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...the genes for blood type reside on chromosome 9 (this was in Matt Ridley's "Genome") and the genetic sequence that produces type A are nearly the same as type O (which produces no distinctive protein) -- type O is the same sequence as type A, with the first base pair (or maybe it was the first "word" of three base pairs; I'm too lazy to look) missing. Type A and type B are very similar, but produce different proteins. Type B is about 15 per cent of the population; type A is 40 per cent, as is type O; type AB is the result of having the A and B coding on the same chromosome and is about 5 per cent of the population.

And there's more:

More Biochemistry and "Bombay phenotype"
by Robert J. Huskey
An interesting situation arises when an individual is not able to make the "H" antigen. Such a person cannot produce the "H" antigen and even if the "A" or "B" enzymes are present, cannot make "A" or "B" antigen since there is no precursor for the antigens to act upon. An individual who cannot produce the "H" antigen will appear to have blood type "O" since this blood type is a negative category (not-A, not-B, not-A and not-B). This was first documented in Bombay and has become known as the Bombay phenotype and has been exploited on the popular soap opera General Hospital.

To my eye type A seems to be the original bloodtype, producing the others through mutation. The exceptions may be the MN bloodtype system, which comes from the Indian subcontinent, and are analogous (but not the same thing as) to A & B (codominant), AFAIK (it's often difficult to find anything about the MN system, which is in a very small number of people worldwide). Strange, isn't it, that a completely unrelated bloodtype system exists (still exists? recently sprang into existence?) after centuries, even millennia, of contact, intermarriage, and commerce, all over the world?

GENOME
the autobiography of
a species in 23 chapters
by Matt Ridley
(from chap 9)
The different kind blood group you have determines your susceptibility to certain diseases. For example, people with A blood are less likely to get diarrhoea than people with B blood. People with O blood are more susceptible to getting diarrhoea than anybody else. People with AB blood are virtually immune to diarrhoea because of their resistance. Nobody really yet knows how AB genotype protects them from this disease. "Since people with the O blood are the most susceptible to the disease, shouldn't they die out according to natural selection?' you are probably asking. That is true but there are a couple of things that keep the O group alive and one of them is malaria. People with O blood are more resistant to malaria than other groups. Another thing is that the O group is less likely to get certain cancers. These benefits cancel out the negative effect that the O blood group has on the diarrhoea disease so, this balance has kept the group from disappearing.