Mutations, inheritance and genetic screening

Sample questions

Original practice questions written in the style of the exam.

1. Question 14 marks

   Explain how a mutation in the CFTR gene can lead to frequent lung infections in a person with cystic fibrosis.

   Mark scheme

   • The CFTR protein (chloride ion channel) is faulty / absent, so chloride ions are not secreted out of the epithelial cells (1)
   • Less water leaves the cells by osmosis (and more sodium ions are absorbed into the cells) (1)
   • So the mucus is thick / sticky and cilia cannot move it out of the airways (1)
   • Bacteria trapped in the mucus are not removed, so they multiply and cause infection (1)
2. Question 23 marks

   A couple who are both carriers of a recessive genetic disorder are offered either chorionic villus sampling (CVS) or amniocentesis during a pregnancy.

   Compare these two methods of prenatal testing.

   Mark scheme

   • CVS can be carried out earlier in pregnancy (about 10–13 weeks) than amniocentesis (about 15–20 weeks) (1)
   • CVS samples cells from the placenta, whereas amniocentesis samples fetal cells from the amniotic fluid (1)
   • Both carry a small risk of miscarriage, which is slightly higher for CVS (1)