... AB blood types. The fact that an AB type exists at all told early investigators that every individual actually carries two alleles, or traits that determine blood type-one inherited from each parent. Further studies proved that if each parent contributed an O allele, the child would be type O, but if one parent contributed an A and the other an O, the A dominated. Likewise, if one parent contributed a B and one an O, the B dominated. A and B do not dominate one another, though, so if one parent contributes an A and the other a B, the child displays both traits and is AB. Obviously, having no A or B factors, two O parents can only have OO children (usually just called O). Two AB parents can never have an O child, as neither of them has an O allele to pass along. Two parents of types ...

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