I have recently been asked the following:
Can a woman type O with a husband type A have an AB child?
Of course, the simplified answer would simply state “No” leaving the child wondering whether or not his dad is his dad and likely believing his mother cheated. Of course, this possibility always exists … BUT:
One thing to keep in mind is that unless we know all of the details, it is not wise to give an answer finalizing the attempt to figure out paternity. Human errors are very high when it comes to blood typing and depending on who developed the tests, there is a possibility of mistakes being all over the place.
Because of that, I would like to take an opportunity to answer this one with the assumption that the blood types mentioned are correct in all 3 and that the woman was faithful and truthful.
As long as the data provided is correct, the only option would be that the husband is actually cis-AB. According to recent studies, the most common type of cis-AB was cis-AB/O with the A2B3 phenotype. The B3 is something you can also refer to as a “weak B”. So it is actually possible that when tested, the B does not show up. So the father could show up as an A actually being a cis-AB. And when a cis-AB and an O form a cis-AB/O child, the blood type of the child will show up as AB.
Always demand retesting when the question of paternity is being raised before you come to conclusions based on the results of blood types.