DNA Testing for Half Siblings
Updated on March 18, 2024
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DNA Testing for Half Siblings

More people than ever are using DNA testing to help them find their relatives. Some are even surprised to learn that they have half-siblings when they get their results.

If you believe you have half-siblings that you’ve never met, genetic testing might help you find them. These tests are a great way to look for long-lost relatives because they help you find DNA links with people who may have also taken the same test.

Can a DNA Test Prove Half-Siblings?

Yes, but only to a degree. Half-siblings share roughly 25% of their DNA since they share a parent, but that number can fluctuate.1

DNA testing can help show the potential for a sibling relationship, but it won’t prove it with absolute certainty. This is because half-siblings may actually share the same amount of DNA with you as a cousin, grandparent, aunt, or uncle—so there’s no absolute way to be sure.2

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More comprehensive DNA testing can narrow down the relationship better than basic tests, so if you’re relying on an at-home test to find a half-sibling, you’ll need to dig deeper to prove the relationship. You may have to get more family members to do a DNA test for more definitive answers.

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What is a Half-Sibling?

A half-sibling is a sibling with whom you share a single parent. If your sibling is a child from your step-parent’s relationship with someone besides your parent, you will not be genetically related.

You need to share a biological parent to share their DNA, as that parent will pass DNA down to both of you.

How Much DNA Do Half-Siblings Share?

Half-siblings share about a quarter (25%) of their DNA. Depending on how DNA is passed down from parents to children, some half-siblings will have more in common while others have less.

This difference in shared DNA is more prominent when comparing half-sisters to sister-brother sibling pairs. Half-sisters who share a father will have more DNA in common than a half-brother and half-sister because of how the X and Y chromosomes are passed down.1

Consequently, half-sisters who share a father will share an X chromosome, but their brother won’t. Half-sisters will always have more DNA in common than they do with a half-brother.1

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Source: 123rf

How Much DNA Do Full Siblings Share?

Biological siblings with the same parents share about 50% of their DNA.4

You and your siblings inherit about half your DNA from your mother and the other half from your father.

This means you and your full biological sibling share 25% of your DNA from your mother. If they share the same father, they have about 25% of their father’s DNA. In total, you and your biological siblings have 50% of your parents’ DNA.

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Why Use a Sibling DNA Test?

There are several reasons people want to prove (or disprove) a sibling relationship, such as:

  • Inheritance claims
  • Cases for migration/immigration
  • Research
  • Insurance claims
  • Other legal matters

How Does a Sibling DNA Test Work?

A Sibling DNA Test compares the DNA profiles of two people trying to either prove or disprove their sibling status.

This is usually done via sample collection (either blood, saliva, or a buccal swab) and an analysis of this sample to examine each DNA sequence. Once your samples are analyzed, it will show how much you genetically have in common.

They will establish a genetic match and, depending on the percentage, will determine your sibling status:

  • Full siblings – A genetic match of 50% and up means you are likely full siblings with the same parents
  • Half-siblings (potentially) – A genetic match of around 25% means you may be half-siblings, but it’s best to take more confirmatory tests and even test other family members to be sure
  • Unrelated – Anything in the single digits means you are likely not at all related
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Source: 123rf

Do Full-Blooded Siblings Have the Same DNA?

No. However, many parts of their DNA may overlap.

This is due to genetic recombination.3 Even if you have the same parents, you won’t inherit the same genes in the exact same sequence—unless you are identical twins, where the DNA sequence was copied almost perfectly before the egg split into two.

Remember that siblings and half-siblings share more DNA with you than with people not related to you, and you can determine that through DNA testing.

However, simply sharing DNA with someone doesn’t necessarily mean that person is automatically a sibling. Some sibling DNA, especially when people are half-siblings, resembles that of more distant relatives, such as cousins or aunts and uncles.

To understand this better, it helps to have a clearer picture of how DNA works.

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Source: 123rf

How Does DNA Work?

DNA is packed into cell units called chromosomes. Each cell in your body contains two copies of chromosomes—one from your mother and one from your father. Most of us have 46 total chromosomes in the body.

These chromosomes are numbered one through 23. The 23rd pair is significant because it establishes biological sex.

Biological females (with XX chromosomes) inherit X chromosomes from both parents, while biological males (with XY chromosomes) inherit a Y chromosome instead of an X from their fathers.

It can be challenging to figure out the percentages and imagine what exactly is contributed by a sibling pair’s mother and father.

The Card Analogy

It helps to imagine the process of passing on DNA with a deck of cards in mind. Shuffle the 52 cards in the deck and lay 26 of them face up. The cards you see represent half your mother’s DNA that you inherited.

Now put the entire deck back together, reshuffle, and lay 26 cards face up again to represent your full sibling’s DNA. Chances are good you aren’t going to get the exact same 26 cards, but some of them will be the same.

Both you and your sibling each got 26 out of your mother’s 52 cards/DNA, but you didn’t get the same 26.

To determine the total amount of DNA shared between you and a full sibling, you’d repeat this exercise for your father’s DNA. However, if you are only half-siblings, you’d only do it once, for your mother or your father—whichever parent you share.

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Source: 123rf

The cards that match when you lay out 26 for each of you represent about 25% of your DNA, which is the amount of DNA you share with a half-sibling.

These percentages vary from person to person. You aren’t going to get a match of 13 cards each time, though it will happen occasionally. The 26 chromosomes we get from each parent are random.

Theoretically, you could have them all match or have none of them match with a sibling. So while the “rule” is siblings share about 50% of their DNA and half-siblings share about 25%, they don’t have to share any at all.

We inherit approximately 50% of our DNA from our mothers and 50% from our fathers, but that’s where “exact” percentages end.

So not only can a brother and sister have different DNA, most do, but it’s in random varying amounts.

How to Find a Relative Using DNA Tests

After you take an Autosomal DNA test (A DNA test that examines your autosomal chromosomes, which contain DNA segments you share with everyone you’re related to), your testing company will compare your results to others in its database.

If it finds significant similarities to other people who have taken the same genetic test, you’ll see them in your list of matches (given they want to be publicly found by others).

You may have thousands of matches if the DNA testing company you choose has tested thoroughly in regions you may have descended from. While most will be only distantly related to you, a few will likely be close matches. 

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Updated on March 18, 2024
Angela Natividad
Angela Natividad
Content Contributor
Angela is a full-time digital content manager and editor for Know Your DNA. She also contributes freelance articles to several local and international websites when she has the time. She's always been a voracious believer in finding the truth and ensuring the science is sound.