Blood type is determined by heredity, the blood type relationship between the parent and the offspring originates from genetic factors, if both parents are O blood type, then the baby can only be O blood type, because the gene of O blood type belongs to recessive, two recessive genes born children or recessive genes, so it is still O blood type, it is impossible to birth A type, B type, or AB type fetus, so in doing paternity test, only O blood type The child belongs to the two of them together, there are other blood types, it should not be the child born of the couple. And for a single patient whose parents are type O, or whose parents are type A, this time there may be a child with type A, type B, type AB, and type O.