Type AB blood is rare – it’s found in less than five percent of the population. And it is the 'newest' of the blood types. Until ten or twelve centuries ago, there was little to no Type AB blood type. That is because type AB results from the intermingling of Type A with Type B. Unlike virtually every other gene, which have 'dominant' and 'recessive' variations (alleles), the A and B alleles are 'co-dominant,' meaning they quite happily co-exist with each other. Type AB is the only blood type whose existence is the result of intermingling rather than evolution and environment