Children of parents with blood types AB and B may have blood types A, B, and AB. The human ABO blood group system is controlled by three alleles, A, B, and O. Only two of these three genes may be present in each individual, one inherited from each parent. The A and B genes are dominant and the O gene is recessive. Therefore, the genotype of blood group AB is AB and the genotype of blood group B may be BB or BO. When both parents have genotypes AB and BB, the children have genotypes AB or BB and exhibit either blood type AB or blood type B. When both parents have genotypes AB and BO, the children have genotypes AB, AO, BB, and BO, and exhibit blood type AB, blood type A, and blood type B, respectively. It should be noted that according to the genetic law can be inferred that the child’s blood type may be, can not make a definite judgment of the blood type, need to be clarified through the blood type test blood type.