Cystic fibrosis and genetic screeningEdexcel International A Level Biology: Subtopic test

1. 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.

   (a)

   What is the effect of the ΔF508 deletion on the CFTR protein?

   [1 mark]

   • AThe reading frame shifts so that every amino acid after the deletion is different
   • BOne amino acid is replaced by a different amino acid
   • COne amino acid is missing from the protein but the rest of the amino acid sequence is unchanged
   • DA stop codon is created so that the protein is much shorter

   (b)

   Which ions does the normal CFTR protein transport out of epithelial cells?

   [1 mark]

   • ASodium ions
   • BChloride ions
   • CPotassium ions
   • DCalcium ions

   (c)

   Explain how the faulty CFTR protein causes the mucus made by epithelial cells to become thick and sticky.

   [2 marks]

   Total for question 1: 4 marks

2. 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.

   (a)

   Which change in the airways of these patients most directly reduces the rate of gas exchange?

   [1 mark]

   • AMucus blocks small airways, so air cannot reach some alveoli and the surface area available for gas exchange falls
   • BThe alveolar walls become thinner, so oxygen has further to diffuse
   • CThe concentration gradient of oxygen between alveolar air and blood is increased
   • DCilia beat faster and so move air out of the alveoli

   (b)

   Why do patients with cystic fibrosis suffer frequent bacterial chest infections?

   [1 mark]

   • ATheir bone marrow cannot make white blood cells
   • BBacteria use chloride ions in the mucus as an energy source
   • CTheir airways produce too little mucus, so bacteria reach the alveoli freely
   • DBacteria are trapped in thick mucus that the cilia cannot move out of the airways

   (c)

   Explain why these patients have difficulty obtaining enough oxygen for their blood.

   [2 marks]

   Total for question 2: 4 marks

3. 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.

   (a)

   Use the information to explain why the girl is underweight and passes fatty stools.

   [3 marks]

   (b)

   Explain why the man is infertile, and suggest why women with cystic fibrosis may also have reduced fertility.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A couple are both heterozygous carriers of the CFTR mutation and already have one child with cystic fibrosis. They would like a second child and a genetic counsellor explains three options: in vitro fertilisation (IVF) followed by pre-implantation genetic diagnosis (PGD); natural conception followed by chorionic villus sampling (CVS) at 11 to 14 weeks; or natural conception followed by amniocentesis at 15 to 17 weeks.

   (a)

   Evaluate the three options offered to the couple for testing a second pregnancy for cystic fibrosis.

   [6 marks]

   (b)

   Discuss the ethical and social issues raised by genetic screening for cystic fibrosis, from religious, moral and social viewpoints.

   [6 marks]

   Total for question 4: 12 marks

End of questions