A woman has type A blood. Her daughter has type O blood. What are the possible genotypes and blood types of the child's father?
The father could be blood type O (genotype: oo), blood type A (genotype: Ao), or blood type B (genotype: Bo).
The woman is blood type A, but we know her genotype must be Ao (not AA) because she has a daughter with blood type O. That means the daughter got an o allele from each parent.
That's why dad must have at least one o. He could have three possible blood types (O, A, or B) with one genotype for each (oo, Ao, or Bo). The father cannot be blood type AB because then he couldn't give his daughter an o allele.
If the father had an Ao genotype, the cross below shows the possible children. You can see the couple would have a 25% chance of having a child with blood type O.