Can parent ab positive and b have negative baby?

Yes, it is possible for parents with A-positive and B-negative blood types to have a child with O-negative blood type.

Blood type is determined by the presence or absence of two antigens, A and B, on the surface of red blood cells. A person with A-positive blood has both the A antigen and the Rh factor (a protein on the surface of red blood cells), while a person with B-negative blood has only the B antigen and lacks the Rh factor.

A child inherits one blood type allele from each parent. If the A-positive parent contributes the A allele and the B-negative parent contributes the O allele, the child will have A-positive blood. However, if the A-positive parent contributes the A allele and the B-negative parent contributes the B allele, the child will have B-positive blood.

Finally, if the A-positive parent contributes the O allele and the B-negative parent contributes the B allele, the child will have B-negative blood. If both parents contribute the O allele to the child, the child will have O-negative blood.

Therefore, it is possible for parents with A-positive and B-negative blood types to have a child with O-negative blood, but it is not the most common outcome.