With the Ancestry DNA test becoming more popular, there have been some questions surrounding the results and how each relative is categorized. For example, if you have half-siblings, you may wonder precisely how they show up in your results.
Half-siblings on Ancestry DNA
Half-siblings on Ancestry DNA will show up as “Close Family” or “First Cousins” and are expected to share an average of 1,759 centimorgans with a range of 1,160-2,436 centimorgans, according to data from the Shared Centimorgan Project.
The image below is a good example of how half-siblings show up on Ancestry DNA.
In this article, I’ll walk you through the ins and outs of the results regarding half-siblings, how much DNA they share, how accurate the test is, and more. This information will help you understand your test results, as well as the process as a whole.
How much DNA do half-siblings share?
To compare how much DNA half-siblings share, I’ll first give you a baseline to compare. When it comes to full siblings, they share between 1,613 to 3,488 centimorgans (cMs) of DNA. For half-siblings, they will typically share anywhere from 1,160 to 2,436 cMs of DNA.
In other words, you and your full siblings share 50% of your DNA while your half-siblings will share 25% of your DNA. What’s interesting about this number, though, is that you can share anywhere between 55 and 85 segments with either person. For example, your full sibling could share 55 segments of DNA with you while your half-sibling could share 85 segments.
You’ll notice quite a bit of overlap in the total shared DNA between half-siblings and full-siblings. It’s important to note that companies like Ancestry can easily separate these two relationships if they detect fully identical regions versus half-identical regions.
Now, when it comes to your first cousin, you will share about 12.5% of your DNA with them, on average, which is half of the 25% that you share with your half-sibling. This means that instead of 1,160 to 2,436 cMs of DNA, you’ll share about 680 to 1,150 cMs.
Can a half-sibling show up as a cousin?
While the amount of DNA you share is different between half-siblings and cousins, your half-sibling still may show up as a “first cousin” because your centimorgans may be within the 1,300 range. So, instead of presenting itself as your “Close Family,” it will then fall into the “first cousin” or extended family categories.
Does this mean you’re more related to your half-sibling than your cousin?
Since you do only share 12.5% DNA with your first cousin, then technically, yes, you are more related to your half-sibling than your cousin since you share 25% of your DNA with your half-sibling.
Can a DNA test prove half-siblings?
Yes, a DNA test can prove half-siblings. As a matter of fact, it’s the only accurate way to establish the biological relationship between the people in question. In a half-sibling situation, the siblings share one biological parent. But you need to test the parent. Here is are the steps involved:
- The potential half-siblings need to share 1160-2436 cm.
- If the potential siblings are in range AND share more than 1600 cm, there must not be any fully identical regions. If there are, then it’s more likely a full sibling relationship.
- Each potential half-sibling must share 2500-3720 cm with the parent.
If all three steps are true, then you’ve got yourself a half-sibling relationship.
Half-sibling DNA test results
Your DNA test results are pretty cut and dry regarding your status as a half-sibling or full sibling. For instance, if you see that you share about 25% DNA with your brother or sister, this means that you are half-siblings instead of full siblings. More importantly, Ancestry will detect fully identical regions in full-siblings.
However, if you happen to share 50% of your DNA with your sibling, this means you’re full-siblings and share both parents. And of course, if the DNA test comes back showing that you share no DNA, then you guys are most likely not related.
Remember, though, as far as half-sibling results go, this may appear in your ‘first cousin’ category depending on the number of centimorgans that are shared between the two of you. It’s essential to keep in mind that not everyone in the cousin category is your cousin.
Surprise readings
It goes without saying that over time there have been a lot of DNA surprises. Using something like Ancestry DNA, people have found long lost siblings, and family members. Also, they’ve found out that their full siblings are their half-siblings.
It’s best to keep an open mind when you submit your DNA, as you never know what your results will read. If you come upon something interesting, you can always contact that other person to see if they’re open to further testing.
Can it determine if siblings have the same father?
As far as the Ancestry DNA test goes, it’s not a paternity test; therefore, you cannot use it as legal proof. However, if you’re looking to find whether your sibling shares the same father as you, then you can always ask your father to take the DNA test as well.
Something else you can do is if you have a third sibling involved that you know is your father’s child, they can also take the DNA test. You can then compare the numbers between the three of you to find out whether you all share the same father. But again, the father needs to test as well.
How accurate is a half-sibling DNA test?
Most half-sibling DNA tests are 99.9% accurate. However, as far as categorizing the results are concerned, this may not be accurate at all. There are instances where half-siblings were categorized as grandparent and grandchild, or cousins.
But, as far as the number of centimorgans, or the percentage of DNA that is shared, this is as accurate as it’s going to get. You may need to search for your match in other categories, but rest assured, the half-sibling DNA test results are correct if you have followed the instructions thoroughly.
Summarizing what we’ve learned
It’s common for people to have half-siblings since not all children share the same two parents. When you take the Ancestry DNA test, since half-siblings only share 25% of their DNA, they may be categorized as your first cousin. However, this all depends on the number of centimorgans that are shared between the two individuals. Yet, if you share a higher amount of centimorgans, then your half-sibling will be listed as a close relative.
Hi Marc, interesting article.
I recently matched two males on Ancestry and both of them are brothers I share 1596 cm with one and 1595 cm with the other.
Ancestry’s new update says that’s either half brother or nephew. Could you tell which one from these numbers?
If not, is there a DNA test that could determine the relationship between us? Most likely the father has passed.
Thank you for the help! I appreciate it!
Are you in contact with these matches? You’d have to compare your match lists. A nephew would share 4 of your match clusters whereas a half-sibling would only share 2. So build out the match clusters for you and the two males.
Recently ancestry changed the relaionship staus with one of my cousing so from Cousin to now half Sister Why would they do that
Our DNA results are 1,307 cM | 19% shared DNA
I’m not sure. Both are possible based on 1307cm. If you’re in contact with the match, you can compare your match list and clusters.
I share 28% with a woman I have never met on my dad side is she my half sister
Could also be a niece or full sibling. But if they only share dna with one side of your tree, then most like a half-sibling or an aunt.
I have a DNA match of 1st cousin 1x removed or 1/2 1st cousin, 7%, 515cm. 13 seg. maternal side. could this person’s parent be my mothers 1/2 sibling
515cm could be one of several relationships. You’d have to do some detective work to figure it out. Sorry I can’t be more help.
I recently found a person on ancestry with 2261 cm and 32% DNA match with me. I tested my brother on my maternal side and he is 1700cms and 24% match with me. What is the relation between me and the new person? It is pretty high for a half sibling. Could she be a double cousin?
Could me a few different relationships. Half-sibling is possible, on the high end of the range.
Hi Marc,
Thank you for your great article regarding siblings/half siblings. I share the following DNA with a previously unknow female:
Shared DNA: 1986 cM across 40 segments
Unweighted shared DNA: 1986 cM
Longest segment 147 cM
I have ruled out Aunt or Niece, this result strongly suggests a half-sibling match, could you reply with a comment please
Possibilities are: Grandparent/Grandchild, Aunt / Uncle, Half Sibling, Niece / Nephew
I have this showing on my dna results – could it be my father’s brothers son? Or is it more likely to be my fathers son?
Half brother or nephew
Paternal side
1,789 cM | 26% shared DNA
Your father’s brother’s son would be your first cousin. It’s not that. These are the possibilities: Grandparent Aunt / Uncle Half Sibling Niece / Nephew Grandchild
Hello Sir
Thank you for all your insight and knowledge you’re sharing. Could you explain how this is possible and is it more like they’re siblings. One doesn’t know his mother and the other doesn’t know her father. Please help! The Paternal side is her father…
Half brother or uncle | Paternal side
22% shared DNA: 1,533 cM across 34 segments
1533cm is not enough to be a full sibling.
My recent match came up with 30 percent shared with 2088 cM and 37 segments. This person is around my age so is it likely a half sibling relationship? What else could it be? The system just says close relationship. Thanks!
Most likely a half-sibling, but could be a full sibling too. Also aunt/uncle.
Hi, Mark, thank you for this article. I recently found a 1st cousins and was quit shocked as I thought I knew all of my first cousins. They are all 20 years or more older. I have run through my paternal family with this person without any results. After reading your article I am wondering if they are a half sibling or an half aunt. The DNA match reads: 1st Cousin, 12% shared DNA, 859 cm shared across 21 segments. My dad would have been 95 had he lived and this person is 72. There’s a strong possibility that my dad or one of his two brothers could have had a child they didn’t tell anyone about. I look forward to hearing your thoughts.
Iden
Hey Iden. 859cm wouldn’t be half-sibling. More likely a first cousin, or half uncle/nephew relationship.
Hello Marc,
First of all, I want to express my gratitude for your insightful article. It has been incredibly helpful as I try to understand a recent match I found on Ancestry. Here are the details of the match:
– Relationship: Close Family – 1st Cousin (Paternal side)
– Shared DNA: 21% (1,476 cM across 22 segments)
I’m wondering if this person could be a half-sibling rather than just a cousin, as the DNA share seems closer to what one might expect from a half-sibling (around 25%) compared to a cousin, who typically shares about 12.5% of their DNA. Unfortunately, my father has passed away, so there’s no way to confirm this directly through him.
Given your expertise on the topic, I would greatly appreciate your thoughts on this matter. Thank you in advance for any insights you can provide.
1476 would likely not be a cousin. More likely to be an aunt/uncle/niece/nephew or half-sibling.
Is she my half Sibing or First Cousin, she is 49 years and I’m 58
1334 cm
Shared DNA 19%
Segment 38
Could be either based on 1334.
On my DNA results it shows a have a 1st cousin these are the results: Shared DNA: 911 cM across 37 segments
Unweighted shared DNA: 911 cM
Longest segment 74 cM
It that a 1st cousin?
911cm can also be a half-nephew, half-uncle, great uncle or great-grandparent. Based on their age, you can probably eliminate those last 2.
I matched with a close relative, 20% with 1372 cm and 31 segments. I thought I knew all my first cousins could this be more?
First cousin is possible but 1372 is more likely to be a Grandparent/Grandchild, Aunt/Uncle/Niece/Nephew, or Half-Sibling.
what would a 41%-47% shared DNA with 2309 cm mean relative wise??
Likely a full sibling, half-sibling, or grandparent.
Hello, I’m trying to convince my brother that we are showing up as half siblings. I show I have two half siblings – one on my paternal side and the other on maternal side. My brother on my maternal side shows 1847 cm 26% and the other which I had no idea he existed shows 1746cm 25% on paternal side. This person does not show up at all on my maternal brothers dna. Are both showing as half sibling for me?
thank you
Hi Trina. I’m not sure I understand your question, but if you share 1847cm with your brother, it’s a half-brother. Hope that helps.
I have two kids for my understanding both kids were from the same father and in the DNA the kids are showing 50-57% shared DNA 2810 cm across 40 segments.
The problem is that my first kid look so much as my first parter and the second kid as my second partner. I jumped so fast from one relationship to another so I’m wondering if is possible the kids could had both men DNA? furthermore the older kid shows Ethnicities that the younger one does not have and might seems from where my first partner is coming from. None of the men want to do a DNA to get something more accurate.
I’m worry about my kid because she looks so different to her sister and the rest of the family, but ancestry shows the kids as full siblings.
Any advice?
If they share 2810cm, they are full siblings. If you want more confirmation, look at their matches. They should both have similar matches. Meaning one should not have a large cluster of matches the other does not have.
I just received a 26% DNA match & 1794 cM. It says on my side she could be my half sibling or niece. On her side it labels me as her first cousin, but doesn’t mention half sibling or Aunt. Why would that be? Is it more possible she would be a half sibling than a cousin? Thank you.
That’s strange. Are you sure that’s how you show up on her side? 1794cm is not possible for a first cousin.
Is it possible to share 51% with a half sibling?
No 51% would be a full sibling.
My mother has had a match with very a close friend. The results are 23% shared DNA and 1,628 cM.
Does this result determine that they are half-siblings or could it also be double cousins by any chance?
What would your take be on this result?
Yes, based on the cm amount, either is possible. You’d have to do a bit of digging to figure out the relationship.
My sibling and I have the same mother. I recently checked a distant cousin’s (26cM) shared matches on my father’s side and my sibling came up as a shared match! I cross referenced with the cousin’s shared matches and my sibling showed up a couple more times!
How can that be? I am now confused!
First, 26cm is quite low and can be a very distant relationship. Second, for clarity, this is your half-sibling you’re asking about?
I recently did an ancestry DNA test and it has matched me with a ‘close family’ member who I do not recognise on my paternal side. We share 22% DNA across 1,535cm. Does this mean this person is my half sibling who I did not know existed?
It could be, yes. It could also be grandparent/grandchild, aunt/uncle/niece/nephew.
I have a 30% match with my half brother. We share a mother – different dad’s. This percent is higher the the avg of 25% for this relationship. Can you help me to understand why? Thx.
All genetic relationships have ranges. 30% is on the high end for half sibling, but within range.