You've gotta see
The Blue People Of Troublesome Creek. Blam posted that link, and shows once again why he deserves to have his own topics. ;')
Type O means coding for no blood protein (this is a separate issue from Rh factor), and given its apparent mutation from A, O is a newer type (those books about "Eating Right for Your Blood Type" notwithstanding), as is B, and of course Bombay syndrome. AB results from a mating of A and B (one from each parent), and is about 5 per cent of the population That 40/40/15/5 breakdown doesn't take into account the MN family of course.
MN comes from the Indian subcontinent somewhere. Probably Blam's SundaLand refugees were MN, arrived there, and are now nearly extinct. :'D