I have type AB blood, and my father is definitely O-. Is that truly impossible?

It’s unusual, but not impossible. There are cases where an O parent can truly have an AB child.

For example, someone can appear O when they are actually A, B, or AB genetically. And there are some cases where an AB child can actually be A or B genetically.

And of course, a lab mixup or a faulty human memory is always possible! It never hurts to repeat the bloodwork.

Normal blood type inheritance

Before going into too much detail about the exceptions, it is probably a good idea to understand the rules first. Because it is true that if the rules are followed, an O parent couldn’t have an AB child.

Since only your dad's O status is in question, we'll just deal with the ABO part. (For more information on Rh+/- status, click here.)

There are three main versions of the ABO blood type genes, A, B, and O. Remember, we have two copies of most of our genes – one from mom and one from dad. Below are the blood types with each of their possible gene combinations:

Blood Type	Genes
A	AA, AO
B	BB, BO
AB	AB
O	OO

Let's do an example to show why, if this were the whole story, it would be impossible for you to have an O dad. Let's say your mom was AB and your dad was O. What possible blood types would their children have? A and B.

You can figure this out using a Punnett square. A Punnett square is really just a chart that helps us organize the genes. I put mom’s two gene versions on top and dad’s on the side like this:

	A	B
O
O

I next matched up squares to figure out the possible blood types like this:

	A	B
O	AO	BO
O	AO	BO

So how did you end up AB? There are a number of possibilities, but we'll focus on two:

Possibility 1: Dad isn’t really an O blood type.

There are at least two ways your dad could test as an O but actually be an A, B, or AB genetically.

The first is a very rare blood type called the Bombay blood group. People with the Bombay blood group test as an O, but have the A and/or B genes.

For the A and B genes to do their job, they need another gene, H. If someone has two broken H genes, then their blood cells look like an O person's blood cells even if they have the A and/or B genes.

Let's say someone has an A gene and two broken H genes. Why do they look like an O? First off, we need to remember that that blood test looks at our blood cells not our genes. If someone looked at this guy's genes, they'd conclude he had A type blood.

What a blood test looks at are proteins that are stuck on the outside of the red blood cells. (Remember, genes are just instructions for making certain proteins. So the H gene makes the H protein, the A gene, the A protein, etc.) Blood type A means the A protein is there, B means the B protein is there and O means neither is there.

What this guy's A gene does is make a protein that turns the H protein into the A protein. If he doesn't make any H protein, then he can't make any A protein – there is no A protein on his red blood cells. So, he looks O but is genetically A.

What happens when someone with the Bombay blood group has kids? Most likely their hidden A and B genes will be seen in their children. So for our example above, the results might look like this:

	A	B
O	AO	BO
O(A)	AA	AB

Now as you can see, AB is possible.

Another way for an apparent O to have an AB child is if the “O” parent has one of a couple of rare subtypes of the A gene. These genes with names like Ax (Ao) or Am make very little A protein. So little, in fact, that the protein test sometimes misses it and the person can be mistaken for an O.

There are additional tests that can be done to distinguish these A subtypes from O but they aren't always done (probably because the subtypes are so rare).

Possibility 2: The AB child is a blood chimera.

There are people called blood chimeras who are AB on a blood test, but are genetically either A or B. Some of their blood cells have different DNA as compared with the rest of the cells in their body.

And most importantly for our discussion here, both the A and B genes in their blood can come from a single parent. An O dad and an AB mom could have an AB child.

So how are blood chimeras made? A blood chimera happens when twins exchange blood stem cells while still in the womb. Blood stem cells make new blood cells throughout our life.

The blood chimera we describe here could have resulted from an A twin and a B twin sharing blood stem cells. She has AB blood, but genetically she is either A or B.

You might be saying, but I'm not a twin. Even so, you might have shared your mother's womb with a twin at some point.

Some number of pregnancies start out as twins but end up having only one baby. At some point early in the pregnancy, one of the twins is lost. This is called “The Vanishing Twin Phenomenon.” I have seen numbers as high as 10% of pregnancies are of this type.

A blood chimera could result from this kind of twin pregnancy. About 8% of fraternal twins are blood chimeras.

So, as you can see, either you or your dad could be different genetically from what your blood test tells you. The main thing to remember is that in genetics, there are exceptions to almost every rule!