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

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. 


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.