Professor: If both parents have type O blood then their children can only have type O blood. This is a genetic law. Student: But that' s not true. My father has type B blood and I have type O blood. The student has most likely misinterpreted the professor's remark to imply that ['only people with type O blood can have children with type O blood', 'if both parents have type B blood, then their child will have type B blood', 'people with type O blood cannot have children with type B blood', 'what is true of one child in the family must also be true of all children in that family']