This is a question that comes up often in the genetic genealogy world. If my sister and I really do have the same parents, why don’t we share all of the same DNA matches?
I’m so glad that you asked!
Spoiler alert: It doesn’t mean that you and your siblings don’t share the same two parents. Though it can, if you are only half-siblings. Check your matching centimorgans (cMs) before ruling that out!
We all get 50% of our DNA from our parents, but not the SAME 50% as our siblings.
Imagine that you are blindfolded, reaching each hand into two hats. Each hat contains the DNA (or genes) of your mother and father. Imagine that you take half of the DNA out of each hat.
Then, imagine your sibling doing the same thing (but imagine that all of the DNA was still in the hat… it’s never-ending DNA).
What do you think the odds are that both you and your sibling grabbed the exact same 50% out of each hat? The answer is that it is almost impossible for two (non-identical twin) siblings to have the exact same DNA. It turns out that two siblings share approximately 50% DNA – sometimes a little more, sometimes a little less.
The consequence of this is that since there is some DNA from your parents that you got, and your brother didn’t get, and some that you didn’t get that your brother got, there will be people on your DNA match list that match one of you, and not the other. Even though you are all genealogically related (you are in the same family tree, whether you actually share DNA or not).
The other situation that occurs almost all the time is that one sibling will share more or less DNA with a cousin match than the other sibling.
Both my grandmother and her brother have taken the Ancestry DNA test. They are full siblings. There is a cousin match on both of their pages who is still a mystery to me. Let’s call him “Jack”.
- Jack matches my grandmother at 76 centimorgans (cMs) and is under the 4th-6th cousin category on her match list.
- Jack matches my “Uncle Bob” at 354 cMs and is under the 1-2nd cousin category on his match list.
While I still haven’t figured out how this happened, I do have a theory:
(Note: This is assuming that Jack’s tree is correct. If one of his great-grandparents really isn’t his great-grandparent, and is one of my great-grandparents, well, then this theory wouldn’t apply)
I believe that Jack is related to me in a few different ways, based on the knowledge that my grandmother’s family has been in the U.S. for centuries. This means that there are multiple DNA segments from multiple lines that could have potentially matched my grandmother. In other words, mostly likely both of my grandmother’s parents had several segments that would have matched Jack.
But since DNA is inherited randomly, and we only get 50% of our DNA from each parent, my grandmother randomly got less of the segments that match Jack from her parents. And her brother probably got most or all of them. Randomly.
Maybe someday I will figure out who Jack is. And I’ll write a post about it!
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.
What do you think? Did you find any mysteries among your match list that didn’t match your sibling?