DNA probes and genetic screeningAQA A-Level Biology: Flashcards

Exam questions on DNA probes and genetic screening

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.
   Explain why the patient's DNA is separated into single strands before the probe is added.2 marks
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.
   Suggest two benefits to patients of screening for drug response before treatment begins.2 marks
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.
   Describe how a labelled DNA probe is used to show whether a sample of DNA contains the mutated allele.3 marks