In a disputed parentage case, the child is blood type O, while the mother is blood type A. What are the possible blood type (s) for the father?

The child's blood type is O, the genotype will be OO

The mother's blood type is A, her genotype can be AA or AO

Thus, a cross between the mother AA and assuming the father OO will produce all AO

A cross between the mother being AO and assuming father with OO has a 50% chance of producing OO (O bloodtype)

A cross between the mother's AO and assuming a father heterozygous AO (A bloodtype) has a 25% chance of producing a child with OO blood type O

A cross between the mother if she is AA and assuming a father with AO will not produce a child with blodtype O

A cross between the mother being AO and assuming father with BO (blood type B) has a 25% chance of producing OO (bloodtype O)

Thus the possible bloodtype of the father can be

1. bloodtype A (AO) if mother is AO

2. bloodtype O (OO) if mother is AO

3. Bloodtype B (BO) if mother is AO