10
Apr

Family Tree Research: What DNA Can’t Do For You

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DNA is one of the most amazing tools for those interested in their family trees and genealogy.  DNA can prove beyond a shadow of a doubt who my parents and grandparents are, but there are a few things it can’t do.

DNA cannot replace the need for a paper trail.  The amount of DNA shared by a parent, child, or a full sibling is so high that a simple autosomal DNA test can tell you how two people are related without much guessing.  For relationships more distant – even only as distant as a first cousin – people share the same amount of DNA (on average) as a grandparent or even a grand-uncle.  So in order to be sure about a relationship, you need to have more tools in your toolbox.

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Let’s take one of my cousins, for example.  I learned about him when I started learning about my family tree – I had not met him or communicated with him previously.  He shows up on my DNA list as a 4th-6th cousin match.  Many people might think that this relationship is too far away to be helpful or even that it would be impossible to figure out the relationship.

Fortunately, this cousin is an expert level genealogy researcher.  So when we got in touch, he was able to explain to me exactly who he was and how we were related.  His great-grandfather is my great-great grandfather.  The reason that the DNA test showed such a small relationship is because his great-grandmother is NOT my great-grandmother.

It turns out that his great-grandmother died after childbirth, which is a tragedy that occurred all-too-frequently in the past (and still occurs to this day in many parts of the world).  His great-grandfather remarried and had my great-grandfather.

In technical terms, we are second-cousins once-removed.  He and my mother are second cousins.  However, the amount of DNA that we share is 50% less than it would be, on average, because of his great-grandmother being a different person than my great-great grandmother.  We share 49 centimorgans (cms).  The average shared for this relationship distance is 81 cms, and it can be as high as around 200 cms.  My mother shares 70 cms with him (her second cousin).  A second cousin could share as high as 500 cms (though the average is closer to 200 cms).   The low number in shared DNA is because of the “half” relationship a few generations back, which resulted in less shared DNA and Ancestry’s software thinking that our relationship was more distant.


My grandmother’s family got split up when she was young, so my mother didn’t get a chance to know first and second cousins in her family.  Otherwise, this new cousin for me might have been someone that I would have known while I was growing up.  It’s a good thing that he had already gotten a head start on the family tree the old-fashioned way (pre-DNA) or I might have ignored this cousin match (foolishly!) and missed out on a very wonderful family relationship with my second cousin.

(I am going to put together a whole series about centimorgans, and the amount of shared DNA for different relationship levels, so that way I can explain things in more detail.)

Have you noticed anything that DNA was not able to help you with?  How have you used DNA in your own family research?  Answer in the comments!

If you want a more complete, in-depth guide about how to understand and use your DNA results to find family and build your tree, click HERE to learn about my ebook guide to your DNA results.