Do you wonder if DNA testing other family members will help you in your genealogy research or family search? The short answer is, who you test depends on what your research goal is.
This information is for autosomal DNA testing.
Searching for Birth Family
If you already know one of your birth parents, it would be helpful to ask that parent or a sibling related to you through that parent to take a DNA test. This will help you to quickly establish which of your DNA matches are paternal versus maternal. What if you your parent or sibling won’t take a test? See if you can get as close a family member on that side of the family to test (such as a cousin, grandparent, or aunt/uncle). This will still help.
If you are starting from scratch and have no identified birth family, there will be no one else to test at this point.
DNA Testing for Family Research Purposes
If you are hoping to use DNA testing as a tool for building a concise family tree, you will want to test as many family members as you possibly can. First let me explain why and then what order you should test family members in.
There is this fancy scientific thing that happens when DNA is passed from parent to child. It’s called recombination. You receive 50% of your DNA from Mom and 50% of your DNA from Dad. But the 50% you receive is not the same exact 50% your siblings receive (unless you are identical twins). Below is a visual example of what recombination looks like. It’s a very simplified example since there are many generations that came before your grandparents, but hopefully you get the general idea of how you and your siblings can receive different segments of DNA from your parents. You might get a huge segment of DNA from your maternal great great grandmother whereas your brother might have such a small one he doesn’t get very many DNA matches on that family line.
My brother has more of our Irish family DNA so he picks up DNA matches that I don’t. Without his DNA test, I would have a much more difficult time working on that family line, especially with no DNA tests from our parents.
You can read more about the science of recombination on this ISOGG wiki page.
DNA testing can be expensive, so who should you test first?
Start with the oldest members of your family. There are two reasons to this. First, we assume older people will not live as long as the younger. You want to get their tests as soon as you can. Second, they are a generation closer to your ancestors than anyone else in your family. For instance, your great grandfather will be the child of your 2nd great grandparents. So he will pick up more DNA matches related to those family lines.
Once you have the oldest family members tested for each of your great grandparent lines (likely your great grandparents or grandparents), have your parents test. This will make it much easier to separate all your DNA matches by paternal versus maternal.
If there are no family members living above your parents’ generation, or if your parents are no longer living, you may have to get creative and look for cousins or second cousins to test in order to have each of your great grandparents’ lines covered in the family DNA pool.
Once you have your great grandparent’s family lines covered with at least one DNA test, any other DNA tests you get are bonus tests. What is the benefit of having more tests at this point? You have the opportunity to pick up some additional DNA matches because of recombination.
It’s important to note: you should make sure your family is all starting at the same DNA testing company for easy comparison between family members’ DNA.
For more information about which DNA test you should take, see this page under our Resources tab.