How Does Ancestry DNA Show Half-Siblings (Examples)

This article explains how half-siblings show up on your list of DNA relatives on Ancestry.

I don’t have close relatives who have tested with Ancestry DNA, so I asked friends who do to send me screenshots of their results.

You may be surprised to see how Ancestry shows half-siblings.

How Much DNA Do Half Siblings Share?

The documentation on Ancestry.com says that we typically share 2,400 to 2,800 centimorgans with a full brother or sister.

In contrast, Ancestry states that we typically share 1,450 to 2,050 centimorgans with half-siblings.

But that is one company’s estimate of the range. What about others?

Shared cM project

Blaine Bettinger’s Shared CM Project invites people to submit the shared cM for known relationships. The ranges from this project are based on over 60,000 submissions.

Of course, this isn’t just Ancestry customers. The submissions also come from 23andMe customers, MyHeritage, FamilyTreeDNA, and other testing sources.

The half-sibling range from the 2020 summaries is 1,160 to 2,346 with an average of 1,759.

This is considerably wider than Ancestry’s estimates. The sibling range is 1,613 to 3,488 with an average of 2,613.

Ancestry Won’t Label Your Half-Siblings With A Special Category

The challenge for Ancestry is that other relationships share a similar range of centimorgans.

You can share the same amount of DNA with a half-sibling as with these relationships:

  • Aunt or uncle
  • Niece or nephew
  • Grandparent or grandchild

Taking a look again at the Shared CM Project, their range for a first cousin goes as high as 1,397. This edges into the typical half-sibling range, although it will be less likely.

So, you can understand that Ancestry can’t know whether your relative is a half-sibling, an aunt, or in rare cases…a first cousin.

So, they hedge their bets.

The general category below “Full Sibling” is labeled as “Close Family”. This category contains a lot of possible relationships. Half siblings will usually be listed in this section.

Example of a half-sibling on Ancestry

The picture below shows a close family relative of a tester who confirmed to me that this is actually his half-sister.

Notice how Ancestry assigns a relationship of “Close Family – 1st Cousin”.

Editing relationships

Ancestry customers have the option to change this assigned relationship.

You can edit it to be half-sister if you wish. But you have to tell Ancestry what the relationship is!

Could Ancestry Label A Half Sibling As A First Cousin?

In the previous section, we showed how a half-sibling was labeled by Ancestry with the wide range of “Close Family – 1st Cousin”.

The next level down on Ancestry is “1st – 2nd Cousin”.

Here is an example of what a first cousin looks like on Ancestry. This tester confirmed to me that the relationship is his aunt’s daughter.

But is it possible that your half-sibling could actually be labeled here as a “1st-2nd Cousin”?

This is highly unlikely. Let’s take a look again at the Shared CM project. As I mentioned before, their lowest reported range for half-siblings was 1,160 centimorgans.

That is just above the edge of where Ancestry starts labeling relationships as 1st-2nd Cousin. The Ancestry top threshold here is 1,150 centimorgans.

Well, ten centimorgans isn’t a lot. It’s conceivable that you could be in an extremely rare situation of being an edge case. But it’s highly unlikely.

Can Ancestry Tell The Difference Between Siblings And Half Siblings?

Here is an example of how Ancestry displays two siblings of the tester.

Notice that they have a category for “Full Sibling”. Then, they will assign a relationship of Brother or Sister to the DNA relative.

How can Ancestry be so sure that these are full siblings? In these cases, they can tell from the DNA segments.

Two full siblings inherit DNA from both their parents. In contrast, half-siblings only share DNA that they inherit from one parent.

Your chromosomes come in pairs, with one copy within the pairs inherited from your mother and the other inherited from your father.

Simplified diagram

Let’s try a crude visualization to explain what’s going on. Here is one chromosome pair showing shared DNA between the tester and a full sibling.

There are regions where the two full siblings share DNA on both copies within the pair. This is known as a fully identical region (FIR). About 25% of the chromosome pair is made up of fully identical regions.

The other regions are shared DNA on a single copy within the chromosome pair. These are known as half identical regions (HIR).

Half siblings won’t have fully identical regions that are of significant length (i.e. above chance).

How Ancestry identifies full siblings

This is how Ancestry can identify a full sibling during their chromosome analysis.

They look for fully identical regions and can use this information to label the relationship correctly.

Rare cases: half-siblings who are also first cousins

It’s very unlikely that Ancestry would label a half-sibling as a full sibling.

There is one situation where you will share fully identical regions with a half-sibling. This is where you are related to your half-sibling through both your father and your mother.

Let’s take the situation where John and Mary have a child. John then has a child with Mary’s sister.

Both children inherit DNA from the same four grandparents. They will have some degree of shared DNA on fully identical regions.

How Can You Identify A Half Sibling On Ancestry?

Now that you know that Ancestry won’t label a relationship as a half-sibling, you’re probably wondering how you can figure this out.

The key is not to be confused by the assigned relationships of “Close family-1st cousin”. Don’t assume that an unknown relative in this category is a first cousin.

Let’s go back to the likely relationships:

  • Half sibling
  • Aunt or uncle
  • Niece or nephew
  • Grandparent or grandchild

Age can be very misleading in genealogy, but you can probably rule in or out a grandparent or grandchild relationship with little difficulty.

Maternal vs paternal

The next step is to figure out whether this relative is maternal or paternal.

Do not assume that an unknown half-sibling must be through your father.

Hopefully, you can figure this out through your shared matches on Ancestry. I’ll address that in the next section.

You can’t solely use Ancestry to confirm the relationship

Unfortunately, Ancestry doesn’t have the tools to confirm with certainty that a relationship is a half-sibling.

You’ll need to do some additional sleuthing. I’ll give you some pointers in a later section.

But first, gather all the background info that you can through using shared matches.

Using Shared Matches To Research Potential Half Siblings

We have a tutorial and tips on using shared matches on Ancestry.

Here, I’ll say that you should take a careful look at the matches you have in common with your target relative.

You’re looking for the presence of a known relative on the shared match list. Do you see your maternal first cousin? Or your father’s brother?

Now you can tell whether your suspected half-sibling is maternal or paternal.

Avoid relying on expected relatives not being present

Be careful not to use the absence of a third or more distant cousin as an indicator.

You may not share any DNA with all your third cousins. And the chances of not sharing DNA with your known relatives get higher as the relationship gets more distant.

But what about a known first cousin or closer? Surely that’s a sure-fire indicator? No, it’s not. (unless you were present at the birth).

You may have a case of adoption in your close or wider family.

Will Ancestry Show That Your Expected Full Sibling Is A Half Sibling?

This may seem to be a more common occurrence than it actually is. It’s a favorite sensationalist angle for media stories about DNA testing.

Double-check your interpretation

If your shared centimorgans are on the margin of being too low for a full sibling, then don’t jump to conclusions.

If you want to double-check your interpretation, I suggest the first step is to contact Ancestry Support. If you mention a surprise in your close family results, they will review your results.

This won’t change the number of centimorgans, but they can give you more advice if you’re on the margins.

If you want further professional advice, you could book a consultation with a reputable genetic genealogist.

Alternatively, you can get the help of thousands of extra eyes for free. There are some private Facebook groups with knowledgeable members who will look at results and provide advice.

The DNA Detectives group is one option.

You could create a new Facebook account under a pseudonym to preserve your privacy.

Who has the Non-Parental Event?

I’m assuming in this section that your shared centimorgans are way too low for a full sibling.

The next step is to examine shared matches This will help to see whether it’s the maternal or paternal side that’s “missing”.

As your mind is reeling, remember that it may not be you who has the unknown parent. Do you see known paternal and maternal DNA matches on your own list of relatives?

Then it’s probably your sibling who needs your support. And that may be to close this box, put it aside, and continue to be brothers or sisters.

This situation is known as a non-parental event or an NPE. It simply means that the person who someone believed was their parent is not their biological parent.

Researching an NPE

Resolving the identity of an unknown parent is a big topic which I won’t tackle in this article.

The fastest way to research the biological identity may be to start asking questions within your family. However, this may not be an option or answers may not be forthcoming.

At this point, you can use your Ancestry DNA results as a vital part of your research.

There are plenty of tutorials on this website to help you. You can start with our guide to DNA matches on Ancestry.

It’s also a good idea not to rely solely on Ancestry – unless your biological father or mother has popped up as a relative here.

We also have a guide to the best DNA tests for adopted adults or people with unknown parentage.

But you have already purchased an Ancestry DNA kit. That’s great because you’ve taken the first step I recommend for an overall research strategy.

The next step is to use your Ancestry results to fish in more pools.

There are several reputable DNA sites where you can upload your results and see more DNA relatives. You’ll find the strategy laid out in the guide.

Or you can just check our our guide to which sites accept your Ancestry DNA results for free.

Help! I’ve Got An Unknown Half Sibling On Ancestry

In this scenario, a close DNA match has turned up and you’ve narrowed down the possibilities to a half-sibling.

Remember, don’t rely on age to guess the relationship. An unknown aunt or niece could be the same age as you.

I’ll run through a few scenarios you may encounter.

Why has my match disappeared?

You spot a close DNA match and need to take a walk to clear your head. When you get back to your computer – the relative is no longer on your list!

Check out our article on why and how your ancestry matches can disappear.

In this situation, the best thing to do (and sometimes the only thing) is to wait patiently. If this is a shock for the other person, they may have reacted by opting out of DNA matching.

Once they’ve recovered from their shock, they may choose to opt back in again.

If you’re reading this after the fact, then my advice to screenshot unexpected close relatives is probably too late! But you’ll remember it for next time…

Reaching out

Before you reach out by sending a message, I suggest that you screenshot the match profile page, and particularly the list of shared matches.

Then, you should compose a friendly but calm message. Don’t assume that the other person understands the full ramifications of the amount of DNA that you share.

You can check out our article on sending Ancestry messages for some tips.

4 thoughts on “How Does Ancestry DNA Show Half-Siblings (Examples)”

  1. Actually, half-siblings also being first cousins (because a man fathered children with two sisters) isn’t as strange (or scandalous) as it may sound. Here’s how it usually plays out:
    A man’s wife dies.
    Her sister (his sister-in-law) steps in to help take care of the kids (her nephews/nieces).
    He ends up marrying the sister-in-law and they have more kids together.
    So the children from wife #1 are both cousins and half-siblings to those from wife #2.

    Reply
  2. Greetings Margaret,

    Thanks for this article! I am the rare case that you explained where a paternal half-brother is also my maternal 1st Cousin. This was a shock to both of us but the weird thing is that he shares more DNA with our paternal half-sister than with me. According to Ancestry.com he shares 31%, 2,128 cM across 38 Segments with her. With me he shares 28%, 1,950 cM across 53 Segments. I share 24%, 1,644 cM across 36 Segments with our half-sister (all 3 of us have the same father; my half-brother and I have mother’s who are full-siblings just to make this clear lol :)) I guess the amount of Segments my half-brother/1st Cousin and I have in common shows the closer relationship? I just expected that we would have something like 35%-38% shared DNA since we are half-siblings and 1st Cousins. I know that I don’t even have that many DNA Segments in common with my mother (25 Segments) or maternal grandfather (29 Segments). If this is just how autosomal DNA works then I will be content but I just want to make sure that Ancestry.com didn’t make a mistake. Thank you so much for taking the time to read this.

    Sincerely,
    Mike

    Reply
    • Crikey, comparing a trio becomes quite a mind-bender. To answer your question, Ancestry.com didn’t make a mistake in their processing. Variability is part of inheritance.
      I did a different article on full siblings testing with Ancestry that shows the results of one tester and his two brothers. That’s a different situation to yours with one of your siblings, but you’ll see the variation in shared cm and segments there.
      I should also say – you would all get slightly different percentages on 23andMe or MyHeritage. The testing of samples isn’t an exact science.

      Reply
      • Thank you, Margaret. Just one more quick question, does the 53 Segments of DNA that my “paternal half-brother/maternal 1st Cousin” and I share indicate our closer relationship? Because like I said I don’t even share that many Segments with my mother (25), maternal grandfather (29) or paternal half-sister (36). I understand how random DNA is passed down it is just still puzzling to me that he shares more cM’s of DNA (2,128) with our paternal half-sister than with me (1,950) even though I am related to him through two parents (we share the same father and our mother’s are full siblings) yet he is only related to our paternal half sister through one parent (our father). I guess this is a pretty rare occurrence I would think.

        I already understand how random DNA is passed down to full siblings; how different full siblings inherit different amounts of DNA from each grandparent such as, for example, one sibling could be 25% German, 25% English and 50% Irish while another sibling could be 40% German, 15% English and 45% Irish, etc. I am also aware that once you get to 3rd cousin relationships and more distant that one sibling may match a particular 3rd cousin while the other 2 siblings may not match that person. This has been very valuable in confirming 3rd and 4th great grandparents, etc.

        Once again thank you for writing this article and thank you so much for taking the time to answer me. Have a wonderful evening! 🙂

        Reply

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