Using the ABO blood typing, and that the type was an inherited characteristic, the dominant and recessive genes were determined. Type A is dominant over O, type B is dominant over O, type AB is co-dominant, and type O is pure recessive. Using punnet squares, the probability of different blood types arising from certain parental crosses were listed. Discussed that prior to DNA testing, blood typing was the only way that parental responsibility could be definitively NOT proved. Certain fetal blood types could not exist if the parents genotype was known.