21.5 - Genetic Fingerprinting Flashcards
What is genetic fingerprinting and what is it used for?
Genetic fingerprinting is a diagnostic tool used widely in forensic science, plant and animal breeding, and medical diagnosis. It is based on the fact that the DNA of every individual, except identical twins, is unique.
What does genetic fingerprinting rely on?
Genetic fingerprinting relies on the fact that the genome of most eukaryotic organisms contains many repetitive, non-coding bases of DNA.
What are VNTRs, and why are they important in genetic fingerprinting?
- DNA bases that are non-coding are known as variable number tandem repeats (VNTRs).
- The number and length of VNTRs in an individual form a unique pattern.
- VNTR patterns differ between individuals, except for identical twins.
- The probability of two individuals having identical VNTR sequences is extremely small.
- The more closely related two individuals are, the more similar their VNTRs.
How does gel electrophoresis work in genetic fingerprinting?
- Gel electrophoresis is used to separate DNA fragments according to size.
- DNA fragments are placed onto an agar gel, and a voltage is applied across it.
- The gel’s resistance causes larger fragments to move more slowly than smaller ones.
- Over a fixed period, smaller fragments move further, allowing separation by length.
- If DNA fragments are labelled with radioactive DNA probes, their positions can be identified using X-ray film.
Why do larger genes and whole genomes need to be cut into smaller fragments for genetic fingerprinting?
- Only DNA fragments up to around 500 bases long can be sequenced using gel electrophoresis.
- Larger genes and whole genomes must therefore be cut into smaller fragments using restriction endonucleases.
What are the five main stages in the making of a genetic fingerprint?
1) Extraction
2) Digestion
3) Separation
4) Hybridisation
5) Development
What happens in the extraction stage of genetic fingerprinting?
- Even the tiniest sample of animal tissue (e.g., a drop of blood or a hair root) is enough to provide a genetic fingerprint.
- The DNA is extracted by separating it from the rest of the cell.
- If the amount of DNA is small, it is amplified using the polymerase chain reaction (PCR).
What happens in the digestion stage of genetic fingerprinting?
- The DNA is cut into fragments using restriction endonucleases.
- The enzymes are chosen for their ability to cut close to, but not within, the target DNA (due to specificity)
What happens in the separation stage of genetic fingerprinting?
- DNA fragments are separated according to size by gel electrophoresis under an electrical voltage.
- The gel is then immersed in an alkali solution to separate the double strands into single strands.
What happens in the hybridisation stage of genetic fingerprinting?
- Radioactive (or fluorescent) DNA probes bind to VNTRs.
- The probes have base sequences complementary to the VNTR sequences and bind under specific conditions (e.g., temperature, pH).
- Different probes bind to different target DNA sequences.
What happens in the development stage of genetic fingerprinting?
- An X-ray film is placed over the nylon membrane.
- The film is exposed by the radiation from the radioactive probes (or detected visually if using fluorescent probes).
- The resulting band pattern is unique to every individual except identical twins.
How are DNA fingerprints analyzed and interpreted?
- DNA fingerprints from two samples (e.g., crime scene evidence and a suspect) are visually checked for a match.
- If a match is suspected, an automated scanning machine calculates the DNA fragment lengths based on electrophoresis results.
- The probability of another individual having an identical fingerprint is then calculated.
How is genetic fingerprinting used to determine genetic relationships and variability?
- It can resolve paternity questions, as individuals inherit half their DNA from each parent.
- Each band in an individual’s DNA fingerprint should have a corresponding band in one of the parent’s fingerprints.
- It determines genetic variability within a population.
- A population with similar fingerprints has little genetic diversity, while one with varied fingerprints has greater diversity.
How is genetic fingerprinting used in forensic science?
- DNA at a crime scene (e.g., blood, semen, hair) can be compared to a suspect’s DNA.
- A match suggests presence but does not prove guilt.
- Other explanations must be considered, such as:
- The DNA was left during a different, innocent occasion.
- The DNA belongs to a close relative.
- The DNA sample was contaminated after the crime.
- The probability of another individual having an identical DNA fingerprint must be calculated.
- This calculation assumes random distribution of DNA in the community, which may not apply to isolated ethnic or religious groups
How is genetic fingerprinting used in medical diagnosis?
- It helps diagnose genetic disorders such as Huntington’s disease.
- Huntington’s disease results from a three-base sequence (AGC) repeated multiple times on chromosome 4.
- Fewer than 30 repeats: Unlikely to develop the disease.
- More than 38 repeats: Almost certain to develop the disease.
- Over 50 repeats: Early onset of the disease.
- DNA fingerprinting can compare patient DNA with known disease patterns to predict the probability and timing of symptoms.
- It is also used to identify microbial infections by comparing the DNA fingerprint of the microbe with known pathogens.
How is genetic fingerprinting used in plant and animal breeding?
- It helps prevent undesirable inbreeding in breeding programs on farms and in zoos.
- It identifies plants or animals carrying desirable alleles.
- These individuals can be selected for breeding to increase the probability of offspring inheriting beneficial traits.
- It can determine paternity in animals and establish pedigrees (family trees)
