What are all the possible blood types for any combination of parents?

Well, at least you're not asking for much! I've put all of the most likely possibilities for any pair of parents in a table at the end of the answer. 

As you can see, two B parents are most likely to have either a B or an O child. Now two B parents having a B child is easy to understand. But where does the O come from? To understand this, we need to remember three things about genetics:

1. We have two copies of most of our genes
2. Our genes can come in different versions (called alleles)
3. Genes are recipes for proteins

There is one gene that determines the ABO part of blood type. This gene comes in three versions -- A, B, and O. Each of our two copies of this gene can be different versions.

So someone can have an A and a B, a B and an O, two A's, etc. Here are the six ways these three gene versions can be combined and what each person's blood type would be:

This is where point 3 comes in. The blood type gene makes a protein that sits on the outside of our red blood cells. Most of this is pretty straightforward. Two O's make O blood type and an A and a B make AB. But why do AO and BO make A and B, respectively? Why not AO or BO?

The A version makes a protein that is just a little bit different than the B version. But the O version makes no protein at all. And it is the presence of the proteins that determines our blood type.

Knowing this makes the blood types easier to understand. Someone who has an A and a B version of the blood type gene makes both A and B proteins. And so is AB blood type.

But someone who has a B and an O version only makes the B protein. They are B blood type but can pass the O onto their kids.

So two B parents can make an O child if both parents are BO. How? By each of them passing down their O version.

If mom passes her O and so does dad, then the child will be OO which is O type blood. Each parent has a 50% chance of passing down the O gene. So each child has a 25% chance of ending up with an O blood type.

A quick way to figure this out is using one of those awful Punnett squares from school. The way a Punnett square works is one parent's two gene versions go on top and the other parents goes down the side. Here is an example with two BO parents:

To figure out the possibilities and the chances for each, you just match up the squares like this:

So each child has a 75% of being B and a 25% chance of being O. Note that genetically, not all of the B blood type kids are the same. The BB child will only pass down the B version of the gene. The BO child can pass on their O gene version.

As I said, the blood tests used to determine blood type look at the proteins made and not the DNA. What this means is we can't tell a BO from a BB with these tests. You could with a DNA test but these aren't routinely given.

One part of blood type that we haven't discussed is the +/- part (called Rh). This is in many ways similar to ABO except that there are only two versions, Rh+ and Rh-.

The Rh- gene version is like O in that it doesn't make any protein. So in the same way that two B parents can have an O child, two Rh+ parents can have an Rh- child. (Click here to learn more about Rh factor.) 

Table of possible blood types

As promised, here is the table showing what each combination of parents is likely to have.

Well, one more thing before getting to the table. I want to emphasize that there are exceptions to these rules. So if the table says you can't have the blood type that you have, don't jump to any conclusions! There are documented examples where these rules are broken (see below).

And I mentioned Rh (the +/- part of blood type) is determined by a different gene. It is inherited separately from the ABO part of blood.

Exceptions to the rules

There are a few ways to break the rules and end up with a child with an unexpected blood type! You can read more about the rule-breaking exceptions here:

• Chimerism
• Bombay blood group
• Cis-AB blood type
• Mutations