Human Heredity 9th Edition by Michael Cummings

Exercise 2

Mary and John Smith went for genetic counseling because John did not believe that their newborn son was his. They both wanted blood tests to help rule out the possibility that someone other than John was the father of this baby. The counselor explained that ABO blood typing could give some preliminary indications about possible paternity. Its use is limited, however, because there are only four possible ABO blood types, and the vast majority of people in any population have only two of those types (A and 0). This means that a man may have a blood type consistent with paternity and still not be the father of the tested child. Because the allele for type 0 can be masked by the genes for A or B, inheritance of blood type can be unclear. More modern (and more expensive) genetic tests, such as DNA typing, would lead to a more reliable conclusion about paternity.
Mary's blood was tested and identified as type 0. john's blood was tested and identified as type 0. On the basis of these two parental combinations of blood types, the only possible blood type that their son could be is type 0. The baby was tested, and his blood type was, indeed, type 0.
If not, was it worthwhile doing the test in the first place?