What blood type can bear what child blood type?

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I am 15 weeks pregnant and I recently had a series of bloodwork done. I found out that I was AB+. My father is definitely O- and my mother is unsure of her type. I have seen online many sites that say this is not possible, that my father cannot have O-. Is this correct?

-A curious adult from Texas

May 26, 2005

No, this isn't necessarily correct. There are cases where 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. In both cases, an O parent can have an AB child.

Before going into too much detail about the exceptions, it is probably a good idea to understand the rules first. Since only your dad's O status is in question, we'll deal with the ABO part. For more information on Rh+/- status, click here and 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:

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.

I figured 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:

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

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

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 rare blood type called the Bombay blood group. People with the Bombay blood group have the A and/or B genes but test as an O.

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:

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).

The AB child is a blood chimera. There are people called blood chimeras who are AB 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. Blood stem cells make new blood cells throughout our life.

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

You might be saying, but I'm not a twin. But 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. And yet this answer doesn't delve into every possibility. You can find other possible explanations in our answers here, here, and here.  

The main thing to remember is that the genetics we learn is simplified. It works very well for lots of cases but not all of them. Real world genetics is way more complicated.

By Dr. Barry Starr

More Information

Bombay blood group
Group A subtypes
Blood chimerism

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lone77star says

Anthropologists have long been puzzled by influxes of peoples to Western Europe throughout human prehistory. They don't know where they came from.

But the highest incidence of O-type and Rh-negative is found in the Basques and other Western Europeans. These may well be the children of Atlantis.

The problem that @Farmer Rachel mentions concerning birthing problems with Rh-negative mothers may have helped to keep the Atlantean refugees' birthrates low so that the more fertile Eurasian populations eventually overwhelmed them despite their superior technology.

The Atlanteans, according to Plato, had a great cavalry and a great navy. This may be where many of the myths came from, including the myth of centaurs -- half man, half horse. To the surprised natives of Eurasia, a man on horseback may have looked to be one creature, especially when their level of surprise kept them from noticing the details of equipment and clothing.

The myth of Atlantis is very similar to the myth of Metis and Athena. Metis may well have been Atlantis in its latter days, while Athena represented the refugees born fully mature and with weapons and armor. That Metis and Athena were both female may indicate that Atlantis and the refugees were matriarchal societies. Something Plato might not have included because of patriarchal prejudices.

Another, possibly related group, are the Georgians of the Caucasus. I believe they also have high Rh-negative, speak an agglutinative language with affinities to Basque, and they are the ones who had the Golden Fleece and the golden dragon which protected it. Artifacts from ancient Atlantis? And when Jason's ex-wife left her second husband, king of Athens, she was seen flying away on a golden dragon. If her disappointment with the men in her life led her to form a band of women warriors, what better name to have given them than "Ama-Atlan" (Amazon, or "Mother Atlantis").

We also have scientific evidence of an Atlantis-like event occurring right when Plato's legendary island was supposed to have subsided -- 9600 BC.