cystic fibrosis
Imagine a salt-pump on the surface of your cells that helps keep the thin watery film lining your airways and gut slippery and clean. In cystic fibrosis that pump is broken, so the film turns into thick, sticky mucus. The sticky mucus clogs the lungs and traps bacteria, and it blocks the ducts that carry digestive juices from the pancreas, so food is poorly absorbed.
Cystic fibrosis is an autosomal recessive disorder: a person must inherit a faulty copy of the CFTR gene from each parent to be affected, while people with one faulty copy are healthy carriers. CFTR encodes a channel that moves chloride ions (and with them, water) across cell membranes. The most common mutation, called F508del, deletes a single amino acid and prevents the channel from folding and reaching the cell surface, but hundreds of different CFTR mutations are known.
It is among the most common life-shortening recessive disorders in people of Northern European ancestry. Care has advanced greatly: airway clearance, enzyme replacement and, more recently, CFTR-modulator drugs that help the defective channel work can substantially improve symptoms for many mutations. This entry is educational reference, not medical advice.
A baby's sweat is unusually salty and they have trouble gaining weight; a sweat-chloride test and CFTR gene analysis confirm cystic fibrosis, and the parents learn they are both carriers.
Salty sweat is a hallmark clue because the broken chloride channel also affects sweat glands.
Because carriers are healthy, two unaffected parents can each pass on a faulty CFTR copy; each of their children then has a 1-in-4 chance of being affected. This is the classic pattern of recessive inheritance.