Please forgive me in case my question wouldn't make much sense. I was reading about ABO blood groups on Wikipedia, where I learnt that O is a recessive allele, and that it seems the A allele predates the O allele. My question is, therefore, how is it possible that almost half of the human population is group O?
For instance, I would expect (quite naively I reckon), if only alleles A and O existed, that around 75% of the population should be A, and 25% should be O.