Cystic Fibrosis
Cystic fibrosis is one of the most common inherited diseases in the Western World. It is transmitted in an autosomal recessive manner: with the exception of the single Y chromosome (which is what makes males males) we each have two of each chromosome, so we have two of each gene, one from each parent. In an autosomal recessive disorder both genes must be abnormal (as opposed to an autosomal dominant disorder, where one defective gene will cause disease). If both parents are carriers of an abnormal gene, then their offspring have a one-in-four chance of inheriting both abnormal genes. The carrier rate for the abnormal gene responsible for the development of cystic fibrosis is roughly one in 22, so it's very common indeed. Patients with cystic fibrosis have an abnormal protein responsible for moving chloride ions across membranes. This results in the production of very thick and sticky secretions which predominantly affect the lungs, the pancreas, and (in the newborn) the bowel. 20 per cent of these patients have abnormalities of liver function, probably due to disturbances of secretion of bile, which can produce portal hypertension. There is a screening test available to detect carriers, but it should not be undertaken without genetic counselling.
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