If both parents are type O, the child will also have type O blood. The fact that the parents have type O blood means that the father and mother have neither A nor B antigens on the surface of their red blood cells, so it is impossible for their children to genetically acquire either A or B antigens. There is neither A nor B antigen on the surface of the red blood cells, so there can only be blood type O, but not A, B or AB. Blood type O has the highest percentage in the population, with about thirty to forty percent of the population having type O blood. Blood type is a congenital determinant and cannot be changed later in life. The ABO blood group system is one of many blood group classification systems, and there are other ways to categorize blood types. If you need to know your exact blood type, you can go to the hospital for a checkup.