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?

Explanation:

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