International A Level Biology / Gene expression and genetics
Cystic fibrosis revision notes
Edexcel International A Level Biology (YBI11), chapter 6: Gene expression and genetics, specification point 2.16. Free lesson.
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Cystic fibrosis (CF) is caused by mutations in the CFTR gene, which can make mucus too thick and sticky. This affects gas exchange, digestion and fertility, and the condition is inherited through a recessive allele.
How a CFTR mutation makes mucus sticky
CFTR is a gene for a chloride ion channel protein in the membranes of epithelial cells. These cells line the airways, gut and reproductive tract. The commonest mutation, ΔF508, deletes three bases. Since three bases make one codon, this does not shift the reading frame. Instead, one amino acid, phenylalanine at position 508, is missing. The protein misfolds and is broken down, so very little reaches the cell surface. Without working CFTR, chloride ions fail to leave through the channel, while sodium ions continue moving into the cell. Water moves from the mucus into the cells by…
Why CF can cause breathlessness and lung infections
Cilia normally sweep mucus out of the lungs. Thick mucus is hard to move, so it builds up and can block the bronchioles, the smallest airways. Less air reaches some alveoli, which reduces the surface area available for gas exchange. Poorly ventilated alveoli contain less oxygen, so the concentration gradient into the blood is lower. Both changes slow diffusion, leaving less oxygen in the blood and making a person breathless, especially during exercise. The chain to remember is: mucus builds up → bronchioles block → fewer alveoli are ventilated → smaller surface area and lower…
How CF affects digestion and weight gain
The pancreas makes lipase, amylase and proteases, which digest fats, starch and proteins. Its ducts carry these enzymes to the duodenum, the first part of the small intestine. In CF, thick mucus can block a duct, so enzymes cannot reach the duodenum and food is not fully digested. Mucus also coats the intestine lining, reducing nutrient absorption. Trapped enzymes damage the pancreas, forming fluid-filled cysts and scar tissue. Scarring is called fibrosis. With less digestion and absorption, gaining weight can be difficult. A healthy pancreatic duct is open, so enzymes can flow along it.…
How CF affects fertility
In men, the testes still make sperm, but the vas deferens, the sperm duct, may be blocked with mucus or may never form properly. Sperm cannot travel from the testes into the semen, so most men with CF are infertile. In women, thick mucus plugs the cervix. Fewer sperm can pass through to reach an egg in the oviduct. Fertility is reduced, although many women with CF still conceive. The reproductive-system diagram shows the testes, vas deferens, cervix and oviduct.
How the recessive CF allele is inherited
The CF allele is recessive, so a person needs two copies to have CF. A person with one normal allele and one CF allele is a carrier. Carriers have no symptoms because one normal allele makes enough working CFTR to keep mucus runny. Two carrier parents (Ff) can each pass on either F or f. Their children have a 3 without CF : 1 with CF ratio. Each child has a 1 in 4 chance of having CF, regardless of previous children. Hundreds of different CFTR mutations can cause CF. Some stop the protein being made; others make a channel that reaches the membrane but lets fewer chloride ions through.…