A couple attend for genetic counselling. Their 3-year-old son has cystic fibrosis, confirmed by newborn screening and genotyping; both parents are carriers of a *CFTR* mutation. Their 6-year-old daughter is well, has had a normal sweat test and has never had chest or bowel problems. They ask what the chance is that their daughter carries a cystic fibrosis mutation.
Which one of the following is the correct answer?
Keep going — a set of ten like this one, one at a time.
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