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6.5.3Unit 6: Practical Skills in Biology II

DNA Profiling — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 6 parts Lesson 19 of 48 Unit quiz

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Section 1 of 6Making and Reading a Profile

1
A DNA sample is collected from, for example, saliva, skin, hair or blood.
2
PCR amplifies the DNA, and restriction enzymes cut it into fragments that differ in length between individuals.
3
Gel electrophoresis separates the fragments, and labels make a pattern of bands visible.
4
Because half of a child’s DNA comes from each parent, a child shares about half of its bands with each parent.
5
The more bands two profiles share, the closer the genetic relationship between the people.

Section 2 of 6Paternity Testing

Method
  • Method The child’s profile is compared with the mother’s and with those of the possible fathers.
Maternal bands
  • Maternal bands Bands in the child that match the mother are accounted for by her contribution.
Paternal bands
  • Paternal bands The remaining bands must have come from the father, so the candidate who matches them is the most likely father.
Limits
  • Limits The result gives a probability of relationship and does not prove it with absolute certainty.

Section 3 of 6Forensic Use

Forensic Use
Short tandem repeats
  • Short tandem repeats The regions examined are short repeated sequences, known as microsatellites, a kind of non-coding VNTR.
Crime scene
  • Crime scene A profile from a sample at the scene is compared with those of suspects.
Number of sites
  • Number of sites The more regions analysed, the lower the chance that two unrelated people have the same profile.
Few sites
  • Few sites If only a few regions are tested, closely related individuals may have the same profile by chance.

Section 4 of 6Breeding Programmes

Aim
  • Aim Profiles of captive animals or crop plants are compared to find those that are least closely related.
Breeding choice
  • Breeding choice Individuals that are genetically most different are crossed, which avoids inbreeding.
Individual harm
  • Individual harm Inbreeding can allow harmful recessive alleles to appear, because they are no longer masked by dominant alleles.
Population harm
  • Population harm A smaller gene pool reduces variation, so the population is less able to adapt to change.

Section 5 of 6Worked Example

Question

  • A child’s profile has 6 bands.
  • Three match the mother.
  • Candidate A matches 1 of the other three bands and candidate B matches all 3.
  • Which man is more likely to be the father?

  1. Bands matching the mother account for 3 of the 6 bands.
  2. The remaining 3 bands must be inherited from the biological father.
  3. Candidate A matches 1 of 3, whereas candidate B matches 3 of 3.

Answer Candidate B is the most likely father, because he accounts for all the paternal bands.

Section 6 of 6Exam Checklist & Tips

6.5.3 – Key Points for the Exam

  • DNA profile A pattern of DNA fragments, unique to an individual, produced by electrophoresis.
  • Microsatellite A short sequence of bases repeated several times in non-coding DNA.
  • Inbreeding Breeding between closely related individuals.
  • Gene pool All the alleles in a population.

Common Mistakes

  • Saying that DNA profiling gives a certain match, when it gives a probability based on the sites tested.
  • Stating that a child shares all bands with the father, when it shares about half of its DNA with each parent.
  • Forgetting that analysing more regions of DNA makes the profile more reliable.
  • Describing inbreeding as reducing the number of genes, when it reduces the variation in alleles.

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