DNA Testing for Siblings

What would it feel like to get a phone call from a sister that you never knew you had?
Jared Rosenthal
Published on

Imagine…one day, your phone rings, and someone you’ve never met before says, “I am your long lost brother.”

Imagine…one day, your dad pulls you aside and says, “Son, I have something to tell you. You have a sister.”

Imagine…you have been searching your whole life for a sibling that got adopted by another family, and you finally think you found her.

What would you do? Amazingly, getting a Sibling DNA Test is not always the first thing people do (although it should be, and I’ll tell you why). Long lost siblings who have just found each other seem to do everything but get a DNA Test. They meet, they cry, they get to know each other’s families. They tell everyone. They involve the kids – the new nieces and nephews. And then, months or even years later, they call me.

“I need to DNA test to prove if my sister is really my sister.” Well, just from the sound of that statement, I can tell right away that they’ve invested heavily in the relationship already. They refer to the other person as a sibling. But they are not 100% sure.

When people wait to get the test, they are putting themselves out on a high wire that keeps getting higher every day. When we find a true match, the long lost siblings are usually elated. But when we find a mis-match, after they’ve spent years thinking that they are truly full or half siblings, it can be devastating.

DNA Testing for Siblings: what you need to know:

  • If you think you are half siblings, but know that you are not full siblings, then you either share DNA from one parent or none. That means half of your DNA is definitely going to be different. So we have to figure out, from the half that’s left, if you have a match. The hard part is that we don’t know which half is from the non-common parent. Because of this, the test becomes stronger if your known non-comment parents (usually the two mothers) test also. This way, we can exclude the half of your DNA that comes from the parent that we know is not in common between the two of you.
  • If you think you are full siblings, but may really just be half-siblings, then you definitely share DNA from the known, common parent. It is a much stronger test if that parent also gets swabbed.
  • No matter what, we can run a sibling study with just the siblings; however, in a small percentage of cases, if no parents are tested, we may not get a conclusive result. If this happens, it’s because the world of DNA is a game of randomness, and even though the two of you may share a parent, you may have randomly received completely different sets of genes from that common parent. It’s rare, but it could happen. You don’t know unless you test
  • There are 3 possible outcomes of a sibling DNA test: related, unrelated, or inconclusive. You can always add a parent to get swabbed later if the results come out inconclusive. In most cases, we get a conclusive result even without the parent.

When to get a Sibling DNA Test

There are two possible answers:
1) Test before you think: if you found someone that might be your long lost brother or sister, don’t wait. Get it done now. The longer you wait, the harder you’ll fall if it turns out negative.
2) Think before you test: if the person you always thought was your sibling may, in fact, not really be your full (or half) sib, then think hard about how you’ll handle the outcome of the test before getting it done.

WRITTEN BY

Jared is the Founder of Health Street, the creator of the Who's Your Daddy DNA truck, and the host of VH1's Swab Stories.

WRITTEN BY

Jared is the Founder of Health Street, the creator of the Who's Your Daddy DNA truck, and the host of VH1's Swab Stories.

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DNA Testing

Read Health Street's dramatic and informative DNA testing stories.

FEATURED IN

DNA Testing

Read Health Street's dramatic and informative DNA testing stories.