DNA probes and genetic screeningAQA A-Level Biology: Subtopic test

1. 1
   A laboratory is developing a test for the allele that causes a heritable condition. The test uses a short, single-stranded DNA probe with a fluorescent label, whose base sequence is complementary to part of the allele. A sample of a patient's DNA is cut into fragments, separated into single strands and mixed with the probe.

   (a)

   Why can the probe be used to locate the allele in the patient's DNA?

   [1 mark]

   • AIt has the same amino acid sequence as the protein made by the allele
   • BIt cuts the allele at a specific restriction site
   • CIt has a base sequence complementary to part of the allele, so it forms hydrogen bonds with it
   • DIt is joined to the allele by covalent bonds made by DNA ligase

   (b)

   What is the term for the binding of the probe to the complementary sequence in the patient's DNA?

   [1 mark]

   • ADNA hybridisation
   • BTranscription
   • CTranslation
   • DDNA ligation

   (c)

   Explain why the patient's DNA is separated into single strands before the probe is added.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Variant alleles of the CYP2D6 gene make a version of a liver enzyme that activates the cancer drug tamoxifen only slowly, so the drug is less effective at the standard dose in patients who have two copies of the variant allele. A hospital now tests each breast cancer patient's DNA with labelled DNA probes before choosing a treatment.

   (a)

   What is the term for choosing a treatment to suit a patient's genotype?

   [1 mark]

   • AGene therapy
   • BGenetic fingerprinting
   • CRecombinant therapy
   • DPersonalised medicine

   (b)

   Which result would lead the doctor to choose a different drug or a higher dose for the patient?

   [1 mark]

   • AThe patient has two copies of the common allele
   • BThe patient has two copies of the variant allele
   • CThe patient is over 50 years old
   • DThe patient has no BRCA1 mutation

   (c)

   Suggest two benefits to patients of screening for drug response before treatment begins.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A couple are planning to have children. They are offered genetic screening for cystic fibrosis, an autosomal recessive condition caused by a mutated allele of the CFTR gene. Their DNA is tested with a labelled DNA probe for the mutated allele. Both of them are found to be carriers, each with one copy of the mutated allele.

   (a)

   Describe how a labelled DNA probe is used to show whether a sample of DNA contains the mutated allele.

   [3 marks]

   (b)

   A genetic counsellor explains the risks to the couple's children. Calculate the probability that their first child is a carrier, and the probability that both of their first two children have cystic fibrosis. Show your working.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A health authority is considering offering all women over 25 a screening test, using labelled DNA probes, for a mutated allele of the BRCA1 gene, which increases the risk of breast cancer. About 1 in 400 women in the population carry the allele. A woman with the allele has a 65% chance of developing breast cancer by the age of 70, compared with 12% for a woman without it. A negative result does not rule out breast cancer from other causes.

   (a)

   Evaluate the proposal to screen all women over 25 for the BRCA1 allele.

   [6 marks]

   (b)

   A woman has one copy of the dominant BRCA1 allele and has tested positive. Explain how this information could be used in genetic counselling and in personalised medicine.

   [6 marks]

   Total for question 4: 12 marks

End of questions