Can child be AB if Father is o positive and mother a negative?

The answer is yes, the child can be AB if the Father is O positive and the mother A negative.

The child's blood group is determined by the alleles inherited from both parents. The Father is O positive, which means he has the alleles OO. The mother is A negative, which means she has the alleles AA or AO.

If the mother has the alleles AA, then she can only pass on A alleles to her child. If the father passes on an O allele, then the child will have the blood group A (AO).

If the mother has the alleles AO, then she can pass on either A or O alleles to her child. If she passes on an A allele and the father passes on an O allele, then the child will have the blood group A (AO). If she passes on an O allele and the father passes on an O allele, then the child will have the blood group O (OO).

Therefore, it is possible for the child to have blood group AB if the father is O positive and the mother A negative.