Parents with blood types A and B can give birth to children with blood type O. They can also give birth to children with blood types AB, A, and B. The common ABO blood group system for human blood types is controlled by three genes, A, B and O, present on chromosome pair 9 and determined by two of the three genes. The genes are divided into dominant and recessive genes, with the A and B genes being dominant and the O gene being recessive. When both genes are dominant, the blood type is dominant; when both genes are recessive, the blood type is recessive; when one gene is dominant and the other gene is recessive, the blood type is dominant. Therefore, the blood genotypes of parents with A and B blood types may be AA, AO as well as BB and BO. If the parents’ blood genotypes are AO and BO, the child’s blood genotype may be OO, i.e. blood type O. If the blood type needs to be checked, blood can be drawn at the hospital without fasting requirement. If the blood biochemistry test is also performed, it is recommended to fast after 12:00 pm the night before the test.