I watched this amazing video from the Rootstech 2018 conference and it made me feel that my post DNA in a Nutshell was incomplete. In that introductory post I talked about how we inherit DNA and provided a link  that has some great pictures. Even if you don’t want to read the article, looking at the pictures can really give you a good idea of how we get the DNA that we get.

One thing people understand about DNA is that if we share it, we’re related. Many people might not realize that if you don’t share DNA with someone, it doesn’t mean you aren’t related. According to the ISOGG wiki, Up to 10% of 3rd cousins (people who share great-great-grandparents) and up to half of 4th cousins (people who share great-great-great-grandparents) do not share any common DNA. As the DNA gets mixed up and handed down, it’s possible you didn’t inherit any of the DNA a particular ancestor had, but a cousin did. It’s also possible that you both have DNA from a particular ancestor, but that it is not the same DNA in the same spot. The algorithms that the companies run don’t know that your segment on chromosome 2 and your third cousin’s segment on chromosome 6 came from the same great-great-grandparent. You and your third cousin may be related, but if you don’t carry the same segments, the computer will never know it. This is important to know when it comes to understanding the intermediate topic of triangulation.

The use of the term “cousin” by companies is a very liberal one. When I see the term “cousin” used for matches I like to think  of it as generations rather than as a reference to a specific person. Since it is an estimate based on the amount of DNA you share, and the amount of DNA people in a particular relationship can vary, a 1st cousin can refer to an actual first cousin, a great-aunt/uncle, or even a first cousin once or twice removed. The further you get past close family (parents, siblings, aunts/uncles, nieces/nephews) it becomes much harder to pin down the actual relationship you share with a given DNA match.

Besides the difficulties of a varying range of DNA assigned to particular relationships, as mentioned in the previous post, things like half-relationships (where you only have one common ancestor rather than a common ancestral couple) as well as endogamy (where you might be related to someone in more than one way) and pedigree collapse (where you have intermarriage in your family tree) can make relationships look closer when the actual common ancestor is actually much further away than suggested. And since every company does their calculations differently about what counts towards the total amount of DNA you share with someone, different companies may give different results and some may conclude that you aren’t actually related to someone while another company says you are.

All this to say is that you shouldn’t worry if you are new to all this and everything isn’t as precise and exact as you expected. If you don’t share DNA with a third cousin who has also tested their DNA, it doesn’t necessarily mean something is amiss. If your testing company says your great-aunt is your 1st cousin, it’s not because your great-aunt is actually your 1st cousin. If you find a DNA match and your testing company says this person is your second cousin, but you can’t see the overlap in both of your trees, you may have to dig further back.  Let go of the expectation of certainty and embrace the variety.