We can illustrate this point with one of the best known examples: the disease cystic fibrosis (CF). This disease has long been known to be caused by mutations in a single gene. These mutations are recessive, but if an individual carries two copies of the gene both of which are defective, then they will develop CF. The defective gene is inherited in a classical Mendelian fashion for a single gene, so if both parents are carriers (i.e. heterozygous for the defective gene) then one in four of their children, on average, will develop the disease, and half of their children, on average, will be carriers. The symptoms of the disease were well known for many years, but the underlying cause of the disease – the nature of the gene, its role in its normal non-defective state, and how mutation of the gene causes the disease – were mysteries until the gene was successfully cloned. We will return to this classic story in Section 6.10.