All revision notes topics

Cystic fibrosis and genetic screeningEdexcel International A Level Biology: Revision notes

Section 1

Cystic fibrosis and the CFTR protein

Cystic fibrosis (CF) is a recessive genetic disorder caused by mutations in the CFTR gene on chromosome 7. A person has CF only if they inherit two faulty alleles, so the parents of an affected child are usually both carriers (heterozygous). The most common mutation, ΔF508, deletes three bases and so removes one amino acid (phenylalanine) from the protein.

The CFTR protein is a chloride ion channel in the cell surface membranes of epithelial cells. In CF the protein is faulty or does not reach the membrane, so chloride ions are not secreted into the mucus. The water potential of the mucus is not lowered enough for water to move in by osmosis, and the mucus becomes thick and sticky instead of thin and watery.

A cross between two carriers gives 1 in 4 children with CF, 1 in 2 carriers and 1 in 4 unaffected non-carriers.

Key termscarrierCFTR proteinΔF508
Common mistake

Do not say the mucus is 'too dry' without explaining why. The chloride channel fails, so water does not move into the mucus by osmosis.

Section 2

Effects on gaseous exchange

Thick mucus builds up in the bronchioles and alveoli. The cilia cannot move it out of the airways, so:

  • some airways and alveoli are blocked, so less air reaches the gas exchange surface and the surface area for gas exchange falls
  • bacteria are trapped in the mucus, causing repeated chest infections, inflammation and lung damage
  • mucus and damaged tissue increase the diffusion distance, so oxygen enters the blood more slowly

Patients feel breathless and the amount of air they can breathe out in one second is reduced.

Key termsdiffusion distancesurface area

Section 3

Effects on the digestive and reproductive systems

Digestive system. Thick mucus blocks the pancreatic duct, so pancreatic juice, with its amylase, protease and lipase, cannot reach the small intestine. Food is poorly digested, fats are passed out in the faeces and fewer nutrients are absorbed, so patients are underweight and grow slowly.

Reproductive system. In males the vas deferens is blocked with mucus or fails to form, so sperm cannot travel from the testes to the semen and most men with CF are infertile. In females thick mucus in the cervix can block sperm from entering the uterus, so fertility is reduced.

Key termspancreatic ductvas deferens
Exam tip

For each system, link it as: mucus blocks a tube, so a product cannot pass (enzymes, sperm), so the function fails.

Section 4

Uses of genetic screening

Genetic screening tests DNA for known alleles.

  • Carrier identification: a blood or cheek swab sample is tested for the CFTR mutation. A person who is a carrier can be counselled about the 1 in 4 risk to children if their partner is also a carrier.
  • Pre-implantation genetic diagnosis (PGD): eggs are fertilised by IVF. When the embryos are at about the 8-cell stage, one or two cells are removed and their DNA tested. Only unaffected embryos are implanted.
  • Prenatal testing of a fetus in the uterus: chorionic villus sampling (CVS) takes a sample of placental tissue at about 11 to 14 weeks; amniocentesis takes amniotic fluid containing fetal cells with a needle at about 15 to 17 weeks. Fetal DNA is then tested.
Key termsPGDCVSamniocentesis

Section 5

Implications of prenatal screening

Both CVS and amniocentesis are invasive and carry a small risk of miscarriage, slightly higher for CVS. CVS gives an earlier result; amniocentesis is later, so a termination would be later too.

A positive result forces parents to decide whether to continue or terminate the pregnancy, or to prepare for a child with CF. Results can cause anxiety while waiting. PGD avoids the need for termination but needs IVF, which is expensive, demanding and not always successful. Tests may also give false results, so counselling is essential.

Key termsinvasive testgenetic counselling

Section 6

Ethical and social issues

Discuss from several viewpoints and reach a balanced conclusion.

  • Religious: some faiths hold that life begins at conception, so discarding embryos or terminating a pregnancy is wrong. Other believers accept screening to prevent suffering.
  • Moral: the right of the embryo or fetus to life against the suffering of a child with CF and the parents' autonomy; concern about a slippery slope to selecting non-medical traits (designer babies); and whether screening suggests that people with CF are less valuable.
  • Social: possible discrimination by insurers or employers if genetic information is shared; confidentiality; stigma for carriers; psychological stress; and the cost of screening compared with lifelong care.
Key termsautonomyslippery slope
Common mistake

Do not give only one viewpoint. Name religious, moral and social issues and give a balanced conclusion.

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Cystic fibrosis and genetic screening

  1. Cystic fibrosis (CF) is caused by mutations in the CFTR gene on chromosome 7. The most common mutation, called ΔF508, is the deletion of three bases from the gene. The CFTR protein is found in the cell surface membranes of epithelial cells lining the airways, pancreatic ducts and reproductive tract.
    Explain how the faulty CFTR protein causes the mucus made by epithelial cells to become thick and sticky.2 marks
  2. A hospital respiratory clinic monitors a group of young patients with cystic fibrosis. Many have a persistent cough, produce large amounts of sticky sputum and are admitted several times a year with bacterial chest infections. Lung function tests show that the volume of air they can breathe out in one second is lower than normal.
    Explain why these patients have difficulty obtaining enough oxygen for their blood.2 marks
  3. A cystic fibrosis clinic treats patients of all ages. One patient, a 9-year-old girl, is underweight despite eating a high-energy diet and passes pale, bulky, fatty stools. A scan shows that her pancreatic duct is blocked with thick mucus. Another patient, a 29-year-old man with cystic fibrosis, produces sperm normally in his testes, but no sperm are found in his semen. His wife does not have cystic fibrosis.
    Use the information to explain why the girl is underweight and passes fatty stools.3 marks
See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).