The law of inheritance of blood type follows the Mendelian law of inheritance, that is, the blood type of the offspring comes from the parental generation, one chromosome comes from the father and one comes from the mother. For example, if the mother’s blood type is O and the father’s blood type is O, one chromosome of the offspring’s blood type comes from the mother’s O gene and the other from the father’s O gene, and the offspring’s blood type can only be O, but not A, B or AB. For example, if mom’s blood type is A and dad’s blood type is O, A is mom’s expression type and her genotype may be AO or AA, while dad’s O genotype is 00, the offspring’s blood type chromosome may be one A from mom and one O from dad, combining to form AO, and the blood type is A. If the offspring inherits mom’s O gene and dad’s O gene, the combined genotype is OO, and the expression type is O. That is, if mom has A blood type and dad has O blood type, the offspring’s blood type may be A or O.