Why does people with blood type O make up the highest percentage in most regions?”
Type O blood is recessive. If you have one of the A or B alleles, you get A or B blood (or AB, if you get both). But that doesn’t mean they’re common. In fact, if you’ve got a large majority of O, you’ll tend to keep to be mostly O. Consider it this way: if there is no A or B alleles, you’ll never have anything except type O. That’s actually the case in South America, where there is practically no A and very little B. If nothing happened, that would never change. The A and B alleles started off in some particular place (the A in Europe, and the B in western Asia). It’s only fairly recently that world-wide mixing of populations were able to spread these genes. As populations mix, it will tend to decrease the amount of O blood out there, but not to zero. If a marriage has one OO person and one BO person, you’ll get 25% BO children and 75% OO children. Eventually, the population will reach an equilibrium at that level. Even though the B is dominant, the fact that most people have O limits
