Can parents with blood group A positive and B positive have a O negative baby?

Yes, parents with blood groups A positive and B positive can have a baby with blood group O negative. The ABO blood group system allows for various combinations of alleles, and if both parents carry the recessive O allele, they can pass it on to their child. However, for the baby to be Rh negative (O negative), both parents must carry the Rh negative allele as well. Thus, while it is possible, it depends on the specific genetic makeup of the parents.