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!
Redwood's Medical Edge 
All I know is my husband is blood type A and I’m blood type B.
Both our children are O. I originally thought they would have to be A or B or AB. But I was wrong!
The most fascinating study of family blood types was in the book Fatal Vision about the Green Beret doctor, Jeffrey McDonald, who killed his family. Everyone in his family, including him, had a different blood type. The prosecutor was able to trace McDonalds steps and what he did by the blood types found in each room.
To this day, I still find this book fascinating to re-read!
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