).

6) Jennifer Wiebe, “DNA in a Nutshell,” Jennealogie (https://maltsoda.wordpress.com/2018/02/13/dna-in-a-nutshell/ : accessed 17 December 2022).

7) Jim Bartlett, Segment-ology (https://segmentology.org/ : accessed 17 December 2022).

8) Jim Bartlett, “What is a segment?” Segment-ology (https://segmentology.org/2015/05/07/what-is-a-segment/ : accessed 17 December 2022).

9) Jennifer Wiebe, “Don’t touch the settings!,” Jennealogie (https://maltsoda.wordpress.com/2018/04/20/dont-touch-the-settings/ : accessed 17 December 2022).

10) Jim Bartlett, at “Genetic Genealogy Tips & Techniques,” Facebook (https://www.facebook.com/groups/geneticgenealogytipsandtechniques :
16 December 2022), comment on post by Jennifer Wiebe 16 December 2022.

11) Jennifer Wiebe, “We all have two family trees,” Jennealogie (https://maltsoda.wordpress.com/2018/04/13/we-all-have-two-family-trees/ : accessed 17 December 2022).