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Can you please explain why half-siblings share 25% of their genes? If 50% comes from each parent, then they should, in theory have 50% from the common parent. I know this is not so, but I do not know why.

-A curious adult from Peru

September 5, 2013

These percentage things are always tricky.  While we do get 50% of our DNA from each parent, we don’t get the same 50% as our siblings. 

In general, there is about a 50% overlap between the DNA you got from your mom and the DNA your brother or sister got from that same mom.  So you and your sibling share 50% of 50% of mom’s DNA or 25%.

I always find this kind of explanation to be a bit confusing.  An easier way to see what is going on is to do a simple experiment with a deck of cards.

Cards Are Like Genes…

Imagine that a standard deck of cards is the shared parent’s DNA.  So I don’t have to keep saying shared parent, I’ll say it is mom’s DNA.  Keep in mind, though, that this works for dad too!

You can do this experiment with a real deck of cards or with a random card generator like this one.  For this answer, I’ll use the random generator. 

First shuffle mom’s DNA and deal out 26 cards.  These 26 cards represent the DNA you got from your mom.  It is half of her DNA.

Here is my first set of cards from the random card generator:

Next you add the cards back into the deck and “reshuffle” to get your sibling’s DNA.  Here is what I got on my second deal:

The first thing to notice is that the cards are not all the same.  Even though you and your sibling each got 26 out of mom’s 52 cards, you didn’t get the same 26.  You each got half the deck of cards but not the exact same half. Hopefully this helps explain why the two of you don’t share the same 50% of mom’s DNA.

Now let’s see how many cards the two of you share in common:

What I’ve done is put little yellow lines through the cards that don’t match.  As you can see, the two of you have 13 of these cards in common.

So in this case, the two of you share 13 out of 52 of mom’s cards.  In other words, the two of you share 25% of your mom’s DNA.  (Note that both are still 50% related to their mom.)

If we are dealing with a half sibling, this is all the two of you would be related.  You’d have different dads and they wouldn’t be related at all.  In our analogy, one might have “DNA” from an UNO deck and the other a different card game like Sorry

So the two of would you share 13 out of 26 from mom and 0 out of 26 from dad.  As half siblings, you would share 13/52 cards or 25%.

In some ways I got very lucky with that first couple of deals.  Because the cards are chosen at random, there is no reason I had to get a 13 out of 26 match in this case.  Theoretically there is even a chance that they could all match or that none of them might!  Let’s see what happens when we are dealt a third hand of 26 (this would be a third sibling):

Again this sibling is 50% related to the shared mom…these 26 cards came from her deck of 52.  But let’s line up all three and see how these three siblings match up:

This time I didn’t put little lines through the cards that don’t line up so you’ll have to take my word for the numbers. (Or check to see if I’m right…I may have missed one!) 

The first thing to notice is that we didn’t get a 13 out of 26 match each time.  This is because the 26 that we get is random…as I said, it is theoretically possible to have none or all of them match.  Here we got a 13/26, a 12/26 and even a 10/26.  If this were exactly how DNA worked, in the last case the half siblings would only share about 19% of their DNA.

This rarely happens in real life though.  Most of the time, half siblings are pretty close to 25% related. 

This is because mom’s DNA is made up of way more than 52 “cards.”  The end result of a larger deck of cards is that we tend to get much closer to the expected percent relationship than we do when we use a standard deck of cards. 

So half siblings are almost always pretty close to 25% related and rarely anything like 19% or less.  Below is an image from our previous answer that gives a feel for the range of percentages of DNA that a grandparent and a grandchild might share.  Half siblings would be similar.

As you can see, the vast majority of cases will be between 24% and 26% and most of those between 24.5% and 25.5%.  There is a chance that they might not be related at all but it is vanishingly small.  In fact, anything less than 22% and more than 28% is pretty unlikely!   

Only Parents and Children Always Share an Exact Amount of DNA

So there you have it.  Most everyone shares the 50% of their DNA with their mom and 50% with their dad.  In other words, sons and daughter are pretty much always 50% related to mom and 50% to dad.  But this is where the exact percentages end.

Other kinds of relatives share around the same amount of DNA.  So siblings share around 50% of their DNA, half-siblings around 25% and so on.  And once you get out to fifth cousins, you can have levels that vary a lot from the predicted 3% because the little differences can build up with each generation.   

By Dr. Barry Starr, Stanford University

It isn't obvious at first why siblings share half their DNA (or half siblings one quarter of their DNA).