Genetic fingerprintingAQA A-Level Biology: Revision notes
Section 1
VNTRs: the basis of the technique
An organism's genome contains many variable number tandem repeats (VNTRs): short sequences of bases repeated one after another in non-coding DNA. The number of repeats at each VNTR location varies between individuals, so the fragments containing them differ in length. Because there are many VNTR loci, the probability that two individuals have the same VNTRs is very low (identical twins are the exception). The more closely related two people are, the more VNTR fragments they share.
Section 2
Making a genetic fingerprint
- Extract DNA from the sample, such as blood, hair roots or saliva.
- Amplify the VNTR regions by PCR: heat to separate strands, cool so primers bind either side of the VNTR, and a heat-stable DNA polymerase (Taq) adds nucleotides. Repeat the cycles to make many copies. The DNA may also be cut with restriction endonucleases.
- Separate the fragments by gel electrophoresis.
- Make the fragments visible, with labelled probes or stains, to show a pattern of bands.
PCR is used because forensic samples often contain only a tiny amount of DNA.
Section 3
Gel electrophoresis
The DNA samples are placed in wells at one end of an agarose gel. When a current passes through the gel, DNA moves towards the positive electrode because the phosphate groups make it negatively charged. Smaller fragments move faster and further through the gel than larger ones, so the fragments separate by length. The final pattern of bands is the genetic fingerprint.
Do not say large fragments travel furthest. Size affects speed, so the smallest fragments are nearest the far end.
Section 4
Interpreting gel results
Relationships: a child gets half its bands from each parent. Bands in the child that are not in the mother must come from the father.
Worked example: a child has bands of 900, 560, 420 and 280 bp; the mother has 900, 650, 420 and 310 bp. The 560 and 280 bp bands are not in the mother, so the father must have both. A man whose DNA shows both is a possible father; a man with neither is not.
Variability: a population in which most individuals have the same pattern has low genetic variability.
Section 5
Uses of genetic fingerprinting
Forensic science: compare DNA at a crime scene with a suspect's; a match with a very low chance probability links them, and no match excludes them.
Medical diagnosis: identify heritable disorders or carriers, and identify the strains of bacteria that cause infections.
Animal and plant breeding: find how closely related individuals are and measure variability, so that closely related individuals are not bred together (avoiding inbreeding) and plants or animals with desired alleles are chosen.
Genetic relationships: paternity and maternity testing.
In an explain question, link each use to a reason: for example, closely related animals are identified so that they are not bred together.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Genetic fingerprinting
- 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.Explain why two unrelated people are very unlikely to have the same genetic fingerprint.2 marks
- 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.Use the data to explain which man is the child's father.2 marks
- 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.Explain why scientists might use genetic fingerprinting in a breeding programme for animals.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).