This makes an O child not possible.A person that has an O allele and an A allele will be blood type A since A is dominant and O is recessive.A person that has an O allele and a B allele will be blood type B since B is dominant and O is recessive.If you know the mother is an O and a child is an O. The father cannot be AB, since an AB father and an O mother can only have children of type A or Type B