The mother in question is blood type A+. Her genotype at the ABO location is either AA or AO. Her Rh genotype is either Rh+/Rh+ or Rh+/Rh-. The information that the maternal grandmother is also blood type A+ and a brother is blood type O tells us that the maternal grandmother of the child has genotype AO, since she is type A but donated an O allele to one of her children.

The mother wants to know the potential blood types of the father of her son. The son is blood type A+. Unfortunately for this particular case, the mother cannot distinguish between any potential fathers from blood type alone. Note from the table that this mother could have created a child with type A blood with a father of any of the four possible blood types, type A, type AB, type B, or type O. Likewise, the father of the child could be either Rh+ or Rh-.

It should be apparent from this discussion that blood type is not a very good test for paternity. In some cases, unambiguous information can be obtained, i.e. a type AB male cannot father a type O child. However in most cases, the results are uncertain.

If determining the paternity of a child is important, there are very sensitive DNA test currently available that can establish paternity to a certainty in excess of 99.99%, or exclude someone as the biological father with absolute certainty. Elsewhere in the Biology Project is an exercise to follow the inheritance of DNA markers in a paternity study.