I have been interested in how a child's blood type is determined for some time. My wife and I are both type A, but my child's blood type is type O. As a child, I learned that:
A+A=A
A+O=A
A+B=AB
B+O=B
O+O=O
It was a simple story like that, but of course, if that were the case, something like ours couldn't happen. The reaction would be, "Whose child is that?"
I was told, "No, no, since both the father and mother are type AO, it's possible for the child to be type O."
That doesn't really convince me. So I did a quick Google search and found this page.