How did half-siblings end up with AB and O blood types?

July 7, 2012

A curious adult from New York asks:

"My mother had AB blood, I have AB Blood, and my daughter has AB blood. Her half brother by her mother has O. How is this possible?"

This is a perfect question for a geneticist. To figure out how this is possible (which it is by the way), we need to step back and do a bit of review on how genes work. Here are the basics of what we need to know:

  1. We have two copies of each of our genes, one from mom one from dad
  2. These genes can come in different versions
  3. Sometimes one version dominates another

So your AB blood type is actually two different versions of the blood type gene. You have an A version and a B version, one from each of your parents.

As you pointed out in your question, there's another version of the gene called O. This version is interesting because you can actually think of it like a zero or nothing. This has to do with what the O gene version does at a molecular level on our blood cells.

An A and an O gives an A blood type and a B and an O gives a B blood type. In this case, that's what it means that some versions dominate others. The A and B versions override the O, but not each other.

Here are all six of the possible combinations of these three gene versions and their corresponding blood type:

  • AA or AO – A blood type
  • BB or BO – B blood type
  • AB – AB blood type
  • OO – O blood type

Now we are almost ready to show how your result came about. The last thing we need to know is that parents pass only one copy of their gene to their child.

So an AO parent (who is A blood type) will pass either an A or an O version to their child. And an AB parent will only pass an A or a B.

This is all the information we need to explain how your daughter is AB and her half brother is O. Let’s get started!

From AB Parent to AB Child

Let's figure out the answer to this question step by step. We'll start with you and your daughter and then work our way towards her half-brother's blood type. This is way easier than trying to do it all in one fell swoop.

Ok, so your blood type is AB. Like I said before, that's actually a separate A and B version of the blood type gene.

To make this all a bit simpler, let's imagine each gene version as a different block. So the A version is a block with the letter "A" on it, the B version is one with the letter "B" on it, and the O version is a block with an "O" on it.

Everyone has two blocks that are some combination of "A," "B" and/or "O." Here's what yours would look like:

A and B blocks.

This is your only possible combination because there is only one way to get the AB blood type. Now, let's think about your daughter.

Remember, she gets half her genes from you and half from her mother. So let's add her and her mother into the picture:

Parent child inheritance.

We know your daughter has the AB blood type so I wrote that in. We also know that only one of those versions of the gene comes from you. Remember, you can't pass an AB down to your daughter by yourself… you can only give her one of your two blocks.

Let's say you give her the A block. If you give her the A, that means the B must come from her mother since she gives your daughter the other half of her blood type. You could very well have given your daughter your B block (in which case her mother would have given the A), but we’ll stick with just one of these scenarios for now.

This is what we have so far then:

Parent child inheritance.

Do we know what her mother's other block is? Not yet. At this point, it could be A, B or O.

Since she can only give her daughter one of her blocks, it means we don't know what her other block is. Until we look at your daughter's half brother that is...

Now the real detective work can begin! You wanted to know how your daughter's half brother has O blood type. First of all, the "O" blood type actually means that you have two O's (just like two A's would be called an A blood type). So let's add the half brother and his father to this tree.

Half sibling inheritance.

Ok, now let's figure out what the missing blocks are. Before, we didn't know what the mother's second block was, but now we know, right? It has to be an O since she gave an O to her son.

And what about the half brother's father? We know what one of his blocks has to be… it's gotta be an O. But we don't know what the other block is based on this tree. It could be an A, B or another O.

Half sibling inheritance.

And finally, for good measure, let's add your parents to the tree. At this point you can probably do it yourself, but just to remind you, here's the important information and rules to remember:

  1. We have two copies of each of our genes, one from mom one from dad
  2. These genes can come in different versions
  3. Your mom is AB

We can't figure out your father's whole blood type from this information, but we can make a guess about half of his blood type because we know yours. And to make this easier, let's say your mom passed her "B" version of the gene to you. Now you can figure it all out! But just in case, here’s the full family tree:

Family tree.

If you had a sibling with a B blood type we could narrow down what's in your father's second box, too. And now that you understand that, you actually understand how all genes are passed down from parents to their children!

Author: Elizabeth Tanner

When this answer was published in 2012, Elizabeth was a Ph.D. candidate in the Department of Genetics, studying preventing drug-resistance in viruses in Karla Kirkegaard’s laboratory. She wrote this answer while participating in the Stanford at The Tech program.

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