SCSA Human Biology Gene pools
15 sample questions with marking guides and sample answers · Avg. score: 72.5%
A computer simulation was used to observe genotypic changes in the gene pool of 20 randomly selected rabbits. The simulation was set with these parameters:
- each rabbit's coat colour was either black or white
- black alleles were dominant; white alleles were recessive
- the number of rabbits was constant in each generation and breeding was random throughout the population
- an environmental factor was chosen in the simulation to provide selection pressure.
The table shows the results of the simulation at the start and after 20 generations.
| Initial population genotypes | Population genotypes after 20 generations |
|---|---|
| BB BB BB BB BB BB | BB BB |
| Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb | Bb Bb Bb Bb Bb Bb Bb Bb |
| bb bb bb bb | bb bb bb bb bb bb bb bb bb bb |
Contrast the initial allele frequency with the allele frequency after 20 generations to draw a conclusion about the effect of the selection pressure on the rabbit population.
Reveal Answer
Initial allelic frequencies were B 0.55 and b 0.45.
Allelic frequencies after 20 generations were B 0.3 and b 0.7.
B decreased (from 0.55 to 0.3) and b increased (from 0.45 to 0.7).
This selection pressure was in favour of white rabbits as both genotype and allelic frequencies shifted toward the white phenotype and the white allele.
| Descriptor | Marks |
|---|---|
Provides the correct initial and final allele frequencies | 1 |
Identifies consequentially correct change in allele frequency | 1 |
States a consequentially valid conclusion | 1 |
Which of the following are all selection pressures which could reduce genetic variation in a population?
disease, increased competition and climate change
increased food availability, disease and climate change
increased competition, reduced environmental pollutants and disease
reduced land availability, increased food availability and climate change
Reveal Answer
disease, increased competition and climate change
Disease, increased competition, and climate change are all strong selection pressures that can cause significant mortality, eliminating disadvantageous alleles and reducing overall genetic variation.
increased food availability, disease and climate change
Increased food availability is a relaxed selection pressure that reduces competition, allowing more individuals to survive and typically maintaining or increasing genetic variation.
increased competition, reduced environmental pollutants and disease
Reduced environmental pollutants represents a relaxation of selection pressure, which allows a wider variety of phenotypes to survive rather than reducing genetic variation.
reduced land availability, increased food availability and climate change
Increased food availability relaxes selection pressure, meaning more individuals with diverse traits can survive, which does not reduce genetic variation.
Alpha and Beta Thalassemia have similarities and differences in their inheritance patterns. Which of the following is correct?
Both are autosomal dominant conditions controlled on multiple gene loci.
Alpha Thalassemia is fatal for homozygote individuals while Beta Thalassemia is not.
Both are autosomal recessive conditions that result in affected individuals having four defective globin genes.
Beta Thalassemia is fatal for heterozygote individuals while Alpha Thalassemia is not.
Reveal Answer
Both are autosomal dominant conditions controlled on multiple gene loci.
This is incorrect because both Alpha and Beta Thalassemia are inherited as autosomal recessive conditions, not autosomal dominant.
Alpha Thalassemia is fatal for homozygote individuals while Beta Thalassemia is not.
This is correct. Homozygous Alpha Thalassemia (loss of all four alpha genes) causes hydrops fetalis, which is typically fatal in utero. Homozygous Beta Thalassemia is not fatal in utero because fetal hemoglobin relies on gamma chains rather than beta chains.
Both are autosomal recessive conditions that result in affected individuals having four defective globin genes.
This is incorrect because while Alpha Thalassemia involves four genes (two loci on chromosome 16), Beta Thalassemia involves only two genes (one locus on chromosome 11).
Beta Thalassemia is fatal for heterozygote individuals while Alpha Thalassemia is not.
This is incorrect. Heterozygotes for Beta Thalassemia (Beta Thalassemia Minor) are typically asymptomatic or experience only mild anemia, making it non-fatal.
Natural selection is often referred to as 'survival of the fittest'. Which of the following is the best definition for 'fittest' as used in this statement?
the ability of an organism to survive and reproduce
adaptations that produce favourable traits
selective breeding to promote favourable traits
heritable traits that will be passed on to offspring
Reveal Answer
the ability of an organism to survive and reproduce
Correct. In evolutionary biology, fitness specifically refers to an organism's ability to survive in its environment and successfully reproduce, passing its genes to the next generation.
adaptations that produce favourable traits
Incorrect. While adaptations and favorable traits can increase an organism's fitness, they are the characteristics themselves rather than the measure of reproductive success.
selective breeding to promote favourable traits
Incorrect. Selective breeding refers to artificial selection driven by humans, whereas survival of the fittest describes the natural process of evolution.
heritable traits that will be passed on to offspring
Incorrect. Heritable traits are the genetic characteristics passed down, but fitness is the actual measure of how successfully an organism survives and reproduces using those traits.
Speciation occurs when
the gene pool of an existing species becomes too small to support a viable population.
selection pressures cause significant changes to the allele frequencies of a population.
genetic drift is no longer occurring within populations.
gene flow is no longer occurring between populations.
Reveal Answer
the gene pool of an existing species becomes too small to support a viable population.
This scenario describes a population bottleneck or the path to extinction, rather than the divergence process required to form a new species.
selection pressures cause significant changes to the allele frequencies of a population.
While selection pressures drive microevolution by changing allele frequencies, speciation specifically requires reproductive isolation to separate a lineage into distinct species.
genetic drift is no longer occurring within populations.
Genetic drift is a constant evolutionary force in finite populations, and its cessation is not a mechanism for speciation.
gene flow is no longer occurring between populations.
Speciation requires reproductive isolation; when gene flow stops between populations, they can diverge genetically through mutation, selection, and drift until they become distinct species.
Allele frequencies were monitored in two large populations of field mice from neighbouring forests over a 10-year period. Results are shown.
Forest X
| Year | Genotype AA | Genotype Aa | Genotype aa | Allele frequency A | Allele frequency a |
|---|---|---|---|---|---|
| 2013 | 52 | 146 | 102 | 0.42 | 0.58 |
| 2014 | 48 | 144 | 108 | 0.40 | 0.60 |
| 2015 | 55 | 147 | 98 | 0.43 | 0.57 |
| 2016 | 60 | 150 | 90 | 0.45 | 0.55 |
| 2017 | 58 | 142 | 100 | 0.43 | 0.57 |
| 2018 | 58 | 148 | 94 | 0.44 | 0.56 |
| 2019 | 59 | 152 | 89 | 0.45 | 0.55 |
| 2020 | 60 | 148 | 92 | 0.45 | 0.55 |
| 2021 | 65 | 149 | 86 | 0.46 | 0.54 |
| 2022 | 66 | 149 | 85 | 0.47 | 0.53 |
Forest Y
| Year | Genotype AA | Genotype Aa | Genotype aa | Allele frequency A | Allele frequency a |
|---|---|---|---|---|---|
| 2013 | 0 | 0 | 300 | 0.00 | 1.00 |
| 2014 | 0 | 0 | 300 | 0.00 | 1.00 |
| 2015 | 0 | 0 | 300 | 0.00 | 1.00 |
| 2016 | 0 | 15 | 285 | 0.03 | 0.98 |
| 2017 | 3 | 46 | 251 | 0.09 | 0.91 |
| 2018 | 14 | 60 | 226 | ||
| 2019 | 31 | 91 | 178 | 0.26 | 0.75 |
| 2020 | 48 | 104 | 148 | 0.33 | 0.67 |
| 2021 | 60 | 122 | 118 | 0.40 | 0.60 |
| 2022 | 66 | 137 | 97 | 0.45 | 0.55 |
Calculate the allele frequencies for forest Y in 2018. Show your working.
Reveal Answer
Frequency (a) =
Frequency (A) =
| Descriptor | Marks |
|---|---|
Provides appropriate working | 1 |
Calculates the frequencies of both alleles | 1 |
Identify temporal trends in allele frequency for forests X and Y and infer reasons for the observed differences.
Reveal Answer
Allele frequencies remained relatively constant in forest X over the 10-year period, with a slight increase in allele A over time. This suggests genetic drift is occurring and the changes are likely due to chance rather than the environment favouring a particular phenotype.
This contrasts with forest Y, where the frequency of allele A increases significantly over time after it first appears in 2016. The allele may have been introduced to forest Y due to migration (mice moving in from a neighbouring forest) or mutation. Either way, it is evident that the allele A provided a selective advantage to mice in forest Y, resulting in its frequency increasing over time.
| Descriptor | Marks |
|---|---|
Identifies allele frequencies remain relatively constant in forest X over time | 1 |
Infers a reason for the temporal change in forest X | 1 |
Identifies allele A first appeared in forest Y in 2016 | 1 |
Infers this is due to migration (gene flow) or mutation | 1 |
Identifies the frequency of allele A increases over time in forest Y | 1 |
Infers allele A provides a selective advantage to mice in forest Y | 1 |
Koalas were once widespread in Australia. Due to a variety of factors, their population decreased and fragmented into small pockets, forcing them to inbreed. They have recently been hit by devastating epidemic diseases.
Explain why koalas face an increased extinction risk from disease.
Reveal Answer
High genetic diversity may allow for some members of the population to survive diseases and later reproduce and pass on their resistance to increase the survivability of the population.
However, inbreeding creates low genetic diversity, which makes koalas vulnerable to extinction due to disease.
| Descriptor | Marks |
|---|---|
Explains how genetic diversity can prevent extinction during rapid environmental change, e.g. disease | 1 |
Describes why koalas have low genetic diversity (inbreeding) | 1 |
States that koalas are more vulnerable to extinction due to low genetic diversity | 1 |
Sickle-cell anaemia is a severe and potentially fatal blood disorder. However, the allele persists in some populations where people who carry only one copy of the allele show resistance to malaria.
Which of the following best explains the higher prevalence of the disorder in these populations?
random genetic drift
heterozygous advantage
gene flow
founder effect
Reveal Answer
random genetic drift
Genetic drift refers to random fluctuations in allele frequencies, whereas the persistence of the sickle-cell allele is a non-random, selective process driven by malaria resistance.
heterozygous advantage
Individuals with one copy of the sickle-cell allele (heterozygotes) have a survival advantage against malaria, which keeps the allele frequency high in the population despite the severe effects of having two copies.
gene flow
Gene flow is the transfer of genetic material between populations, which does not explain why the allele is specifically maintained at high frequencies in malaria-endemic regions due to survival benefits.
founder effect
The founder effect occurs when a small group breaks off from a larger population, leading to reduced genetic diversity, rather than a trait being maintained through natural selection.
Fossil evidence seems to show that the morphology of the Queensland lung fish has remained relatively unchanged for the past 100 million years.
Describe the features of the theory of natural selection to explain how this may have occurred.
Reveal Answer
In a population, some individuals will have inherited traits that help them survive and reproduce. Because the helpful traits are heritable, and because organisms with these traits leave more offspring, the population will become adapted to its environment.
In the case of the lung fish, if the environment remains relatively unchanged (i.e. no new predators or competitors, still a water-dwelling organism), there is no environmental selection pressure to select for any new mutations in morphology, so there is minimal change in the species.
| Descriptor | Marks |
|---|---|
Describes natural selection in terms of natural variation in a population includes traits that may be positive for survival in a given environment | 1 |
Describes natural selection in terms of states that these traits survive in the population if there is a selection pressure | 1 |
With reference to the lung fish states that the environment must have been relatively unchanged as no new morphological traits have been selected | 1 |
Compare microevolution and macroevolution.
Reveal Answer
Both involve changes in allele frequencies, but in microevolution, this change occurs within a species, whereas for macroevolution, the change is at or above the species level. This means that in microevolution, the descendant is in the same taxonomic group as the ancestor, whereas in macroevolution, the descendant is in a different taxonomic group.
| Descriptor | Marks |
|---|---|
provides a similarity | 1 |
provides a difference | 1 |
states the significance | 1 |
Use the following information to answer the question.
Populations of the koala (Phascolarctos cinereus) can be found along the eastern coast of Australia. In the late 1800s, habitat destruction and hunting reduced the number of koalas dramatically. In an effort to save the koala from extinction, a small number of individual koalas were introduced to French Island, Victoria. The number of koalas on the island quickly grew. Scientists measured the genetic diversity within the island population and found it to be low when compared to populations of koalas in New South Wales and Queensland.
The population of koalas on French Island
will show more variation in traits than the populations in New South Wales.
will have the same number of alleles per trait as the populations in Queensland.
may be vulnerable to future changes in selective pressures acting on the population.
will have more harmful mutations than the populations in New South Wales and Queensland.
Reveal Answer
will show more variation in traits than the populations in New South Wales.
The passage states the French Island population has low genetic diversity, which means it will show less variation in traits, not more.
will have the same number of alleles per trait as the populations in Queensland.
Low genetic diversity implies that the population will have fewer alleles per trait compared to the more diverse populations in Queensland.
may be vulnerable to future changes in selective pressures acting on the population.
Populations with low genetic diversity have less variation to draw upon, making them less adaptable and more vulnerable to environmental changes and new selective pressures.
will have more harmful mutations than the populations in New South Wales and Queensland.
While low genetic diversity can increase the expression of existing recessive harmful mutations due to inbreeding, it does not cause the population to have a higher number of harmful mutations overall.
Atlantic salmon (Salmo salar) are fish that are farmed in the waters of Tasmania. A selective breeding program was introduced that focused on increasing the growth of the fish, increasing the resistance of the fish to disease and maintaining the colour and oil content of the fish meat. The selective breeding program resulted in at least a 10% increase in growth of the fish in each generation. Increased resistance to disease has been noted as fewer fish in each generation have required treatment for disease.
In this selective breeding program
offspring will be produced by random mating of parents.
genetic variation within the fish population will increase.
the frequency of alleles for the desired phenotypes will increase over generations.
maintaining the colour and oil content of the fish meat will be advantageous for the fish.
Reveal Answer
offspring will be produced by random mating of parents.
Selective breeding relies on non-random mating, as humans specifically choose which individuals reproduce based on their desired traits.
genetic variation within the fish population will increase.
Selective breeding typically decreases genetic variation because only a small subset of the population with specific traits is chosen to reproduce, eliminating other alleles.
the frequency of alleles for the desired phenotypes will increase over generations.
By consistently selecting individuals with desired traits to reproduce, the alleles responsible for those phenotypes become more common in the population over successive generations.
maintaining the colour and oil content of the fish meat will be advantageous for the fish.
Maintaining meat colour and oil content is advantageous for human consumers and commercial value, not for the biological fitness or survival of the fish.
Which of the following statements does not support the theory of natural selection?
Organisms reproduce at a rate greater than can be supported by the environment.
Genotypic variation is exhibited in the phenotypes of individuals.
Selective agents act on the alleles in a gene pool.
Similar individuals mate and produce offspring with favourable traits.
Reveal Answer
Organisms reproduce at a rate greater than can be supported by the environment.
Overproduction of offspring creates competition for limited resources, which is a fundamental premise of Darwin's theory of natural selection.
Genotypic variation is exhibited in the phenotypes of individuals.
Natural selection relies on phenotypic variation, which arises from genotypic differences, allowing the environment to select for advantageous traits.
Selective agents act on the alleles in a gene pool.
While selective pressures act directly on phenotypes, they ultimately determine which alleles are passed on, altering allele frequencies in the gene pool over time.
Similar individuals mate and produce offspring with favourable traits.
This statement describes assortative mating rather than natural selection. Natural selection is driven by environmental pressures determining which traits are favorable for survival and reproduction, not simply by similar individuals choosing to mate.
Speciation can occur when
the gene pool of a population becomes too small to support natural selection.
individuals are able to move between two different populations and introduce new alleles.
two populations are separated, but some individuals are able to interbreed freely.
populations of the same species become isolated from one another for a long period of time.
Reveal Answer
the gene pool of a population becomes too small to support natural selection.
A small gene pool typically leads to genetic drift or inbreeding depression, which reduces genetic diversity rather than driving the formation of a new species.
individuals are able to move between two different populations and introduce new alleles.
This describes gene flow, which actually prevents speciation by keeping the genetic makeup of the two populations similar.
two populations are separated, but some individuals are able to interbreed freely.
If individuals can interbreed freely, gene flow is still occurring, which prevents the populations from diverging enough to become separate species.
populations of the same species become isolated from one another for a long period of time.
Speciation occurs when populations become reproductively or geographically isolated, preventing gene flow and allowing them to diverge genetically over time.
Allele frequencies are most likely to stay constant in
large populations with low levels of migration.
small populations with low levels of migration.
large populations with high levels of migration.
small populations with high levels of migration.
Reveal Answer
large populations with low levels of migration.
Large populations minimize the effects of genetic drift, and low migration limits gene flow, creating conditions closest to Hardy-Weinberg equilibrium where allele frequencies remain stable.
small populations with low levels of migration.
Small populations are highly susceptible to genetic drift, causing random fluctuations in allele frequencies regardless of migration levels.
large populations with high levels of migration.
High levels of migration introduce gene flow, which alters allele frequencies by bringing in new genetic material from other populations.
small populations with high levels of migration.
This scenario leads to rapid changes in allele frequencies due to the combined destabilizing effects of genetic drift (from small size) and gene flow (from high migration).