Full-Sibling or a Half-Sibling? DNA Details

Share the knowledge!

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.

Do you have a new DNA match that looks like they might be a sibling to you?  Are you examining DNA details to try to figure out whether they are a full or half-sibling?

Alternatively, maybe you and your siblings all did DNA tests and wonder if the results show you as full or half-siblings.  No matter what the case is, this article will help you resolve your questions.

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.

Let’s get started, shall we?

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

Most of the time, it’s easy to figure out if your sibling is a full or half-sibling.  If you have both done an autosomal DNA test like the ones offered at Ancestry DNA, 23 and Me, FTDNA, or My Heritage DNA, you can compare the amount of DNA that you share.

By looking at the amount of DNA shared, measured in centimorgans or percentages, 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.

Below, we will discuss in detail exactly what to look for.

How much DNA do siblings share?

Shared DNA is often measured and reported in centimorgans on DNA results for the purpose of determining a relationship.  The following are the basic guidelines for comparing the amount of DNA that you share with your sibling:

• 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 likely a half-sibling.  If you share 2500, then they are likely a full sibling.

On 23andMe, the amount of shared DNA is first reported as a percentage, though you can access the shared centimorgan data.  Full siblings usually share between 32-54% of their DNA and half-siblings share between 18-32%.

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 can to take before making a definitive decision about whether you share both parents, or one.

Have additional family members test

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).

Check your X chromosome

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.

Find out if you have fully-identical regions

Two full siblings will have many fully identical regions on all of their chromosomes, whereas two half-siblings will not have any.  This is because full siblings inherit DNA from both of their parents, and some of that DNA inherited from both parents will happen to match their full siblings on both copies of their chromosomes.

Fortunately, there is an easy way to compare your DNA to your sibling’s DNA at the chromosome level.  It doesn’t take any special training, either.

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?

Yes!  And this is important to know because you might wonder if your DNA match is not a sibling (full or half) at all.

Instead, your DNA match might be another close relative.

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

Families are certainly complicated, as is shared DNA.  There are often many possibilities to explore.

What to do if you have discovered your sibling is a half-sibling

If you have recently discovered that your sibling is a half-sibling through DNA testing, you might wonder what you should do.  Should you tell your sibling?

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.

This is a very delicate situation, as I’m sure you are aware.

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

Don’t jump to conclusions about anything

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 had 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.

Feel free to consider talking about your own biological roots

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.

Never make the decision to disclose someone else’s biological roots

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 you did.

The information is there for them to discover, should they choose to explore it.

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.

Conclusion

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.

Thanks for stopping by today!

Share the knowledge!