How to Determine Blood Type

In the age of DNA testing, blood typing seems to have fallen by the wayside in use in novels but I think it can still be very valuable and add an element of suspense and surprise. A child’s blood type may be the first clue to a parent that they may not be biologically related.

Blood type is determined from two allele’s. An allele is a “form of genetic information that is present in our DNA at a specific location on a specific chromosome”.

There are four blood types: A, B, AB, and O. Blood type A can be designated either by AA or AO. Blood type B can be designated by BB or BO. This will be clearer below.

This is the easiest way I’ve been shown to determine a child’s potential blood type. In the square below, the top horizontal portion is one parent, the vertical side is the other parent. Each box with a single letter is the one allele that parent will give their child. A child receives one allele from each parent so you need to “cross multiply” each square to determine blood type.


In the above instance you have one parent that is blood type A (designated AO) and one that is blood type O (designated OO). As you can see, their child would have a 50% chance of having blood type A and a 50% chance of having blood type O.

Let’s look at another example. Take a look at what happens when both parents are blood type AB.


In this case, their biological child would have a 25% chance of being blood type A, a 50% chance of being blood type AB, and a 25% chance of being blood type B.

How can this work for your fiction? Let’s look at this example. You’re writing a novel that centers around a child diagnosed with leukemia. The child needs a bone marrow transplant. The presumed parents are blood type A (AO) and blood type AB. You have a child with blood type O. Can this child be the biological offspring of these two parents?


Give your answer in the comments section.


Resources: This resource includes a blood type calculator!

How is Blood Type Determined?

There’s nothing more fun as a suspense author then to have a twist in your novel. Some medical twists can be intriguing and one that is in such a category is looking simply at blood type. Knowing a child’s blood type can give you an idea if their parents are really their parents.

b7ab9-bloodbagLet’s say a child has AB blood type, but his parents are both blood type O. I can tell you right now that those parents are not that child’s parents. How? Because you inherit a letter from each of your parents. So this child can be A, B, or AB blood type, but never O.

A parent that is just A or B blood type have an O they can pass on. It might be easier to view them as AO blood type. So if both parents were blood type A— their children could actually just be blood type O from inheriting that “O” from each of their parents. What blood type could they never be?

Type B.

Here is a chart that looks at how blood types are determined. Scroll to the bottom of the page.

As a writer, have you ever used blood type to reveal that a child is not living with his biological parents when he thought he was?