A Staff Report from the Straight Dope Science Advisory Board

How does inheritance of blood types work?

February 4, 2000

Dear Straight Dope:

My father is very stubborn. He refuses to have his blood typed, but my mother would like to know what it is, just in case. She knows that her blood type is B+, and my brother's blood type is AB.  With that info can you determine my father's?

Dear Straight Dope:

My husband wants to know why he and I both have blood type A and our biological daughter is an O.

This is going to take some explaining.

There are three blood genes: A, B, and O.  (I'm going to ignore the + and - part of this.)  A and B are dominant, and O is recessive.  You inherit one blood gene from your mother and one from your father.  The combination of genes determines your blood type.  There are four possibilities: A, B, AB, and O.   Here's how it works:

A + A = A
A + O = A
A + B = AB
B + B = B
B + O = B
O + O = O

With that in mind, we turn to your question, Confused.  Your brother is an AB.   That means he got an A from one parent and a B from the other.  Your mom is a B, so we know your brother got his B gene from her. (As a B, she could only have given him a B or an O, and if he'd gotten an O he wouldn't be an AB.  This may make you Really Confused, but think about it a bit, it'll come clear.)

Since your brother got his B gene from mom, he got his A gene from your father.   Your father, therefore, may be A or AB.  He's definitely not B or O.  So while we don't know exactly what your father's blood type is, we've narrowed it down.

Now it's Jill's turn.  The only way you can get type O is to have two O genes (because O is recessive).  Assuming the mailman wasn't involved, the fact that both you and your husband are A and your daughter is an O means that both parents have one A gene and one O gene.  (This is the only way your daughter could end up with type O blood; if one of you had two A genes, she could only have had blood type A.) Thus, there was a 25% chance your daughter would get two A genes, making her blood type A. There was a 25% chance she'd get two O genes, making her blood type O. And there was a 50% chance she'd get an A and an O (25% for getting an A from each of you), again making her blood type A.

So it was a 1-in-4 chance for your daughter to be type O--certainly not a rarity.   That should reassure your husband.  Had your daughter been a B, you'd have had some explaining to do.

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