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Blood Types

Whose blood do we carry, our moms or dads? Or it doesn't matter because it could be from either one?

-A curious adult from Massachusetts

November 18, 2004

Well, your blood is definitely all your own. But because of how the genetics of blood type works, it could seem like you have your mom's blood type, your dad's blood type, or a mix of the two.

For every gene, you get two copies -- one from your mom and one from your dad. Even though the two copies are for the same gene, you can get different versions from each parent. These different versions are called alleles.

Alleles are a little like cars -- the same make and model of car can come with either two doors or four doors. The cars do the same thing (get you where you're going!) but they are slightly different.

Now let's talk about the genes for blood type. There are two main genes. One gene is for the ABO type. This gene codes for a protein that is on the surface of your blood cells. The different versions or alleles for the blood type protein are called A, B, and O.

The other gene is for another protein on your blood cells called the Rh factor. The alleles are called plus (+) and minus (-). You can learn more about the Rh factor and its implications for health in the answer to another question here:

It doesn't matter what type your blood is unless you need a blood transfusion. In that case it is very important that you receive blood that is of the same type as yours (or type 0-, which I'll talk about at the end). If you get the wrong type of blood your body will see the transfused blood cells as foreign attackers and will destroy the new blood cells, making you very sick.

The two genes for blood type are inherited separately, so we'll talk about them individually, and then put it all together at the end.

Let's take an example of a mom who has blood type AB, and a dad who has blood type A. You might notice that the mom has two letters in her blood type. This is because she has the two different alleles she got from her parents (A and B). But then why does the dad only have one letter? It's because he either has two A alleles or he has one A allele and one O allele. This is just the way people write these things.

There isn't any way to tell what kind of type A blood the dad has unless we know the blood types of his parents. For now we're going to assume that he has one A allele and one O allele, and we're going to write out both alleles when we talk about the possible blood types of the kids to make things easier for us.

To figure out what blood type the kids of these two parents can have, we can use a genetics tool called a Punnett square. It's really just a chart that helps us organize the genes. We'll put the mom's alleles across the top, and the dad's alleles down the side.

Each box in the chart represents a possible combination of the alleles donated by each parent (each box gets one entry from mom, one entry from dad). We can say what percent chance a child has of having a particular blood type by how many times that blood type shows up in the chart (one out of four is 25%, two out of four is 50%, etc.).

If we fill in the possible mixtures of alleles each kid could get in our example, we get a chart that looks like this.

So the possible blood types are AA, AB, AO, and BO. Like I explained for the dad's blood type before, people with two A alleles (AA) and people with one A allele and one O allele are both called type A (the same thing goes for type B). So, the kids of these parents have a 50% chance of being what we call type A, a 25% chance of being type B, and a 25% chance of being blood type AB.

You can do the same with the Rh factor gene. Let's say that the mom was Rh-positive (with one + allele and one - allele, though she could also have two + alleles) and the dad is Rh-negative (meaning both of his Rh alleles are -).

There are two possibilities for the Rh-status of the kids. They have a 50% chance of being Rh-positive (+/-) and a 50% chance of being Rh-negative (-/-).

Let's put it all together. The mom was blood type AB+, and the father was blood type AO-. A Punnett square for this is shown below.

The possibilities for their child's blood type are. AA+, AA-, AB+, AB-, AO+, AO-, BO+, and BO-. (The Punnett square can also tell us that there is a 1 in 4 chance for either A+ or A-, and a 1 in 8 chance each for AB+, AB-, B+, and B-.)

You can see that two of these, AB+ and AO- (in blue), are exactly like one of the parents. All the others are mixtures of the two.

This is just one of the many examples of how the instructions for making you that are in your genes make you like your parents, but at the same time unique.

I said at the beginning that if you needed a blood transfusion that you could either get the same blood type as your own, or else you could get O- blood. But why?

People with O- blood are called universal donors because they can give blood to anyone. The reason is that the O and the - actually mean that their blood cells don't have any versions of either of the two proteins coded for by these gene on their surface. So there is nothing for a person's body to see as a foreign attacker. Unfortunately for these people, it also means that their bodies will see all other blood types as foreign attackers and that they can only receive other O- blood.

Is there a universal acceptor blood type? Yes, it is AB+. These people are lucky because their bodies are used to seeing both the A and B versions of the protein, and they are used to seeing the Rh+ factor. So no matter what kind of blood they receive, their bodies won't see it as a foreign attacker. Only about 3% of the population is this lucky (http://chapters.redcross.org/br/northernohio/INFO/bloodtype.html).

By Erin Cline