Answer: Blood types O, A, and B are possible for the father.
Explanation: Since the mother has type A blood, there are two possible genotypes that she can have: AA or AO. Since her son has type O blood, he only has one genotype: OO.
For blood types,
Type A - AA or AO
Type AB - AB
Type B - BB or BO
Type O - OO
* A and B are dominant over O.
Now we have to do crosses between the mother and the father:
AA mother with AB father: child with type AA or type AB blood
AO mother with AB father: child with type AA, AO, AB, or BO blood
AA mother with OO father: child with AO blood
AO mother with OO father: child with AO or OO blood
AA mother with AA father: child with type AA blood
AA mother with AO father: child with type AA or AO blood
AO mother with AA father: child with type AA or AO blood
AO mother with AO father: child with type AA, AO, or OO blood
AA mother with BB father: child with type AB blood
AA mother with BO father: child with AB or AO blood
AO mother with BB father: child with AB or BO blood
AO mother with BO father: child with AB, AO, BO, or OO blood
The father can have blood types O, A, or B.
