Why do Type AB and O parents have children who are Type A or B?

Believe it or not, in this case having a blood type different from either parent is by far the most common result. In most cases, an O parent and an AB parent will have only A or B kids.

I added a table at the end of this answer that has all blood type combinations for two parents plus the possible blood types for their children. Exceptions to these rules are extremely rare. (Though not impossible. Isn’t genetics fun!)

What I’ll do for the rest of this answer is go through why an O parent and an AB parent will have only A or B kids. As you’ll see it has to do with one gene, two copies, three versions, and four blood types.

As Easy as 1, 2, 3 (4)

One Gene

Your set of genes has the instructions for making and running you. Each gene has the instructions for one small part of you. The gene responsible for whether you have A, B, O, or AB blood type is called the ABO gene.

Two copies

We have two copies of most of our genes—one from mom and one from dad. The ABO gene is no exception. The vast majority of us have two copies of this gene.

Three versions

As humans, we all have the same basic set of genes. What makes you different from me is that we have different versions of some of our genes.

So there isn’t a blue and a brown eye gene for example. Instead there is a gene that comes in a brown and a blue version. (Well, that is a simplification. It actually comes in a brown and a not-brown version.)

The ABO gene comes in three versions: A, B, and O.

Four Blood Types

So there is the single ABO gene that we have two copies of. And that single gene comes in three different versions. This means there are six possible combinations that anyone can have. Here they are:

If you got an A from one parent and an A from the other, then you are the AA baby in the upper left. And an A from one parent and an O from the other, you are the AO in the upper middle. And so on.

These six combinations lead to the four different blood types like this:

As you can see, there are two ways to get an A blood type (AA and AO), and two ways to get B (BB and BO). There is only one way to get AB (AB), and only one way to get O (OO).

That is blood type in a nutshell! Your blood type depends on which version of the ABO gene you got from mom and which one you got from dad.

Now let’s see how you ended up with A and B children.

One Shall Pass

From the previous section we can see that you have an A and a B and that your partner has two O’s. Maybe something like this:

(I made the mom AB and the dad OO here but it works the other way too.)

Remember, you only get to pass down one copy to your child. Your partner can only pass an O while you can pass an A or a B. Here are the two possibilities:

What this means is that each of your kids will either be BO (the B from you, the O from your partner) or AO (the A from you and the O from your partner). They can only have an A or a B blood type!

As an aside, each child has a 50% chance of getting an A from you and a 50% chance for getting a B. Which one you pass down is selected at random.

So that’s why your kids have a different blood type from you and your partner.

Here is that table I promised of each possible combination of parental blood types and the predicted blood types of their children: