blood is actualy determined by 2 genes not one
it is tough to explain
ever heard of the "Bombay Phenotype"
an AB mom gave birth to an O
anyway long story short
AB is the rarest
and going by your logic
O is not hard to breed
and O mom and O dad will always give O
it's not that hard at all
but to get AB you'll need a pure A dad (or mom) and a pure B partner for him/her to get you AB
that is rare
chances otherwise are rare
and an AB child will not give birth to AB kids
O on the other hand will