Genetic fingerprinting

Question 1

What is genetic fingerprinting

Answer 1

• 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.

Question 2

What are variable number tandem repeats (VNTRs)

Answer 2

DNA bases which are non-coding

Question 3

Describe why variable number tandem repeats are important in genetic fingerprinting

Answer 3

• 95% of human DNA is currently not known to code for any characteristic but may be functional.
• Variable number tandem repeats (VNTRs) are DNA bases which are non-coding.
• For every individual, the number and length of VNTRs has a unique pattern.
• they are different in all individuals except identical twins
• The more closely related two individuals are, the more similiar the VNTRs are.
• Therefore VNTRs are very useful in genetic fingerprinting.

Question 4

What does gel electrophoresis do

Answer 4

Gel electrophoresis is used to separate DNA fragments according to their size.

Question 5

Describe the process of gel electrophoresis

Answer 5

• The DNA fragments are placed on to an agar gel and a voltage is applied across it.
• The resistance of the gel means that the larger the fragments, the more slowly they move.
• Therefore, over a fixed period, the smaller fragments move further than the larger ones.
• In this way DNA fragments of different lengths are separated.
• If DNA fragments are labelled (by radioactive DNA probes) their position can be determined by placing a sheet of X-ray film over the agar gel for several hours.
• The radioactivity from each DNA fragment exposes the film and shows where the fragment is situated on the gel.
• Only DNA fragments up to around 500 bases long can be sequenced in this way.
• larger genes and whole genomes must therefore be split into fragments by restriction endonucleases.

Question 6

List the five stages in the making of a genetic fingerprint

Answer 6

1) Extraction
2) Digestion
3) Separation
4) Hydridisation
5) Development

Question 7

Describe and explain the first stage in the making of a genetic fingerprint : Extraction

Answer 7

• Even the tiniest sample of animal tissue, such as a drop of blood or hair root is enough to give a genetic fingerprint.
• whatever the sample,m the first stage is to extract the DNA by separating it from the rest of the cell.
• As the amount of DNA is usually small, its quantity can be increased by using the polymerase chain reaction.

Question 8

Describe and explain the second stage in the making of a genetic fingerprint: Digestion

Answer 8

• The DNA is the cut into fragments, using the same restriction endonucleases.
• The endonucleases are chosen for their ability to cut close to, but not within, the target DNA.

Question 9

Describe and explain the third stage in the making of a genetic fingerprint: Separation

Answer 9

• The fragments of DNA are next separated according to size by gel electrophoresis under the influence of an electrical voltage.
• The gel is then immersed in alkali in order to separate the double strands into single strands.

Question 10

Describe and explain the fourth stage in the making of a genetic fingerprint: Hybridisation

Answer 10

• Radioactive (or fluorescent) DNA probes are now used to bind with VNTRs
• The probes have base sequences which are complementary to the base sequences on the VNTRs and bind to them under specific conditions, such as temperature and pH.
• The process is carried out with different probes, which bind to different target DNA sequences.

Question 11

Describe and explain the fifth stage in the making of a genetic fingerprint

Answer 11

• An X-ray film is put over the nylon membrane
• the film is exposed by the radiation from the radioactive probes (if using fluorescent probes, the positions are located visually).
• because these points correspond to the position of the DNA fragments as separated during electrophoresis, a series of bars is revealed.
• The pattern of the bands is unique to every individual except identical twins.

Question 12

List four key uses of genetic fingerprinting

Answer 12

1) Genetic relationships and variability
2) Forensic science
3) medical diagnosis
4) Plant and animal breeding

Question 13

Explain why genetic fingerprinting is useful in examining genetic relationships and variability- paternity tests

Answer 13

• DNA fingerprinting can be used to resolve questions of paternity
• Individuals inherit half of their genetic material from their mother and half from their father.
• Therefore each band on a DNA fingerprint of an individual should have a corresponding band in one of the parents DNA fingerprint.
• This can be used to establish whether someone is the genetic father of a child.

Question 14

Explain why genetic fingerprinting is useful in genetic relationships and variability- examining populations

Answer 14

• Genetic fingerprinting is also useful in determining genetic variability within a population.
• The more closely two individuals are related, the closer the resemblance of their genetic fingerprints.
• A population whose members have very similiar genetic fingerprints has very little genetic diversity.
• A population whose members have a greater variety of genetic fingerprints has greater genetic diversity.

Question 15

Explain why genetic fingerprinting is useful in forensic science

Answer 15

DNA is often left at the scenes of crimes, so genetic fingerprinting can establish whether a person is likely to have been at the scene of a crime.

Question 16

What is the limit of genetic fingerprinting in forensic science

Answer 16

• Genetic fingerprinting can establish whether someone is likely to have been present at the crime scene but cannot prove that they actually carried out the crime.
• Even if a close match to the suspects DNA is found at a crime scene, other possible explanations must be investigated such as:
• The DNA may have been left on some other, innocent occasion.
• The DNA may belong to a very close relative.
• The DNA sample may have been contaminated after the crime, either by the suspects DNA or by chemicals that affected the action of the restriction endonucleases used in preparing the fingerprint.
• The probability that someone else DNA might match that of the suspect must also be calculated- this calculation is based on the assumption that the DNA which produces the banding pattern is randomly distributed in the community.
• This is not always the case- it may not apply where religious or ethnic groups tend to have partners from within their own, small community.

Question 17

Describe how genetic fingerprinting can help in diagnosing Huntington’s disease

Answer 17

• Huntington’s disease is a genetic disorder of the nervous system.
• It results from the three-base sequence AGC at one end of a gene on chromosome 4 being repeated over and over again (a ‘genetic stutter’)
• People with fewer than 30 repeats are unlikely to get the disease, while those with more than 38 repeats are almost certain to do so.
• If they have over 50 repeats, the onset of the disease will occur earlier than average.
• A sample of DNA from a person with the allele for Huntington’s disease can be cut with restriction endonucleases and a DNA fingerprint prepared.
• This can then be matched with fingerprints of people with various forms of the disease and those without the disease.
• In this way, the probability of developing the symptoms, and when, can be determined.

Question 18

How can genetic fingerprinting identify a microbial infection

Answer 18

You can compare the genetic fingerprint of the microbe found in patients with the genetic fingerprint of known pathogens to identify it.

Question 19

How can genetic fingerprinting identify a microbial infection

Answer 19

You can compare the genetic fingerprint of the microbe found in patients with the genetic fingerprint of known pathogens to identify it.

Question 20

Describe how genetic fingerprinting is useful in plant and animal breeding

Answer 20

• genetic fingerprinting can be used to prevent undesirable inbreeding during breeding programs on farms or zoos.
• it can also identify plants or animals that have a particular allele of a desirable gene.
• Individuals with this allele can be selected for breeding in order to increase the probability of of their offspring having the characteristic that it produces.
• Another application is the determination of paternity in animals and thus establishing the pedigree of an individual.

Question 21

List the steps in interpreting the results of genetic fingerprinting

Answer 21

• DNA fingerprints from two samples are visually checked.
• If there appears to be a match, the pattern of bars of each fingerprint is passed through an automated scanning machine, which calculates the length of the DNA fragments from the bands.
• It does this by using data obtained by measuring the distances travelled during electrophoresis by known lengths of DNA.
• The odds are calculated of someone else having an identical fingerprint.
• The closer the match between the two patterns, the greater the probability that the two sets of DNA have come from the same person.