Cystic fibrosis and genetic screening

How CF affects the body

CF is caused by recessive alleles of the CFTR gene, which codes for a chloride ion channel in the membranes of epithelial cells. In CF the channel does not work, so less chloride, and therefore less water, moves out of cells. Mucus becomes thick and sticky.

• Gas exchange: mucus blocks bronchioles, reducing the surface area for gas exchange; bacteria trapped in mucus cause repeated infections.
• Digestion: mucus blocks the pancreatic duct, so enzymes do not reach the small intestine; food is poorly digested and absorbed. The pancreas can be damaged.
• Reproduction: in most men the vas deferens is blocked or absent, causing infertility; in women thick cervical mucus can reduce fertility.

Genetic screening

• Carrier screening: testing people (for example with a family history) to see if they carry a recessive allele.
• Preimplantation genetic diagnosis (PGD): embryos made by IVF are tested by removing one cell; only unaffected embryos are implanted.
• Prenatal testing: amniocentesis samples amniotic fluid containing fetal cells, usually at about 15 to 20 weeks. Chorionic villus sampling (CVS) samples placental tissue, usually at about 11 to 14 weeks. Both carry a small risk of miscarriage (about 1 in 200 in NHS guidance).

Ethical and social issues

• Religious and moral views: some believe embryos have a right to life, so discarding embryos (PGD) or terminating a pregnancy is wrong.
• Results may lead to difficult decisions; false positives and false negatives can occur.
• Concerns about 'designer babies', discrimination (for example by insurers or employers) and the confidentiality of genetic information.
• Benefits: couples can make informed choices and prepare; treatment can start early.

Worked example

Explain why people with CF often have poor growth.

1. Thick mucus blocks the pancreatic duct.
2. Digestive enzymes cannot reach the small intestine.
3. Food is not fully digested, so fewer nutrients are absorbed.

Answer: Blocked pancreatic duct means poor digestion and absorption of nutrients.

Check yourself

1. What is the normal role of the CFTR protein?
   • An enzyme that digests mucus
   • A carrier for glucose
   • A channel that moves chloride ions out of epithelial cells
   • A hormone receptor
   Show answer

   A channel that moves chloride ions out of epithelial cells. CFTR is a chloride ion channel.

2. Why does CF reduce gas exchange?
   • Mucus blocks bronchioles, reducing the surface area available
   • Alveolar walls become thinner
   • More capillaries form
   • Haemoglobin is faulty
   Show answer

   Mucus blocks bronchioles, reducing the surface area available. Blocked airways mean fewer alveoli are ventilated.

3. How does CF affect digestion?
   • The stomach makes too much acid
   • Villi grow larger
   • Mucus blocks the pancreatic duct, so enzymes do not reach the small intestine
   • Bile is not made
   Show answer

   Mucus blocks the pancreatic duct, so enzymes do not reach the small intestine. Pancreatic enzymes are trapped.

4. Which test samples cells from the placenta?
   • Amniocentesis
   • Chorionic villus sampling
   • Carrier screening of the father
   • A blood pressure test
   Show answer

   Chorionic villus sampling. CVS samples chorionic villi of the placenta.

5. Why might a couple choose CVS rather than amniocentesis?
   • It has no risk of miscarriage
   • It samples amniotic fluid
   • It tests embryos before implantation
   • It can be done earlier in pregnancy
   Show answer

   It can be done earlier in pregnancy. CVS is usually at 11 to 14 weeks; amniocentesis at 15 to 20 weeks.

6. What is preimplantation genetic diagnosis?
   • Testing amniotic fluid at 16 weeks
   • Testing a cell from an IVF embryo before it is implanted
   • Testing adults for carrier status
   • Treating CF with drugs
   Show answer

   Testing a cell from an IVF embryo before it is implanted. PGD tests embryos before implantation.

7. Which is an ethical objection to PGD?
   • It is done during the third trimester
   • It cures cystic fibrosis
   • It increases fertility
   • Embryos found to be affected are discarded
   Show answer

   Embryos found to be affected are discarded. Some believe embryos have a right to life.

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)
• MedlinePlus Genetics: Cystic fibrosis
• GOV.UK / NHS Fetal Anomaly Screening Programme: CVS and amniocentesis information for parents

Last updated 7 November 2026. Spotted a mistake? Tell us.