Many people are receiving unexpected sibling matches. Everyday on social media, “surprises” are being reported so often that they are no longer surprising – unless of course you’re the people directly involved and then it’s very personal, life-altering and you’re in shock. Staring at a computer screen in stunned disbelief.
Conversely, sometimes that surprise involves people we already know, love and believe to be full siblings – but autosomal DNA testing casts doubt.
If your sibling doesn’t match at all, download your DNA files and upload to another company to verify. This step can be done quickly.
Often people will retest, from scratch, with another company just for the peace of mind of confirming that a sample didn’t get swapped. If a sample was swapped, then another unknown person will match you at the sibling level, because they would be the one with your sibling’s kit. It’s extremely rare, but it has happened.
If the two siblings aren’t biologically related at all, we need to consider that one or both might have been adopted, but if the siblings do match but are predicted as half-siblings, the cold fingers of panic wrap themselves around your heart because the ramifications are immediately obvious.
Your full sibling might not be your full sibling. But how can you tell? For sure? Especially when minutes seem like an eternity and your thoughts are riveted on finding the answer.
This article focuses on two tools to resolve the question of half versus full siblingship, plus a third safeguard.
Half Siblings Versus Step-Siblings
For purposes of clarification, a half sibling is a sibling you share only one parent with, while a step-sibling is your step-parent’s child from a relationship with someone other than your parent. Your step-parent marries your parent but is not your parent. You are not genetically related to your step-siblings unless your parent is related to your step-parent.
Ideally two people who would like to know if they are full or half siblings would have both parents, or both “assumed” parents to compare their results with. However, life is seldom ideal and parents aren’t always available. Not to mention that parents in a situation where there was some doubt might be reluctant to test.
Furthermore, you may elect NOT to have your parents test if your test with your sibling casts doubt on the biological connections within your family. Think long and hard before exposing family secrets that may devastate people and potentially destroy existing relationships. However, this article is about the science of confirming full versus half siblings, not the ethics of what to do with that information. Let your conscience be your guide, because there is no “undo” button.
Ranges Aren’t Perfect
The good news is that autosomal DNA testing gives us the ability to tell full from half-siblings by comparing the siblings to each other, without any parent’s involvement.
Before we have this discussion, let me be very clear that we are NOT talking about using these tools to attempt to discern a relationship between two more distant unknown people. This is only for people who know, or think they know or suspect themselves to be either full or half siblings.
Because the ranges of the amount of DNA found in people sharing close family relationships varies and can overlap. In other words, different degrees of relationships can be expected to share the same amounts of DNA. Furthermore, except for parents with whom you share exactly 50% of your autosomal DNA (except males don’t share their father’s X chromosome), there is no hard and fast amount of DNA that you share with any relative. It varies and sometimes rather dramatically.
Anonymous (April 3, 2019). Full or Half Siblings? [Blog post]. Retrieved from https://dna-explained.com/2019/04/03/full-or-half-siblings/