Half Sibling DNA

One of the most common questions we are asked is “If 50% of the genes comes from each parent, then shouldn’t there be 50% from the common parent?”

You do get 50% of your DNA from each of your parents, however you don’t get the same 50% as your siblings.

Half-siblings only share 25% of their genes

Generally speaking, there is about a 50% overlap between the DNA you received from your mother and the DNA your brother or sister got from that same mother. Therefore, you and your sibling share 50% of 50% of mom’s DNA or 25%. The same principle is true if the parent you share is your father.

Half sibling DNA illustration with a deck of cards.

For this example we are going to use playing cards that represent the genes that are inherited be each of us from our biological parents.

You can do this experiment with a real deck of cards or with a random card generator. For this example we used a random generator. This deck of cards represents the mother’s DNA (this also works for the father). First shuffle the cards and deal out 26 cards. These 26 cards represent the DNA you got from your mother. It is half of her DNA.

Here is the first set of cards that were dealt 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 came up on the second deal:

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

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

We have put little yellow lines through the cards that don’t match. As you can see, the two of you have 13 out of 52 of these cards in common. This helps demonstrate that 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 need to be related.  You would share 13/52 cards or 25%. The two of would you share 13/26 from mom and 0/26 from dad. You’d have different dads and wouldn’t be related at all. In our analogy, one might have “DNA” from a deck of cards from a “Sorry” game and the other a different card game like “Life”.

In some ways we got very lucky with the first couple of deals. Because the cards are chosen at random, there is no reason we 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 a third hand of 26 (this would be a third sibling) is dealt:

Again this sibling is 50% related to the shared mom. These 26 cards came from her deck of 52.

Let’s line up all three and see how these three siblings match:

Notice is that there is not a 13 out of 26 match each time. This is because the 26 is random. As mentioned earlier, it is theoretically possible to have none or all of them match. You can see that in this illustration there are matches of 13/26, 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. 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.

Only parents and children always share an exact amount of DNA

Our hope is that this helps with your understanding of half sibling genetics. Most everyone shares the 50% of their DNA with their mother and 50% with their father. In other words, sons and daughters 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 second cousins, you can have levels that vary a lot from the predicted 3% because the little differences can build up with each generation.

Our DNA testing helps determine which genes, or “cards”, are shared.

You don’t have to rely on guessing which cards you share through DNA testing. The more “cards” (genes) that are examined the more accurate the testing. Most half-sibling tests only examine 16 markers. Using the example above you can see that there may be many genes missed and it would be hard to obtain an accurate result. Bio-Gene DNA can perform low marker tests like these, but we won’t. We insist on performing higher marker testing for more accurate results.

If either of the mother’s DNA is available to test, then we will use the DNA from the mother’s sample to eliminate that portion of DNA in one or more of the potential siblings and the focus on the paternal DNA that is left to determine the relationship.

If the mother’s DNA sample is not available, or you do not wish to include her sample, then we can test using a process that is specifically designed for sibling to sibling DNA testing without the mother’s involvement. Our laboratory’s state of the art equipment and complex statistical algorithms allow us to obtain the most accurate answers possible with just two siblings.