Half-Siblings On Your Family Tree? Try This DNA Strategy

Melanie Mohler

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How can half-siblings of your relatives be “better” than whole ones (at least, from a genetic genealogy perspective?) Here’s what Joy learned about using descendants of half-siblings to cluster the shared matches she wanted.

How can half-siblings be better than whole ones? At least, from a genetic genealogy perspective?

Half-relatives generally share about half the expected amount of DNA that “whole” relatives do. Despite sharing LESS of the DNA, these half-relatives can be TWICE as useful in your family tree!

In a recent Your DNA Guide live event, Your DNA Guide Diahan Southard explored a question from participant Joy Stubbs: Who were the parents of Caroline? Caroline is a 2x great-grandmother of Joy’s husband. Joy was trying to identify his 3x great-grandparents, and we share her story here with her permission.

Joy’s husband tested at AncestryDNA, and has a robust family tree. We went to Ancestry’s ThruLines tool, which constructs a family tree with your DNA matches on it, based on your tree data. ThruLines isn’t always accurate, but is a great place to start.

What are we looking for on ThruLines? A Best Known Match: someone else who descends from Caroline, but through one of her other children. ThruLines shows that three of Caroline’s other children have descendants who have tested: John, Clarissa and Chestina. I’ve circled the DNA matches we could use as Best Known Matches.

Diahan is MOST interested in matches who descend from John. Why? Because he was a half-sibling to the others.

Using half-siblings to find the right DNA matches

Of course, all relatives (half or whole) are great! DNA matches who descend from Caroline can point us to other DNA matches who may descend from her unknown parents. But in this situation, a half-relative is golden.

Why? CR and JS, who descend from full siblings Clarissa and Chestina share DNA that comes from Caroline’s husband. To look for Caroline’s parents, we need to get Caroline’s husband’s DNA out of the mix, and normally that requires an extra step. John, as a half-sibling, doesn’t have that shared DNA from Caroline’s husband. So looking at his descendants takes that extra step away. 

Take-home lesson: when dividing your DNA matches into groups who are related to each other (genetic networks), watch for those half-siblings. They can be so useful!

There was more to talk about with Joy’s case study in the live event. We used several Best Known Matches who descended from John to create a genetic network of people with Caroline’s DNA. We explored those matches and talked about next steps to take to find her parents. And we did it in real time, with everyone watching, so they could learn how to use their own DNA to answer questions like this.

Wondering about the courses Your DNA Guide offers? Check them out here to see how you can learn valuable DNA skills like the ones Joy used  here to use on your own DNA research!

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  1. Bruce McKenzie

    I , am the oldest child , I have a half brother and a half sister, who are full siblings to each other. I am trying
    to track down my Dad , who is my brother and sisters biological father, His name was Hector, and he was placed in an orphanage in Glasgow , Scotland I have been using my brothers DNA to try and track him down.
    Is it possible that the DNA which my sister inherited from her father could give a different result than her brothers?

    • Diahan Southard

      Great job on everything you have found so far. There is definitely a benefit to having more people test. Since everyone inherits a slightly different segment of their parents’ DNA, testing multiple of your siblings will increase the number of DNA matches you connect with on your dad’s side. We have a more in-depth explanation on our website here: https://www.yourdnaguide.com/ydgblog/siblings-dna-tests


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