In this post, our Science Team gives you the answer to the 2019 HSC Biology paper.
Our Biology Team has been hard at work putting together the solutions to the 2019 HSC Biology Exam. Read the solutions below and see how you went!
|1||D||Haemophilia is genetic while all the other options are caused by a pathogen.|
|2||C||The body produces antibodies that match the antigens present in the vaccine.|
|3||D||The predator acts as a selection pressure, reducing the number of dark beetles over time.|
|4||D||A nucleotide is composed of one base, one sugar and one phosphate group.|
|5||B||Phagocytes engulf and digest foreign particles.|
|6||C||Cochlear implants bypass the ear canal, ossicles and hair cells to electrically stimulate the auditory nerve.|
|7||A||While antibiotic W is fairly effective against both types of bacteria.|
|8||B||The data is not continuous. A and D represent the number of animals not the number of species.|
|9||A||Internal fertilisation occurs in an enclosed space (reproductive tract) to prevent the gametes from drying out.|
|10||C||The number of infected cattle will be dependent on the quarantine policy of each island.|
|11||B||The dominant phenotype will be visible in both homozygous and heterozygous individuals.|
|12||D||Type 2 diabetics maintain blood glucose levels above that of a non-diabetic.|
|13||B||Genetic drift is the change of allele frequency in a population due to random chance events.|
|14||A or B||Assuming the template strand is provided, the complimentary mRNA strand would be GCG UAG UAC GAU so the matching anticodons would be CGC AUC AUG CUA.|
|15||C||A germ line mutation occurs in DNA during meiosis in the production of a gamete. If the mutation occurs in non-coding (‘junk’) DNA it is less likely to have any impact.|
|16||D||Student X has been exposed to antigens from the pathogen before so will experience a secondary response. Student Y has not been exposed to the antigens before so will experience a primary response. According to the graph the primary response would not reach 1000 mg/L.|
|17||A||The inheritance pattern must be dominant as there are two affected parents with an unaffected child. It cannot be sex linked recessive as there is an affected female with an unaffected son.|
|18||B||This is a positive feedback loop as the response to the stimulus results in a further increase to the stimulus.|
|19||A||Cas9 is an enzyme as it is a reusable molecule that catalyses a reaction (breaking bonds between DNA bases).|
|20||D||gRNA has nucleotides complementary to the target DNA so that the Cas9 enzyme binds and cuts at the exact position required.|
An increase in body temperature is [detected by the hypothalamus in the brain]. Nerve impulses travel to blood vessels and sweat glands causing [vasodilation] and sweat to be produced by sweat glands. This causes excess blood flow to the skin and [evaporative cooling]. The result is heat loss from blood and a fall in body temperature.
|Somatic mutation||Germ-line mutation|
|Location||Any body cell other than sex cells||Sex cells in the gonads only|
|Effect on offspring||None – mutation is not passed on to offspring||The mutation may be passed to offspring|
|Example||Mutation in a skin cell resulting in skin cancer||Trisomy 21 – an extra copy of chromosome 21 in every cell of the offspring|
Educational programs are initiatives developed by the Government and non-government organisations in order to minimise the incidence of disease amongst the community. Initiatives include awareness advertisements on TV and radio, on the sides of buses, in schools, doctors’ surgeries or in school programs such as life education. These initiatives influence lifestyle changes that will reduce the incidence of non-infectious disease. For example, epidemiological studies have identified a relationship between U.V. exposure and skin cancer, as well as cigarette smoking and lung cancer. In response, the Australian Government has developed campaigns to increase awareness and influence lifestyle changes to prevent the incidence of lung cancer. After the introduction of the ‘quit now’ campaign in 1997-98 it was found that there was a 1.8-3.1% reduction in the prevalence of smoking within 18 months of the start of the campaign.
Artificial insemination is the collection and manual placement of sperm into the female reproductive tract. This is often used to selectively breed two organisms of the same species with favourable traits. For example, impregnating a mare without requiring the stallion to be in the same location. Artificial insemination can increase biodiversity at the genetic level as it allows crosses that would not normally occur due to distance or physical differences. However, sperm from the same male can be used many times and this can lead to increased levels of these genes within the population i.e. decreased genetic diversity.
Transgenic species in agriculture initially increase genetic diversity by introducing genes from one species to another e.g. Bt cotton that has a gene from a bacterium. A disadvantage is that transgenic organisms are usually cloned which reduces biodiversity as it means large areas of crop are genetically identical. Bt cotton has an advantage and may outcompete native species, reducing ecosystem biodiversity.
(a) Humans usually have two sex chromosomes, XX in females and XY in males. This karyotype has only one sex chromosome (X).
(b) The karyotype above is an example of aneuploidy as a result of non-disjunction during meiosis. Each daughter cell produced during meiosis should have N chromosomes, but in some cases non-disjunction occurs where chromosomes do not separate properly during Anaphase I or Anaphase II producing some daughter cells with N+1 and some cells with N-1. In this case, during fertilisation a N-1 gamete has combined with a normal N gamete to produce a 2N-1 karyotype in the zygote. This specific karyotype would result in Turner syndrome.
In the regions with a high percentage of the population able to digest lactose (e.g. Northern Europe, Western Africa) it is likely that in the past cow’s milk was used as an important food source. There would have been periods where other sources of food failed and individuals that could not digest lactose perished and did not pass their genes to the next generation. Through this process of natural selection, those with a mutation that allowed them to produce the lactase enzyme for longer would have survived and passed on their genes. In areas where milk is less available, the mutation is not likely to be passed on and become common in the population.
While there is a protein coding gene responsible for the production of the lactase enzyme, it is not on this gene where the mutation has occurred. What has changed is when the gene is switched on and off, which is managed by other DNA regions (e.g. promotors, inhibitors, silencers).
(a) Budding is a form of asexual reproduction.
(b) As yeast reproduces and its biomass increases, the yeast converts glucose into carbon dioxide and ethanol in a fermentation reaction. Therefore, carbon dioxide gas can be used as an indicator of reproduction rate.
1. Grow a culture of yeast by adding 500ml of growth media (containing 20g of sucrose) and 20g of yeast to a 1L flask. Incubate at room temperature for 24 hours.
2. Pour 100ml of growth media into four 500ml flasks. Label the flasks 4oC, room temperature (~24 oC), 30 oC and 40 oC.
3. Mix the culture produced in step 1, take four 1ml samples and add them to each of the flasks.
4. Place the 4 oC flask into the fridge, the 30 oC flask into an incubator set at 30 oC and the 40 oC flask into an incubator set to 40 oC. Leave the room temperature flask on the bench. Incubate the flasks for 6 hours at the assigned temperatures.
5. For each flask, mix and take three 1mL samples. Swirl each sample evenly over a separate agar plate.
6. Incubate plates for 2 days at room temperature and use a microscope and mm grid to count the number of colonies formed on each plate. Record results.
7. Steps 1-6 should be repeated 3 times.
(a) The patient inherited H and r alleles from the father, h and R from the mother.
|Chromosome I||Chromosome II|
|Before crossing over||r r|
|After crossing over||r r|
Key: r = Stargardt disease, R = normal retina, H = Huntington’s disease, h = normal nerve cells
Water loss must be controlled in plants or the plant will not be able to perform photosynthesis and it will lose turgor pressure and wilt. When there is sufficient water available, the guard cells of the stomata swell up (become turgid), this bends them and opens up the pore for gas exchange. When water levels drop the guard cell shrinks, closing off the pore. Closing is triggered by the stress hormone abscisic acid and prevents evaporation of water (but also prevents gas exchange for photosynthesis).
If the original parents were pure breeding (homozygous) the F1 generation will all be heterozygous. For seed shape, the F2 generation shows the typical 3:1 ‘Mendelian’ ratio of phenotypes of offspring seen when crossing two heterozygous parents with alleles that have a simple dominant/recessive relationship. For example if R = round seeds and r = wrinkled seeds
When the F1 generation is bred together 3 genotypes are produced: RR (25%), Rr (50%) and rr (25%). However due to the presence of the dominant allele only two phenotypes are produced: Round (75%) and wrinkled (35%).
By contrast crossing two heterozygous parents from the F1 generation has produced three different phenotypes for feather colour. This indicates a case of incomplete dominance or co-dominance (as shown in the punnet square below). If FB = black feathers and FW = white feathers
In a case of co-dominance the F2 generation will have 25% birds with black feathers, 25% with only white feathers and 50% of birds will have speckled (black and white) feathers. In the case of incomplete dominance 50% of birds would have grey feathers rather than speckled..
(a) The bacterium that causes the plague (Yersinia pestis) uses fleas as a vector in order to bypass the first line of defence (skin). When a person is bitten the bacterium leaves the flea and enters the blood stream of the human host.
(b) Infectious diseases can spread by direct or indirect transmission. In direct contact transmission there is some physical contact between an infected person and a susceptible person e.g. mucus droplets spread the common flu virus. This causes diseases to spread quickly in densely populated areas. In indirect contact transmission there is no direct human-to human contact e.g. bite from insect vector causes Malaria. The spread of the disease may be easier to control.
(a) Between 1900 to 2010 there has been an increase in the human population from 1.2 to 6.8 billion and an increase in the actual number of people at risk of Malaria from 0.9 billion to 3.4 billion. Despite this, the percentage of the population at risk of Malaria has actually decreased from 75% to 50% and the number of countries that experience cases of Malaria has declined from 140 to 88. By contrast, reported cases of dengue fever has increased from 1950 to 2010 and the disease has spread to many more countries and continents in that time (e.g. South America, Africa, Australia).
(b) Since the world population has increased, with many people living in high density areas, this provides an opportunity for diseases such as Dengue fever and Malaria to spread. However, Malaria is a disease that has occurred in the human population for thousands of years and by contrast dengue is a relatively new disease occurring between 100-800 years ago and has been restricted geographically until the 1950s, this makes most of the human population susceptible hosts that do not have immunity to the disease. While Malaria is caused by the plasmodium parasite, Dengue fever is a virus that has a variety of strains and can mutate easily, further preventing populations from developing immunity. This may help explain distribution of Malaria has reduced (from 140 countries reporting down to 88), while between 1950 and 2010 the number of countries reporting dengue fever has increased.
Historically there has been more awareness of Malaria and the mosquito vector that spreads it. There was a Global Malaria Eradication program started in 1955 which used techniques such as DDT pesticide spraying, draining of swamps and distribution of bed nets. There was a 14% decrease in the population at risk between 1946 and 1975 indicating that the programs succeeded in eradicating the disease in certain areas. Dengue fever has not attracted the same attention and coordinated global elimination programs. In fact it is referred to as a ‘neglected tropical disease’.
International trade and travel development (e.g. frequency of international flights) has increased rapidly since the 1950s which can transport infected people and the mosquito vectors that spread these diseases. However, another difference between Malaria and Dengue is that there is no effective antiviral treatment for Dengue. This means more infected individuals that are sick for longer and can spread the disease to others. By contrast, Malaria can be treated effectively with medicines such as Coartem or chloroquine which prevents the disease from spreading further (for both Dengue and Malaria, vaccinations have had minimal effectiveness to date).
(a) In the nucleus the gene/s for amyloid beta are copied onto an mRNA strand by RNA polymerase in the nucleus (Transcription). In the cytoplasm a Ribosome will undergo Translation of the mRNA strand into a string of amino acids. The tRNA molecules with anticodons that match the codons on the mRNA deliver amino acids to the ribosome that joins them together with peptide bonds. This creates a polypeptide chain or multiple chains which will be folded and modified to become amyloid beta protein.
(b)(i) Alleles are alternative forms of a gene. While an individual can only carry 2 alleles for a gene, for many genes there are more than two different alleles for a gene in the population. For example clover has 7 different alleles for leaf pattern resulting in 22 different possible genotypes for leaf pattern.
(ii) The e2, e3 and e4 alleles are each associated with a risk level for Alzheimer’s, and the different alleles interact with each other to produce different risk levels. Allele e3 produces an average risk, while e2 seems to be a protective factor for this disease, reducing the likelihood of developing Alzheimer’s by 40% (e2 is dominant over e3). Inheriting a copy of e4 increases the risk of developing Alzheimer’s, even in combination with e2. Inheriting two copies of the e4 allele makes developing Alzheimer’s 14.9 times more likely. Therefore, inheriting one or more e2 alleles reduces risk of Alzheimer’s while one or more e4 alleles increases the risk of Alzheimer’s developing.
(c) Of the three groups, individuals with untreated Herpes Simplex Virus have a higher risk of dementia and this risk increases with the number of years that pass. After 11 years of follow up the cumulative risk of dementia for this group more than twice that of the group that was treated with antivirals. For those with HSV infection that was treated with antivirals the risk increases much more slowly and after 11 years the cumulative risk is 1. The risk of dementia still increases over time for those without any HSV infection but only very slightly (less than 0.5 after 11 years).
(d) Infectious diseases are caused by a pathogen and can be passed from person to person while non-infectious diseases cannot be passed on to another person except through inheritance. None of the information provided in Q33 suggests that a person with Alzheimer’s disease symptoms can pass this disease on to another person through direct or indirect contact. However, the table in 33 (b) shows that the genotype of an individual has a very strong influence over whether an individual develops Alzheimer’s. An individual with two harmful e4 alleles has 14.9 times higher risk of Alzheimer’s and with two e2 alleles the disease is 40% less likely. This indicates it should be classed as a non-infectious disease.
The graph in 33(C) shows there is an increased risk of dementia after Herpes Simplex Virus infection, but Alzheimer’s is not the only cause of dementia, so it is unclear exactly how the graph relates to the risk of Alzheimer’s disease. The graph also shows that even those that have not experienced HSV infection show a gradual increase in risk of dementia over time which indicates that HSV is not the only cause of the disease.
A way to establish whether Alzheimer’s is caused by a pathogen would be to use Koch’s postulates which were developed in order to identify which microbes are associated with a specific disease. If the postulates were to be followed:
In addition to genetic factors and HSV infection, Alzheimer’s disease has also been associated with bacterial infection, gender, head injury and cholesterol levels. Therefore, there seem to be multiple factors that affect the risk of Alzheimer’s and a single pathogen alone is not responsible. At this point Alzheimer’s disease should be classified as a non-infectious disease. Further studies are needed to reveal the specific role of pathogens in development of the disease.
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