Full-Sibling or a Half-Sibling? DNA Details

Do you have a new DNA match that looks like they might be a sibling to you?  Maybe your known sibling did a DNA test and you’d like to know whether or not you are full or half-siblings?  In this post, I’ll explain how to know whether your DNA match or known sibling is a full or half-sibling.  Plus, I’ll tell you my perspective on what to do if you have discovered that you and your siblings are half-siblings.

Before we get started, I feel like it is really important to emphasize that a sibling relationship is one of the most important relationships that we have in our lives.  If you have grown up with a sibling and you find out via a DNA test that you are half-siblings instead of full-siblings, it’s important to remember that your relationship with your sibling doesn’t need to change because of this information.  Our family is whoever is in our heart, and while a DNA test can help us explore our biological heritage, we shouldn’t let it affect our relationships with people who we have known as family our entire lives.

How to figure out if your sibling is a full or half-sibling?

Most of the time, it’s pretty easy to figure out if your sibling is a full or half-sibling.  That is, if you have both done an autosomal DNA test (atDNA) like the ones offered at Ancestry DNA, 23 and Me, FTDNA, or My Heritage DNA.  By looking at the amount of DNA shared between you and your sibling you should be able to easily figure out whether or not you share both parents.

For every relationship, there is a range of shared DNA that is seen for that relationship.  Sometimes, this range leaves some room for interpretation, but fortunately this is not usually the case between siblings.

How much DNA do siblings share?

Shared DNA is usually measured in centimorgans for the purpose of determining a relationship:

  • Full siblings will share between: 2300-3900 centimorgans
  • Half-siblings will share between: 1300-2300 centimorgans

If you know for sure that your DNA match is a sibling, but you are not sure if they are a half-sibling or a full sibling, you can use the above guidelines to figure out which relationship applies.  For example, if you only share about 1400 centimorgans with your sibling, then they are a half-sibling.  If you share 2500, then they are a full sibling.

Note:  It is important to note that there are occasional statistical outliers.  Sometimes, DNA defies all probabilities and statistical calculations and two full siblings can share slightly less DNA than expected, or two half-siblings can share slightly more DNA than expected.  This means that a definitive conclusion cannot be made about whether two people are full or half-siblings based only on shared DNA if they share between 2200-2400 cMs.

What if I share between 2200-2400 cMs with my sibling?  Can we be sure that we are full or half-siblings?

As I wrote before, there is a small range of shared DNA that can leave us with some doubt as to whether or not two people are full or half-siblings.  This range is somewhere between 2200-2400 cMs, and so if your relationship with your sibling falls within this range, there are a few more steps you’ll have to take before making a definitive decision about whether you share both parents, or just one.

  • Have any other family members taken a DNA test?  If so, you might be able to figure out which matches you share in common with your sibling, and which ones you don’t.  Two full siblings should share all of their close family members in common (from other siblings, parents, aunts, uncles, and first cousins, for example).
  • Two female siblings with the same father will share a full X chromosome.  You can check X chromosome information by uploading both of your DNA files to Gedmatch and using the X chromosome One-to-One tool to compare your X chromosome with that of your sister.
  • Two full siblings will have many fully identical regions on all of their chromosomes, whereas two half-siblings will not have any.  You can compare your chromosomes with that of your sibling by uploading your DNA to Gedmatch (your sibling should do the same), and using the One-to-One comparison tool and choosing the “Graphics and position” option, which will visually display your identical regions by chromosome.

Are there any other relatives that share the same amount of DNA as a sibling?


This is why I mentioned previously that the ranges for shared DNA between siblings apply only if you are sure that you are siblings.  This is because there are several other types of relationships that overlap with the amount of shared DNA that both full and half-siblings share:

  • Full siblings share between 2300-3900 centimorgans, but parents share an average of 3600 cMs
  • Half-siblings share between 1300-2300, but so do aunts, uncles, nieces, nephews, grandparents, and grandchildren – as well as double-first cousins

Uh-oh – you’ve discovered your sibling is a half-sibling and not a full sibling.  What should you do?

This is a tough situation, as I am sure you are aware.  Did you discover that you and your siblings are half-siblings?  Are you 100% sure?  Not sure what to do with this information?  While I don’t have all of the answers, I do have a couple of suggestions that might help you figure out the best way forward.

Disclaimer:  I’m not an attorney, a doctor, or a therapist.  Only you know what choice is right to make in your own situation.

  • First, be careful not to jump to any conclusions about which of you (you or your sibling) doesn’t match both of your biological parents and the nature of the events that led to this occurrence.  Families are complicated, and being human is hard.  Any one of our parents are capable of having been human and having a lapse in judgement at any point in time.  Perhaps we don’t have all of the information, and so it’s best to wait until coming to a final conclusion about what happened until we do.
  • If you have discovered that your own ancestry is not what you had believed it to be, then I believe that you are completely within your rights to divulge this information as you see fit.  Keep in mind that you can explore your roots with discretion, if you feel like it is best to do so.  It’s your information to keep, or to tell.
  • If you have discovered that someone else’s ancestry is not what they thought it was (like a sibling, for example), I strongly recommend that you not say anything to them about it unless they explicitly ask you if you are full or half-siblings AND you really believe that your sibling really wants to know the truth.  If it were my own sibling, I think I would avoid answering the question until they came to the conclusion on their own.  I would never offer or volunteer this information to someone.  If they want to know, they can access their own results and use Google to learn about DNA just like everyone else has.

If I discover something about my own ancestry, I can talk about it if I want to or I can keep it a secret if I want to.  If I learn something about someone else, even a close family member, I don’t talk about it unless they specifically ask me to tell them what I think about something.  I would not tell my siblings anything surprising about their parentage unless they found out on their own and want to talk about it.


I hope that this post has given you some ideas as to how to figure out how whether or not your DNA match is a sibling, and if they are, whether or not they are a full or half-sibling.  If you have any questions about something that you read in this post, or if you would like to add your experience with a sibling DNA match, I would love to hear from you in the discussion below.

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