8.4.3 Genetic fingerprinting

• What are variable number tandem repeats (VNTRs)?

  ● Repeating sequences of nucleotides / bases (eg. GATA)

  ● Found within non-coding sections of DNA at many sites throughout an organism’s genome
• Why are VNTRs useful in genetic fingerprinting?

  ● Probability of two individuals having the same VNTRs is very low

  ● As an organism’s genome contains many VNTRs and lengths at each loci differ between individuals
• Explain how genetic fingerprinting can be used to analyse DNA fragments

  1. Extract DNA from sample (eg. blood cells) and amplify by PCR
  2. Cut DNA at specific base sequences / recognition sites (either side of VNTRs) using restriction enzymes
  3. Separate VNTR fragments according to length using gel electrophoresis (shorter ones travel further)
  4. Transfer to a nylon membrane and treat to form single strands with exposed bases
  5. Add labelled DNA probes which hybridise / bind with complementary VNTRs (& wash to remove unbound probe)
  6. Then you need to show the bound probe
• How do you show the bound probe

  • To show bound probe, expose membrane to UV light if a fluorescently labelled probe was used
  • OR use autoradiography (expose to X-ray film) if a radioactive probe was used
• Compare and contrast genetic fingerprinting with genetic screening

  ● Both use PCR to amplify DNA sample

  ● Both use electrophoresis to separate DNA fragments

  ● Both use labelled DNA probes to visualise specific DNA fragments

  ● Genetic fingerprinting analyses VNTRs whereas genetic screening analyses specific alleles of a gene
• Explain how genetic fingerprinting can be used to determine genetic relationships

  ● More closely related organisms have more similar VNTRs, so more similarities in genetic fingerprints

  ● Paternity testing - father should share around 50% of VNTRs / bands with child (due to inheritance)
• Explain how genetic fingerprinting can be used to determine genetic variability within a population

  • Differences in VNTRs arise from mutations, so more differences show greater diversity within a population
• Explain the use of genetic fingerprinting in the fields of forensic science

  ● Compare genetic fingerprint of suspects to genetic fingerprint of DNA at crime scene

  ● If many bands match, the suspect was likely present at the crime scene
• Explain the use of genetic fingerprinting in the fields of medical diagnosis

  ● Some VNTR patterns are associated with an increased risk of certain genetic disorders eg. Huntington’s
• Explain the use of genetic fingerprinting in the fields of animal and plant breeding

  ● Shows how closely related 2 individuals are, so that inbreeding can be avoided

  ● Breed pairs with dissimilar genetic fingerprints