Download Genetics Option - Worked Examples

BIOLOGY
OPTIONS
Genetics: The Code Broken?
Worked Examples
CHAPTER 1 Revision Questions
8. Draw a ‘flow chart’ to summarise the main events from nucleus to cytoplasm in Question 7 [i.e. the transfer of
information from DNA through RNA to produce a polypeptide].
Answer
Travels to
cytoplasm
Ribosome
mRNA
Translation
Transcription
Gene
(DNA)
Polypeptide
Amino
acids
tRNA
Marking Criteria
Shows at least 6 events correctly linked
3
Shows 3 or more events correctly linked
2
Shows 3 or more events
1
CHAPTER 2 Revision Questions
7. Today, DNA fingerprinting is used in everyday language. Famous stars, Mick Jagger, Elizabeth Hurley and Boris
Becker to name a few, have used it to decide paternity cases. It has also been used in crime investigations, for
example Wee Waa in country New South Wales where all males (population was made up of approximately 350
males) were DNA tested after an elderly woman was raped. The suspect was identified. Television dramas and
movies that involve crime investigations prioritize (place at the top of the list), ‘DNA tests’. Top breeders also use
DNA fingerprinting as a means of ‘verifying’ ownership of a pedigree animal.
Critically analyse the importance and accuracy of DNA fingerprinting.
Answer
[Critically analyse means add a level of accuracy, depth, knowledge and understanding, logic, questioning, reflection
and quality to an identification of components and the relationship between them, drawing out and relating
implications]
DNA fingerprinting has been a crucially important breakthrough in crime analysis, enabling forensic scientists to
identify a suspect from a tiny sample of blood, semen, saliva or other tissue. Before this, they could only identify blood
groups and a few proteins, which could provide only a very rough identification. The technology has also brought a
much greater degree of certainty to other areas, such as paternity testing and pedigreeing of animals.
DNA fingerprinting involves the use of restriction enzymes and Southern blotting, or DNA probes, to compare
particular base sequences in 2 or more DNA samples.
The accuracy of the technique depends on two factors: the analysts must have a sufficiently large sample of
undamaged DNA, and they must follow rigorous experimental techniques, particularly in avoiding contamination. If
there is insufficient DNA, or if it has been degraded by exposure to the weather or to chemicals, the results can be
uncertain. Any contamination of the sample by microorganisms or by the experimenter’s DNA can produce false
results.
DNA fingerprinting is particularly good at providing negative assessments, eg proving a crime suspect innocent, or
showing that a person is not a particular child’s parent or that an animal’s pedigree has been faked. Already a number of
convicted criminals – some of whom have served sentences of several years – have been released after being proved
innocent by DNA tests.
Positive identification is less certain, even if there is a perfect match between two DNA samples. At best, this can
provide an extremely high probability that the samples came from the same source. The reason for this is that a number
of people may show the same patterns on a DNA test. The probability that two similar bands are evidence of the same
person depends on how common the bands are, but working out the probability is difficult because different ethnic
groups often have different genetic patterns; without testing every population in the world, it may be difficult to be
certain, and during court trials, experts have disagreed about the significance of a result. Because of these uncertainties,
judges and juries have become confused, and some suspects have been set free after being found guilty on the DNA
evidence (eg O.J. Simpson).
Often DNA fingerprinting is done by using a single restriction enzyme (or a single probe), which requires only a
tiny DNA sample but which cannot identify a person with great accuracy. Using several enzymes (or probes) is much
more time-consuming and requires a much larger sample, but it produces far more accurate results.
Another potential problem with the technique is the threat to civil liberties which results from the establishment of
national or international databases of DNA information. This has made many people wary of allowing their DNA to be
analysed.
Despite these problems, however, DNA fingerprinting has already had enormous benefits for crime investigators,
immigration authorities, pet breeders and others, and will undoubtedly be even more beneficial as it is used more and
more in the future.
Marking Criteria
Explains the accuracy of DNA fingerprinting, makes a judgement about its importance and backs this up
7-8
with relevant information about the successes and limitations of the technique
Explains the accuracy of DNA fingerprinting, makes a judgement about its importance and gives some extra
5-6
relevant information
Explains the accuracy of DNA fingerprinting,
3-4
OR makes a judgement about its importance and gives some extra relevant information
Gives some relevant information about the accuracy and importance of DNA fingerprinting
1-2
CHAPTER 3 Revision Questions
6. Evaluate the importance of cross-breeding experiments in studying the inheritance of linked genes, and explain what
is meant by a testcross.
Answer
[Evaluate means make a judgement based on criteria; determine the value of]
Cross-breeding experiments have been essential in determining the presence and inheritance of linked genes, because
(prior to the use of DNA sequencing and gene probes) linkage could only be determined by examining the frequency
with which certain gene combinations are inherited. Linked genes are inherited together more often than is predicted by
the Mendelian ratios.
A testcross is used to determine whether an organism is homozygous or heterozygous for a particular trait. It is a
cross with a homozygous recessive individual. For example, to find whether the genotype of a pea plant with round peas
is RR or Rr, it should be crossed with an rr plant; if any offspring are wrinkled, this means that the original plant must
be Rr, but if they are all round, the original was probably RR – the more offspring are examined, the more certain this
conclusion is.
Marking Criteria
Explains and makes a judgement about the importance of cross-breeding experiments, and gives the features
4
of a testcross.
Explains or makes a judgement about the importance of cross-breeding experiments, and gives the features
3
of a testcross.
Explains the importance of cross-breeding experiments,
2
OR gives the features of a testcross.
Gives some information about cross-breeding experiments or a testcross
1
CHAPTER 4 Revision Questions
5. (a)
Should genes be patented by anyone? Support your answer.
Answer
Genes should not be patented because they are naturally occurring DNA sequences. Researchers may discover genes,
they may even modify them, but they do not invent them.
Marking Criteria
Gives an opinion and supports it with a logical reason
1
(b) Celera patented the probes they used to identify genes. Justify whether this is ethically acceptable or not.
Answer
[Justify means support an argument or conclusion]
A gene probe is a tool which does not exist naturally, although, like all other artificial chemicals, it is made from
naturally occurring substances (DNA). Since Celera actually designed and built the probe, its is ethically acceptable to
allow them to patent it.
Marking Criteria
Gives an opinion and supports it with a logical reason
1
CHAPTER 5 Revision Questions
3. (a)
Describe why viruses are thought to be a good choice as a ‘vector’ in gene therapy.
Answer
[Describe means provide characteristics and features]
Viruses are totally adapted to insert their DNA into host cells and to incorporate it into their chromosomes; these are the
two things that a vector needs to be able to accomplish in order to deliver a gene to an organism for purposes of gene
therapy.
Marking Criteria
Gives two reasons why viruses are good vectors
2
Gives a reason why viruses are good vectors
1
(b) Explain the disadvantages of using viruses in gene therapy.
Answer
[Explain means relate cause and effect, make the relationships between things evident; provide why and/or how]
There are two main disadvantages of using viruses as vectors: viruses can be dangerous, as they normally cause
diseases, and the host’s immune system is designed to locate and destroy viruses.
Marking Criteria
Clearly explains two disadvantages of using viruses
2
Clearly explains one disadvantage of using viruses
1
CHAPTER 6 Revision Questions
5.
Is gene mutation always successful and can DNA repair itself?
Outline the process by which genes may come to the rescue of their damaged members.
Answer
[Outline means sketch in general terms; indicate the main features of]
Observed gene mutations are rare because DNA has the ability to repair itself. There are many genes which produce
enzymes that repair damaged DNA: some have the ability to locate and cut out damaged sections of the DNA molecule,
others add the missing bases, and still others repair the break in the sugar-phosphate backbone. In this way the molecule
is restored to normal and no mutation can be detected.
Marking Criteria
Gives the main features of DNA self-repair
2
Gives one or more features of DNA self-repair
1
CHAPTER 7 Revision Questions
5. Describe the term ‘gene cloning’ and using an example for each, show how gene cloning is being researched and
used to improve the health of humans and how it is used for commercial productivity.
Answer
[Describe means provide characteristics and features]
Gene cloning means making identical copies of a gene. This is normally done by using recombinant DNA technology,
e.g. by inserting the gene into the DNA of a plasmid, causing the plasmid to be taken up by a bacterium, and allowing
the bacterium to reproduce by placing it on a growth medium.
Gene therapy is completely dependent on gene cloning – for example, in therapy for cystic fibrosis, the healthy
version of the CF gene may be inserted in an AAV vector, and many millions of copies of this virus are cloned, together
with the gene, before being inserted in the patient’s respiratory tract.
Commercial applications of genetic engineering also rely on gene cloning. In the Bt cotton project, which aims to
save farmers hundreds of millions of dollars, the Bt gene is cloned in the Agrobacterium vector before being inserted
into the cotton plants.
Marking Criteria
Gives features of gene cloning and explains two examples, one medical and one commercial
5-6
Gives features of gene cloning and explains one or more examples
3-4
Gives some information about and/or examples of gene cloning
1-2
CHAPTER 8 Revision Questions
6. In named species, describe evidence which may indicate the presence of ancestral vertebrate gene homologues in
lower animal classes.
Answer
[Describe means provide characteristics and features]
Homeotic genes in Drosophila are regarded by biologists as ancestral homologues of Hox (homeotic) genes in
mammals such as mice. This is because in both species, these genes:

Have a similar structure,

Contain a very similar 180-nucleotide region called the homeobox,

Have a similar function – controlling development of segments of the body,

Are arranged on chromosomes in the same order as the parts of the body they control.
Therefore the genes in mice (and other vertebrates) could have evolved from those in more primitive groups.
Although mice have homeotic genes on four chromosomes, compared to the Drosophila’s single set, this can be
explained by duplication of the chromosome during the course of evolution; similarly, duplication and deletion of
individual genes could explain other differences between them.
Marking Criteria
Provides correct evidence, gives features and correctly relates this to named vertebrate and invertebrate
3-4
species
Provides correct evidence and gives features
1-2