How Do Half-Siblings Show Up on Ancestry DNA?

Marc McDermott

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.

In general, due to the amount of DNA that is shared, half-siblings will most likely show up as “Close Family” or as “First Cousins.” The image below is a good example of how half-siblings show up on Ancestry DNA.

half siblings 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.

full sibling vs half sibling

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:

  1. The potential half-siblings need to share 1160-2436 cm.
  2. 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.
  3. 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.

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