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Blood type calculator: enter blood types for both parents and find out possible blood types for a child, or blood types for one parent and one child and find out possible blood types for the other parent. Please use this first!
I got a letter two years ago from someone who very much wanted help disentangling her family history. She wrote to me:
I have just found my deceased father's blood group, and it has got me worried. I am AB, my brother is O, my sister is O, and so was my father. My mother was type AB, I think. So the burning question is, Is my father really my father?
As you may know, you have two copies of every gene in each cell of your body, and you get one from your mother and one from your father.
For example (and to oversimplify a lot), there are two forms of the gene for eye color, one for brown and one for blue. If you have both genes for blue, you will have blue eyes; if you have one or two genes for brown, you will have brown eyes. I will write B for brown and b for blue. So blue-eyed people have bb genes, whereas brown-eyed people can have BB, Bb, or bB genes. (The gene from the father comes first, so Bb means you got brown from your father and blue from your mother.)
Two blue-eyed parents can only have blue-eyed children, whereas two brown-eyed parents can have blue-eyed children if both of them are of the Bb or bB types, and both happen to give their child the b gene. (You probably know some exceptions: I am one, because my father had blue eyes and my mother had brown ones, whereas my own eyes are blue. But looking closely shows that there are flecks of brown in my eye color; blue here means 100% true blue.)
Moving on to the ABO blood type system. There are three kinds of genes here, A, B, and O. The A gene will cause a person to have red blood cells with the A protein in them, and the B gene will cause a person to have red blood cells with the B protein in them. The O gene doesn't do either one. So if someone's genes are AA or AO or OA, they will have A protein and be of blood type A. Someone whose genes are BB or BO or OB will have B protein and be of blood type B. Someone whose genes are AB or BA will have both proteins and be of blood type AB. And finally, genes that are OO will have neither protein and be of type O.
In your case, your your father's, brother's, and sister's genes are OO. Your mother is AB or BA and so are you. Your mother gave you either an A or a B gene, and you had to get the other B (or A, as the case may be) from somewhere. Your father is OO, so where did the other gene come from?
But that's not the whole story. Your brother and sister are OO, and your mother could not give them an O gene (since she has only an A gene and a B gene), so where did their O genes come from? One possibility is that you're wrong in thinking your mother was AB.
The most probable explanations are adoption, sperm donation, or something else that makes you and your siblings have different genetic parents. A DNA test of you and your siblings, preferably both of them, will nail this very reliably, and I would encourage you to get one. It turns out that about 15% of human beings, on average, are mistaken about their genetic fathers.
There is another possibility. There is another gene known as the H gene, which comes in two varieties: H (working) and h (not working). (The whole issue of ABO and H versus h does not make any difference to health, of course.) Neither the A nor the B protein can be made in your body unless you have at least one H gene. So people who have hh in their genes always appear to have blood type O, because no A or B protein is being made in their bodies even though the A or the B gene might be physically present. So your father might actually have an A or B gene to give you even though his apparent blood type was O, if he also had hh. However, the h gene is quite rare and the hh combination even rarer, so this isn't a very likely explanation.
Finally, she added:
I would be grateful for any help you can give me. I will always love him either way; I just need to know.
Of course. As an adoptive parent whose daughter has always known that she is adopted, I know that genetics has very little to do with how we feel about our children or how they feel about us.
This post has obviously struck a nerve: it has gotten more comment than anything I've ever written. If you are going to comment to ask a question, three things, please:
- The Rh blood types (+ and -) are separate from the ABO blood types. The only thing to say about them is that two - parents will always have - children; every other combination is possible.
Look in the following A/B/O chart first. Find your mother's blood type across the top, your father's along the side, and your possible blood types in the box.
|A||A or O||Any||Any but O||A or O|
|B||Any||B or O||Any but O||B or O|
|AB||Any but O||Any but O||Any but O||Any but O|
|O||A or O||B or O||Any but O||O|