Bootcamp Bio Punnett Square Error?

[MRZ8]

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Mom and Dad had a child already, who was type O (ii). Out of 4 children, 2 can be type B and 2 can be type O blood. After one was born with type O, there should theoretically be 1 out of 3 chances that another child can be type O. Is this reasoning correct?

I think this reasoning resonated with me when I did a similar Bio Destroyer question.

 

[FeralisExtremum]

This reasoning is not correct - a Punnett square shows you the possibilities of any newborn child, it's not a table that gets checked off one by one as each new child is born. The existing child doesn't factor in to the probability at all, it's just given to you to establish that the father is type BO (because for the existing child to have type O blood, it must have received the type O allele from each parent). Therefore you're running the probability knowing you are crossing BO x OO. 50% chance of producing an OO child, 50% chance of a BO child.