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.

A O
O AO OO
O AO OO

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.

A B
A AA AB
B AB BB

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?

A O
A AA AO
B AB BO

Give your answer in the comments section.

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Resources: http://www.biology.arizona.edu/human_bio/problem_sets/blood_types/inherited.html. This resource includes a blood type calculator!