It is possible to have a child with type O if your parents have blood types A and B respectively. The human ABO blood group system is controlled by three alleles on the chromosome, A, B and O. Only two of these three genes can be found on the chromosome of each person, and one is inherited randomly from each parent. The A and B genes are dominant genes, and the O gene is recessive, i.e., when there is an A or B gene present, it behaves as A or B type, and only when the genotype is OO, it behaves as O type. So the genotype of blood type A may be AA or AO and the genotype of blood type B may be BB or BO. Only parents with genotypes AO and BO may give birth to children with genotypes AO, BO, AB, and OO, which manifest as type A, B, AB, and O, respectively. Special attention should be paid to the fact that inferring the blood type of a child from the blood type of the parents can only make a negative judgment (i.e., to determine which blood type is unlikely to occur), but cannot make a positive inference, so if you need to know the blood type, you need to have a professional ABO blood typing.