Genetic fingerprintingAQA A-Level Biology: Subtopic test

1. 1
   Police find a very small bloodstain at the scene of a burglary. A forensic laboratory produces a genetic fingerprint from the DNA in the stain and compares it with the fingerprint of a suspect. The technique analyses variable number tandem repeats (VNTRs), which are short base sequences repeated one after another in non-coding DNA.

   (a)

   Which statement describes a VNTR?

   [1 mark]

   • AAn allele that codes for a protein
   • BA short base sequence repeated many times in a row in non-coding DNA, with the number of repeats varying between individuals
   • CA site where a restriction endonuclease cuts DNA
   • DA gene that is identical in all members of a species

   (b)

   The stain contains only a tiny amount of DNA. Why is the DNA amplified by PCR before the fingerprint is made?

   [1 mark]

   • ATo remove the introns
   • BTo label the fragments
   • CTo separate the fragments by size
   • DTo make enough copies of the DNA to analyse

   (c)

   Explain why two unrelated people are very unlikely to have the same genetic fingerprint.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A baby's father is to be identified. DNA from the mother, the child and two men, X and Y, was amplified by PCR and the VNTR fragments were separated by gel electrophoresis. The fragment sizes, in base pairs, were: mother 900, 650, 420 and 310; child 900, 560, 420 and 280; man X 780, 560, 380 and 280; man Y 700, 480, 310 and 200.

   (a)

   Which fragment in the child's DNA travels furthest through the gel?

   [1 mark]

   • A900 bp
   • B560 bp
   • C280 bp
   • D420 bp

   (b)

   Why do the DNA fragments move towards the positive electrode during gel electrophoresis?

   [1 mark]

   • AThe phosphate groups make DNA negatively charged
   • BThe bases make DNA positively charged
   • CThe gel is acidic
   • DLarger fragments are repelled by the negative electrode

   (c)

   Use the data to explain which man is the child's father.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A zoo breeding programme for an endangered big cat has a small captive population. Scientists use genetic fingerprinting to analyse VNTRs in each animal. They compared two captive populations, X and Y, each of 30 animals, at six VNTR loci. In population X, 24 of the 30 animals have the same VNTR pattern at all six loci. In population Y, 3 of the 30 animals do.

   (a)

   Explain why scientists might use genetic fingerprinting in a breeding programme for animals.

   [3 marks]

   (b)

   Calculate the percentage of animals in each population that have the same VNTR pattern at all six loci. Use your answers to evaluate which population is at greater risk.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A university genetics laboratory offers genetic fingerprinting to a police force, a hospital and a plant-breeding company. The police compare blood left at crime scenes with suspects' DNA. The hospital uses the technique to diagnose heritable disorders and to identify strains of bacteria that cause infections. The plant breeders test seedlings before choosing which plants to use as parents.

   (a)

   Describe and explain how the laboratory produces a genetic fingerprint from a small sample of blood left at a crime scene.

   [6 marks]

   (b)

   Explain why scientists might use genetic fingerprinting in forensic science, in medical diagnosis and in plant breeding.

   [6 marks]

   Total for question 4: 12 marks

End of questions