Why is type O blood most common?

ABObloodsystemBlood type A involves having a particular antigen (type of protein) on the membrane of your red blood cells. Type B blood has a different antigen on the membrane of its red cells, while type O has neither. The gene for type O is ‘recessive’, because if you have one gene for O and one for A, then you still end up with A antigens on your cell membranes, and the same goes for O and B. In order to be group O, you need both parent cells to be O. But group O is still more common simply because it is the ancestral form. The A and B mutations appeared in the last 20,000 years and haven’t spread through the population yet.

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