13
Apr

How Much DNA Do You Share With a Sibling?

Do you want to know how much DNA siblings share? In this post, learn the percentage and centimorgans of DNA shared between siblings, and more!

The topics covered in this post include:

  • How much DNA do brothers and sisters share
  • Why don’t siblings share 100% of their DNA
  • Whether two siblings can share more DNA with each other than their other full sibling
  • Whether two siblings can share more DNA with each other than their other full sibling
  • Whether two siblings can share more than 50% of their DNA

Many people come to wonder about the usual amounts of shared DNA between siblings after two or more siblings from the same family have done DNA tests.

It’s interesting to examine how much DNA you share with your siblings, and it’s especially informative to learn why we share more with certain siblings than others.

How much DNA do brothers and sisters share?

Full siblings generally share between 33-50% of their DNA. In centimorgans (cMs), we expect siblings to share at least 2300 cMs.

There are some self-reported surveys that report full siblings sharing lower than 2200-2300, but this would be very uncommon.

Below, you can see an example of DNA shared between full siblings. My sister took a DNA test a few years after I did:

this is an example of full siblings sharing 2671 centimorgans of DNA
My sister and I share 2671 centimorgans of DNA with each other

Our 2671 centimorgans of DNA equals approximately 35.8% of shared DNA. Ancestry DNA is confident about our full sibling relationship and reports us to be in the “full sibling” category.

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Why don’t siblings share 100% of their DNA?

If two people share the same parents, wouldn’t we expect their DNA to be 100% identical?

The only situation where two people share essentially 100% identical DNA is when dealing with identical twins. Identical twins are formed from one egg fertilized by one sperm, and so their genes will be identical.

Every other sibling relationship, even fraternal twins, will share somewhere around 33-50% of their DNA with each other.

The reason that siblings don’t share all of their DNA with each other is because each person inherits a somewhat randomly selected 50% portion of their mother’s DNA and 50% of their father’s.

Siblings will inherit randomly selected DNA with from their parents that doesn’t exactly match their siblings.

We can think of DNA inheritance like many, many coin tosses. There are seemingly endless possibilities of how our parents very long genomes can be combined in their offspring.

While it is theoretically possible for two siblings to have inherited identical DNA, or DNA that doesn’t match at all, from their parents, it is so unlikely that we can say it’s basically impossible.

Lots of siblings’ DNA will be identical, but a sizeable portion will be DNA that they got from their parents that their siblings didn’t get.

Coincidentally, this is the same reason that ethnicity estimates between siblings will rarely match. The different DNA that each sibling inherited from mom and dad will result in slightly different ethnicity estimates.

Can two siblings share more DNA with each other than their other full sibling?

Another fascinating phenomenon that we see among siblings is some siblings sharing more DNA with one sibling than another.

For example, you might find that you share 2700 cMs with your sister and only 2600 cMs with your brother. It’s absolutely normal – and expected – that this will happen.

In the image below, you can see two full siblings of our sample DNA tester. She has two full siblings who have done DNA tests, and one shows up sharing slightly more DNA with her than the other.

this image shows how one sibling shares slightly more DNA with their sister than the other sister does
This are two full siblings to the DNA tester – one shares slightly more DNA with the tester than the other does

You might notice in the image that our DNA tester shares just a tad more DNA with her sister than her brother, and you might wonder if this is because they are both female.

Female full siblings do share an entire X chromosome that they inherited from their father, and they would not share this with a brother. This is not, however, the reason that our DNA tester shares more DNA with her sister than with her brother.

The Ancestry DNA test that these three siblings took tests autosomes – our numbered chromosomes – not our sex chromosomes (i.e. Y or X DNA).

Instead, we can just know that our test taker shares more DNA with her sister because of the “luck of the draw” and the way that their randomly selected portions of their parents’ DNA was passed down to them.

Can siblings share more than 50% of their DNA?

Yes, siblings can theoretically share more than 50% of their DNA. This is not very common, however.

Conclusion

I hope that this post has helped you understand how much DNA siblings share, and why they don’t share more (or less) DNA than they do.

If you have any questions about something that you read in this post, or if you would like to share your own experience comparing your DNA with that of your siblings, please join us in the discussion below.

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