Nice answer from David in How does inheritance of blood types work? (I wonder if that worked…) however, one tiny clarification. I would be more inclined to say that we have ONE gene that determines ABO blood types with three ALLELES – those being A, B, and O. Other than that, his description of the inheritance patterns is spot on, and Jill’s husband has no worries.
Why did “Confused” not give his/her own blood type? Stubborn like Dad? Having info on two offspring might have nailed Dad’s type. If he/she were O, Dad would have to be A. Of course, B or AB would leave it inconclusive.
Oops. Left out A. That would also leave us with “either A or AB.”
All well and good, But I was waiting to find out where RH negative comes from. Any ideas?
There are several Rh antigens, but the first one discovered, Rh1, causes the most severe immune reaction (Rh+ presence of Rh1 vs. Rh- absence of Rh1) and is the primary determinant of the Rh trait. The Rh antigens are coded by a closely linked set of three genes on chromosome 1, so their inheritance tree is more complex than that of the blood types.
Rh antigens are present in the cell membranes of red blood cells (erythrocytes.) The immune reaction is caused because a person lacking Rh1 (a person with Rh- blood) will form antibodes to the Rh1 antigen after a first exposure, so if they are exposed a second time to Rh+ blood (e.g. by a blood transfusion) their antibodies will attack the Rh- blood cells and they will get sick.
In a similar fashion, if a woman with Rh- blood is pregnant with an Rh+ fetus, and she has been previously exposed to Rh+ blood (either by a previous blood transfusion, or by a previous pregnancy with an Rh+ fetus and some fetal blood traversed to the mother’s bloodstream during labour), then her antibodies will attack the fetus and cause anemia (in the fetus), which can result in death.
The way I recall it (I’m doing this out of my head, so I’d love to be corrected), blood typing comes from the proteins on the surface of the red cells. A and B are the two types of agglutinogens(sp?). If two incompatible types are put together, the blood cells clump up into a useless mass. O is the lack of agglutinogens. So A blood attacks B blood, and AB can tolerate both. Nothing attacks O.
Rh is another protein of the same type, but there’s only one type. You either have it or you don’t, so they label it positive or negative.
That’s why AB+ is considered the universal receiver–it has all the factors, so none of them will bother it. O- is the universal donor–it has none of the factors so it can’t attack any type of blood.
I hope this made a little sense. 
Arnold posted while I was writing mine. If I’d known, I wouldn’t have bothered. Very clear and complete post.
I realize that SDSTAFF David was probably just being polite, but I for one would appreciate explicit acknowledgement of the fact that Confused’s father, Confused’s brother’s father, and Confused’s mother’s husband could conceivably be three different people. It is reasonable (from the point of view of social convention) to assume that the three are in fact the same man, but we ARE fighting ignorance here (at least, David is) and it is a plain disservice to not point out the assumptions one is making when answering a technical question. If nothing else, doing so might help some casual readers think a little more clearly, or recognize that one’s automatic assumptions cannot always be blithely and unquestioningly accepted.
Or am I the only one who recognized the wording of the question might be an intellectual “trap,” played by a prankster?
Scole – you’re right, we probably should have used the word “allele” or something similar. I was trying to make it simple, and pretty much everybody knows what a “gene” is, but not so many know what an “allele” is. And, frankly, I figure anybody who does know what an allele is already knows how blood types are inherited. 
Saltire, I also wondered why Confused didn’t give his/her own blood type. < shrug >
And, yes, Df, I should have noted that the assumption was that they had the same father.
I’ve always thought that I had a pretty good handle on blood types, except that I thought that type O was the name for the absence of types A and B.
But I have another question - why are there still A and B types (and Rh-positives) around? Since the child of two type A parents can be either A or O, and the child of two type O parents can only be an O (my parents are both O-), it seems like each generation would have a higher percentage of O’s than the previous, eventually leading to an all O-negative population (I guess I’m more evolved that y’all).
They are still around because it only takes one A or B to override the O. And not every A or B type has an O allele along, they may have two As or two Bs.
Thank you for the compliment. I refreshed my memory with the Encyclopedia Britannica (to find out about how many genes coded for RH factors) before I posted my answer. Don’t forget that EB now available online. http://www.britannica.com