The phenotype of the mother is AB, so the genotype can only be AB.
The father's phenotype is O so his genotype should only be OO.
if we cross between AB and OO it gives us either AO or BO, so the possible phenotypes are either A (if the genotype is AO) or B (if the phenotype is BO).
Often by the people that get sick you ask them who they have been around and have video of the people slowly you find them and often a doctor will have some type of evidence that this person had the disease's