How Much DNA Do Half Siblings Share?

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Do you want to know how much DNA siblings share? In this post, you’ll learn the answer, including the percentage of DNA and centimorgans shared between half-siblings.

This is one of the most common questions that people have when they view their DNA results. Many people wonder about the topic when inspecting how much DNA they share with their siblings who also tested.

Others might have received a DNA match who shares a high amount of DNA and are curious about whether their match could be a half-sibling.

No matter the reason that you are reading this article, you are sure to understand more about the how and why of shared DNA between half-siblings.

Do half brothers and sisters share the same DNA?

Half-siblings share less DNA than full siblings. This is because half-siblings inherit 50% of their shared parent’s DNA.

The 50% of the shared parent’s DNA that half-siblings inherit is not identical. Each sibling will inherit a randomly selected half of their shared parent’s DNA through a process called recombination.

Some of the DNA that they inherit will match their half-siblings, but a portion of it will not. The result of this is that half-siblings share far less than 50% of their DNA with each other.

How many centimorgans do you share with a half-sibling?

Half-siblings typically share between 1300-2300 centimorgans of DNA. Their total amount of shared DNA is made up of identical DNA segments inherited from their common parent.

When it comes to measuring shared DNA, the most useful unit of measurement is centimorgans. Centimorgans are a way to measure genetic distance between the start and end locations of the identical shared segments.

While most half-siblings share between 1300-2300 centimorgans (cMs), there have been self-reported instances of siblings of this relationship sharing as little as 1200 and as much as 2400.

The image below shows an example of DNA shared between half-sisters on Gedmatch:

We can see that these sisters with the same mother share a total of 1631 cMs across 51 DNA segments. Gedmatch estimates that their common ancestor is 1.6 generations back, but we know it’s only one (the mother).

What percent DNA do half-siblings share?

Half-siblings share an average of 25% of their DNA with one another. While this is the average, we also see half-siblings sharing as little as 18% and as much as 32%.

If you compare this to full-siblings who share, generally, between 32-54% of their genome, you can see that the percentage is substantially less.

If you know the number of centimorgans you share with your sibling and you would like to change it to percent, simply divide your total number of shared centimorgans by 6770.

For example, if you share 1800 centimorgans with your half-sibling and you would like to know the percentage, simply do this equation:

• 1800/6770

You will find that you and your sibling share 26.6% of your DNA.

Do male half-siblings share more DNA than female half-siblings?

Yes, and no. Male half-siblings will share identical Y-DNA, which accounts for about 2% of total DNA.

This 2% is not included in the total shared autosomal DNA when we discuss centimorgans. This is because autosomal DNA is located on numbered chromosomes, and Y-DNA is on the chromosome that determines whether we are male or female.

Half-sisters who share the same father also share slightly more DNA than brother-sister half-siblings or half-sisters who share a common mother.

Half-sisters will inherit an identical full copy of their father’s X chromosome. The DNA located on the X-chromosome accounts for about 5% of total DNA for females (or 2.5% inherited from a common father, in the case of half-sisters), and about 2.5% of total DNA for males.

Again, the X-DNA from the sex chromosome is not included in the autostomal DNA count.

Some of this “bonus” DNA shared between certain combinations of half-siblings because of shared X and Y-DNA could be offset by shared mitochondrial DNA inherited from a common mother.

However, mitochondrial DNA, or mtDNA, represents less than 1% of total DNA, by most estimates.

How many common ancestors do half-siblings share?

Because half-siblings share one parent and have one different parent, they will share approximately 50% of their ancestors with each other.

This means that half-siblings can be excellent research partners who can help build your common family tree.

Learn more about half-siblings and shared DNA

There is so much more to learn when it comes to DNA shared between full siblings and half-siblings. The following posts will help you learn all you need to know to understand this topic:

• How many DNA segments do siblings share?
• How do half-siblings show up on Ancestry DNA?
• Full sibling or half-sibling?

Conclusion

I hope that this post helped you learn more about how much DNA half-siblings share, and how much of their DNA is the same.

If you have any questions about something that you read in this post, or if you would like to share how much DNA you and your half-siblings share, I would love to hear from you in the discussion below.

Thanks for reading this post 🙂

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