Children born to two parents with blood type a and blood type b may be type a, b, ab and o. Genetically, the genes for blood type a can be aa or ao, and the blood types that are expressed, are all type a, while o is a recessive gene and cannot be expressed. By analogy, the gene for blood type b could be bb or bo. Then the combination of two parents with blood type a and blood type b would result in four blood types, i.e., the expression of type a, b, ab and o. In general, if both parents want to know the specific blood type of their child, they can go to the hospital for laboratory tests and should not deduce it on their own.