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

DNA is one of the most amazing tools for those interested in their family trees and genealogy.  DNA can prove many things beyond a shadow of the doubt, but it does have some limitations – especially when it comes to using autosomal DNA to prove family relationships. 

Family Tree Research_ What DNA Can't Do For You

In this post, you’ll learn why you’ll still need to do more traditional family tree research, even if you took a DNA test.

Ethnicity estimates don’t tell you much about your family history

Most people get started with DNA testing because they are interested in their ethnicity estimate.  Ethnicity estimates are really fun and can occasionally reveal something previously unknown about our family’s ancestry. 

When it comes to building a family tree or understanding details about our roots, however, ethnicity estimates are only vague tools that can point us in the general direction of where our ancestors may have lived.

The reason that ethnicity estimates can’t provide details about our ancestry is because we don’t share DNA with all of our ancestors.  Each child inherits 50% of their DNA from their parents, meaning that some DNA is “lost” each generation. 

Our genomes aren’t big enough to hold DNA from all of our ancestors anyway, so it’s a good thing that some of it is left behind each generation.  (It’s not such a good thing for genealogy, though!)

The end result is that our ethnicity estimates only show results for the DNA that we DO share with our ancestors.

Much more valuable to family tree research is our DNA match list.  Our DNA matches can provide us with information about who our genetic relatives are, where they live, and what their family tree looks like. 

Using this information can help us build our family trees and get a very detailed understanding about our recent and distant ancestry.  Even DNA match lists have limitations, though.

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DNA can tell you very close relationships with no additional research

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.

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.

Centimorgans are important for determining relationships, but they don’t tell the whole story

In technical terms, my cousin and I 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 of, or even known personally, 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 wrote a beginner’s guide to centimorgans, which includes the amount of shared DNA for different relationship levels, so that way you can see how to use this valuable information in more detail.)

Have you done a DNA test yet?

Even though DNA can’t instantly solve many of your family’s mysteries (though it CAN solve plenty!), your search for your heritage will benefit greatly from DNA testing.

I recommend testing with Ancestry DNA, 23andMe, Living DNA and/or Family Tree DNA. You can purchase your DNA tests via the links below.

You can order a DNA test using any of the links below. I may receive a very small commission that helps me support this site, and it is at no extra cost to you, so thank you!


I hope this post gave you some explanation as to why you might still have to build a family tree even if you took a DNA test.  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!

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