Will My DNA Match My Siblings?

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Are you wondering if your DNA will match your siblings?  In this post, learn whether your siblings have the same DNA as you do, how much you share, and more.

One of the most common questions that people with siblings have when considering a DNA test is whether they have the same DNA as their sibling. As it turns out, brothers and sisters do have some DNA that is identical.

Equally as important, some of the DNA that siblings share is different than that of their siblings. This is true whether the siblings share both parents, or one, such as is the case with half-siblings.

I myself have a few full siblings. As far as I know, I don’t have any half-siblings.

When I first got excited about DNA testing, my siblings were glad that I had taken a DNA test. Since we share both parents, they thought that maybe my DNA test meant that they wouldn’t have to ever take a DNA test.

Not so fast.

Brothers and sisters do not have 100% identical DNA, as we came to discover. Only identical twins have 100% of the same DNA.

Every sibling has a variable percentage of DNA shared with all of their siblings, as we will learn below.

How Much DNA Do I Share With My Siblings?

Siblings who share both parents will share between 33-50% of their DNA. If the siblings share one parent, whether the shared parent is the mother or the father, they will share between 18-33% of their DNA.

As you may have noticed, there is a range of the percentage of DNA shared between any type of sibling (i.e. full or half). This means that there is no way to know exactly how much DNA you will share with your sibling until you both take a DNA test.

Unfortunately, we can’t estimate which two siblings share more DNA with each other based on who looks more alike.

The only relationship type that does not typically include a large range of shared DNA possibilities is that of parent and child. This is because children always inherit 50% of both of their parents DNA.

When it comes to any other relationship, even full siblings, the amount of DNA will always vary, falling somewhere in the range of shared DNA percentages that I listed above.

There are two ways to estimate the amount of DNA that siblings share.  Some people want to know a percentage of shared DNA, and others are interested in a specific number of centimorgans (cMs), which is a term used to measure DNA.

Below, you will learn more details about centimorgans (cMs) shared between both full and half-siblings.

How much DNA do full brothers and sisters share?

The sibling DNA match percentage that is typically seen between full siblings is 33-50%. In centimorgans, we usually see siblings share between 2300-3900 cMs.

We usually say that full-siblings share about 50% of their DNA with each other, just to make things simple. Of course, you now know that it is possible for them to share a bit less than that.

This is where things get a bit confusing. If siblings inherit 50% of their DNA from both of their parents, why don’t they share 100% of their DNA?

Each sibling inherited 50% of their DNA from each parent. However, except in the case of identical twins, siblings never inherit the same 50% of each parent’s DNA that their siblings inherited.

While this might sound confusing, stick with me.

Imagine that each parent has a deck of 52 cards (and it automatically replenishes).   Imagine that you randomly pick half of the deck of cards from each parents.

Then, your sibling will choose, at random, 26 cards, too.

If you were to write down the name and type of the 26 cards that you chose and compare it to the list of cards that your sibling chose, you will find that some of them will match. You will also see that others won’t match.

Based on statistics, we know that about 50% will be the same and about 50% will be different.  It’s random, though, and that’s why you never know how many “cards”, or which cards, will match those of your siblings. 

That is, unless you do a DNA test.

1. Full siblings will share a high amount of cMs (centimorgans).  The average will be around 2600 cMs, but it could range from 2300-3900.
2. Some sources say that 3900 cMs is unlikely for a full sibling match, and that it is more likely that they share less than 3100 cMs.
3. The average shared DNA percentage will likely range from 32%-54%.
4. I share 2671 cMs with my full sibling, for example.  My mother shares 2570 cMs and 2501 cMs with two of her full siblings.

DNA is passed down from our parents in a similar way to that of the playing card analogy, albeit in a more complicated and scientific fashion. Each parent has two copies of 23 chromosomes.

Each chromosome of each parent goes through a process called “recombination” to create a new chromosome to become the child’s single copy from that parent. The other parent passes down their unique, recombined copy.

Together, these two copies become the child’s chromosomes. This occurs 22 times, one time for each numbered chromosome.

Fathers will also pass down a full copy (non-recombined) of their Y-chromosome to their sons, and a full copy (non-recombined) copy of their X-chromosome to their daughters. Since females have two copies of the X-chromosome, they will pass a recombined copy of their two copies of the X-chromosome to both their sons and daughters.

Amount of shared DNA between half siblings

When it comes to half siblings, we see them share between about 1300-2300 centimorgans of DNA .  Since half siblings share one parent, and not two, that means that they will only share an average of 25% of their DNA with their half siblings. 

In the card example that I used above describing DNA shared between full-siblings, imagine that one half sibling got 50% of their DNA from the parent that they don’t share.  Then, they had to choose from the deck of cards from the parent that they do share. 

The probability that they chose the exact same cards as their other sibling is almost impossible – meaning that they will share far under 50% of their DNA with their half siblings.

• In terms of centimorgans (cMs), half siblings will share an average of 1750 cMs with each other (18-32%).  The range is somewhere between 1300-2300.
• Other sources say that it is very unlikely that half siblings will share 2300 cMs, and place the upper number as low as 2150.  That’s why it is really important to look at circumstances and documents before making a final decision that a sibling is a “half” sibling.
• Just as an example, my mother shares 1963 cMs and 1584 cMs with her two half siblings.

Half-sibling DNA matches can be confused with first cousins

On the topic of half siblings, it is important to note that there are other relationships that share a similar amount of DNA.  Uncles, aunts, nieces, nephews, grandparents or grandchildren, and a few other relationships, are included. 

If you have a DNA match that shows up in the lower portion of the half-sibling centimorgan range, you will need to use other methods to determine their relationship to you:

• the age of your match
• where they lived when they were born
• available documentation
• DNA testing other relatives, if possible

Read more in this post: First cousin or half-sibling?

Need to get another DNA test?

There is a lot to learn from having multiple siblings test DNA. As you now know, siblings do not have identical DNA.

This means that siblings will have different DNA matches and different ethnicity results. DNA tests taken by full-siblings might reveal different ethnicity regions and distant DNA matches based on the DNA that the other siblings did not inherit.

Important: If your parents are able to take a DNA test, their DNA will show all of the DNA that you and your siblings did not inherit. In other words, if you have to choose between having your siblings or parents take a DNA test, the parents are the best option.

If you are interested in learning how much DNA you share with your siblings, or the ethnicity regions that they inherited (and that you didn’t), you might be interested in my post titled, “Beginner’s Guide to DNA Testing: Ultimate Strategy“.

Conclusion

I hope that this post has helped you understand why your DNA will always match your full and half-siblings. If you have any questions about something that you read here, or if you would like to share your own experience with siblings as DNA matches, I would love to hear from you below in the discussion.

Thank you for being here today!

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