When exploring your DNA Matches, you may have encountered a strange phenomenon: a DNA Match who matches you, but not your brother or sister.
How does that make sense? Everyone you’re related to, your siblings should also be related to! Right? They have the same parents!
Well… yes. But being related to someone doesn’t necessarily mean you share DNA.
Yes, you read that right: not all of your relatives share DNA with you. There are some relatives who share DNA with your brother or sister, but not with you, and vice versa — even though you and your sibling have the same parents!
How can it be that you and your siblings don’t share DNA with all the same relatives?
The answer to this question lies in inheritance patterns.
It works like this.
You and your sibling each inherit around 50% of your DNA from each parent, but the 50% you inherit is random. So you might inherit a segment of DNA through one parent that your sibling doesn’t inherit — and you might share that same segment with a cousin who inherited it from your common ancestor.
The diagram below demonstrates how this might look:
In this very simplified example, the siblings have each inherited different segments of their parents’ DNA — meaning they actually don’t have a single shared segment! Because DNA is so complex, this is extremely unlikely to happen in real life. Usually, you’ll share between 33–50% of your DNA with your siblings. That’s why siblings may look similar, but not identical (except of course for identical twins, who share 100% of their DNA). You and your sibling might both have your mother’s brown eyes, but your sibling might have a facial structure like your dad’s, while yours is more like your mom’s.
It’s the same with DNA Matches. It could very well be that you’ll have a match who shares a segment of DNA with you that’s located in the 50–67% of your DNA that you don’t share with your sibling. In the diagram above, the blue segment on the far left that you share with your father as well as your DNA match was not inherited by your sibling.
You might have different Ethnicity Estimates, too
The difference between the DNA you inherited and your sibling’s DNA also means that in some cases, you’ll inherit an ethnicity that your sibling doesn’t, and vice versa.
You can read more about why that could happen here: Can Siblings Have Different Ethnicity Estimates?
If you were expecting a certain ethnicity result based on your family history, and you’re surprised to discover that your DNA results don’t include that ethnicity — testing a sibling is one way to find evidence of it. You can read this article to learn another way: Where’s My Ethnicity?!: Why an Ethnicity Might Not Show Up in Your DNA (and How to Find Evidence of It Anyway)