Respuesta :
Blood types:
AB +
AB -
A +
A -
B +
B -
O +
O -
It is possible to have a child with O blood despite the parents having different blood types.
While the man tested for B blood, he could have been a combination between type o and B whereas, the woman was a combination of type A and o. Since A/B are dominant, they would 'stand out' contrarily to o.
This means that their children could have AB or O blood.
Answer:
This is possible, the rest of their family tree might have blood type O and it might have been past down in the generations to them and the sperm had blood type O
Edit: i just accidentally rated my own answer