Question

If a man and a woman who both have the ab blood phenotype have a child, the one blood type that child cannot have is

Answer 1

The answer is O. 

Since blood type A and B are codominant, it means that the people who have blood type AB only have genes of A and B. They do not have the gene for blood type O. 

We can be sure that the child will not have blood type O by constructing a Punnett square. We can list all the possible genotypes thus predict their phenotypes. The capital letters represent the genotypes of parents, while others are the positive genotypes of offsprings. 
$\left[\begin{array}{ccc} &A&B\\A&aa&ab\\B&ab&bb\end{array}\right]$

From this, we can see that the only possible blood types their child could have are A, B, or AB. So, O is not possible.