According to Northern Arizona University, blood type cannot prove paternity, although it can disprove it, since there are some parental blood types that cannot combine to produce a child with a certain blood type, genetically speaking. However, even if it is genetically possible for a couple to produce the child's blood type, the man may not be the father because many men share the same blood type.
Blood types O and AB can only result from one particular genotype, while blood types A and B each have two possible genotypes, notes Northern Arizona University. Type O blood results from the genotype ii, and an individual with AB blood has the genotype IAIB. The possible genotypes for type A blood are IAIA and IAi, while someone with type B blood has a genotype of either IBIB or IBi. IA, IB and i represent the possible alleles for blood type. According to Regina Bailey, a biology expert at About.com, an allele is one of the two or more possible versions of a particular gene.
If a mother and her child both have type O blood, they both have the same genotype of ii, notes Northern Arizona University. Therefore, the child inherited an i allele from each parent. Since the blood type AB only results from the genotype IAIB, which does not feature an i allele, it is not possible for the child's father to have AB blood. However, if the man had any of the three other blood types, it would remain possible for him to be the father.