The parents of an AB child can be a combination of A+B, AB+A, AB+B, and AB+AB. The inheritance of the human ABO blood group system is controlled by the three alleles A, B, and O on the top of chromosome nine. Only two of the above three genes can be carried on a pair of chromosomes. There are six groups of genotypes, and four groups of phenotypes. For example, a person with genotype OO has type O blood, a person with genotype AA/AO has type A blood, a person with genotype BB/BO has type B blood, and a person with genotype AB has type AB blood. An AB child needs to inherit one allele of each of the genotypes carried by the parents. For example, an A-blooded father and a B-blooded mother may have genotypes AO/AA and BO/BB, and if they acquire the father’s A gene and the mother’s B gene, they will be AB. Acquiring the father’s O gene and the mother’s B gene, with genotype BO, would show B blood type; acquiring the father’s A gene and the mother’s O gene, with genotype AO, would show A blood type. It should be noted that in forensic medicine, paternity cannot be determined solely on the basis of blood type, but needs to be clarified in a professional medical appraisal organization.