SCSA Human Biology Mutations
15 sample questions with marking guides and sample answers · Avg. score: 67.4%
An error during DNA replication resulted in the following change to mRNA transcripts.
| mRNA before | AUGAAGUUUGGCAUC ... (continued) |
| mRNA after | AUGAAGUUUGCAUCG ... (continued) |
The DNA replication error most likely involved
deletion of cytosine.
insertion of guanine.
substitution of uracil with guanine.
substitution of guanine with cytosine.
Reveal Answer
deletion of cytosine.
Comparing the sequences reveals that a guanine (G) is missing in the "after" mRNA (changing ...GGC... to ...GCA...), which causes a frameshift. Since mRNA guanine is transcribed from cytosine on the DNA template strand, a deletion of cytosine in the DNA would result in this specific error.
insertion of guanine.
An insertion would add a base to the sequence. The comparison shows that a nucleotide has been removed (deleted) rather than added, as the sequence has shifted to the left.
substitution of uracil with guanine.
A substitution replaces one nucleotide with another without changing the length of the sequence. The observed change is a frameshift mutation caused by a deletion, which alters the reading frame of all subsequent codons.
substitution of guanine with cytosine.
This describes a substitution mutation. However, the sequences show that a base was removed entirely, causing the downstream sequence to shift, which characterizes a deletion mutation rather than a substitution.
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 |
The term ‘selectively-advantageous mutation’ means the mutation
provides a survival advantage in a particular environment.
will always be passed to the offspring.
is always changing to adapt to new environments.
provides a benefit to heterozygote individuals in populations.
Reveal Answer
provides a survival advantage in a particular environment.
A selectively advantageous mutation increases an organism's fitness, meaning it provides a survival and reproductive benefit specific to the environment the organism lives in.
will always be passed to the offspring.
Even highly advantageous mutations are not guaranteed to be passed on, as the organism might die before reproducing or the mutation might not be inherited due to random chance during meiosis.
is always changing to adapt to new environments.
Mutations are random changes in DNA and do not intentionally change or adapt in response to new environments; rather, the environment selects for beneficial mutations that already exist.
provides a benefit to heterozygote individuals in populations.
While heterozygote advantage is a specific phenomenon, a selectively advantageous mutation can provide benefits to homozygous individuals as well, not just heterozygotes.
Which event could cause a frameshift mutation?
non-disjunction during meiosis
error during replication
base pair substitution
heat damage
Reveal Answer
non-disjunction during meiosis
Non-disjunction involves the failure of chromosomes to separate during meiosis, leading to an abnormal number of chromosomes (aneuploidy) rather than a mutation within the DNA sequence.
error during replication
Frameshift mutations are caused by the insertion or deletion of nucleotides that are not multiples of three; these errors frequently occur due to strand slippage during DNA replication.
base pair substitution
Base pair substitution involves replacing one nucleotide with another (point mutation), which affects a single codon but does not shift the reading frame of the entire gene.
heat damage
Heat damage typically causes chemical changes like deamination or depurination, which usually result in base substitutions (point mutations) rather than the insertions or deletions required for a frameshift.
The BMP4 gene in African cichlid fish is responsible for
the great range of colours and scale patterns observed in adult fish of different species.
the amount of bone laid down in the jaws of cichlid fish embryos.
convergent evolution between different species of cichlids.
sex determination in cichlid fish living in African lakes.
Reveal Answer
the great range of colours and scale patterns observed in adult fish of different species.
BMP4 stands for Bone Morphogenetic Protein 4 and is involved in skeletal development, not the pigmentation or scale patterns of the fish.
the amount of bone laid down in the jaws of cichlid fish embryos.
BMP4 regulates bone formation. In African cichlids, variations in its expression determine the amount of bone laid down in the jaws of embryos, driving their diverse jaw morphologies adapted to different diets.
convergent evolution between different species of cichlids.
Convergent evolution is a broad evolutionary pattern driven by similar environmental selective pressures, not a process directly controlled by a single gene like BMP4.
sex determination in cichlid fish living in African lakes.
Sex determination in cichlids is controlled by a complex mix of other genetic and environmental factors, whereas BMP4 is specifically dedicated to bone and tissue development.
Species of the genus Homo do not possess the powerful jaw muscles commonly found in the genera Australopithecus and Paranthropus. According to the fossil record, this decrease in the size of jaw muscles coincided with changes in brain size. Scientists have discovered that both the reduction in the size of the jaw muscles and the change in brain size occurred due to mutations.
Describe the various types of mutations, identify the causes and describe how they can occur.
Reveal Answer
A mutation is a change in a gene or a chromosome, leading to new characteristics in an organism or their offspring.
Mutations can be gene mutations or chromosomal mutations. Gene mutations are changes in a single gene which occur during DNA replication, while chromosomal mutations affect all or part of a chromosome. Mutations can also be somatic or germline. Somatic mutations occur in the body cells, meaning reproductive cells are not affected and therefore the mutation is not passed on to offspring. Germline mutations occur in the reproductive cells or gametes, and therefore can be passed on to offspring.
Mutations can be caused by mutagenic agents, which are substances that are known to increase the rate at which mutations occur, such as ionising radiation (e.g. X-rays), mustard gas, formaldehyde, and some antibiotics. Mutations can also be caused by errors in DNA replication, resulting in deletion where part of a chromosome is lost, duplication where a section of chromosome is repeated or occurs twice, etc. Finally, mutations can be caused by errors in cell division, leading to inversion where a broken part of a chromosome joins back but in the wrong way, translocation where part of a chromosome breaks off and re-joins to the wrong chromosome, etc.
Define
| Descriptor | Marks |
|---|---|
Defines a mutation as a change in a gene or a chromosome (leading to new characteristics in an organism or their offspring) | 1 |
Types of mutations
| Descriptor | Marks |
|---|---|
1 mark for each valid point (up to 3 marks):
| 3 |
1 mark for each valid point (up to 3 marks):
| 3 |
Causes - Mutagens
| Descriptor | Marks |
|---|---|
States mutations are caused by mutagens/mutagenic agents | 1 |
Defines mutagens as substances that are known to increase the rate at which mutations occur (e.g. ionising radiation, mustard gas, formaldehyde, some antibiotics) | 1 |
Causes - DNA replication
| Descriptor | Marks |
|---|---|
States mutations are caused by errors in DNA replication | 1 |
1 mark for each valid point (up to 2 marks): Deletion: part of a chromosome is lost; Duplication: section of chromosome is repeated/occurs twice; Insertions: DNA can be duplicated/extra DNA added; Frameshifts: incomplete DNA code/unreadable DNA code | 2 |
Causes - Cell division
| Descriptor | Marks |
|---|---|
States mutations are caused by errors in cell division/meiosis | 1 |
1 mark for each valid point (up to 2 marks): Inversion: broken part of chromosome joins back but in the wrong way; Translocation: part of chromosome breaks off and re-joins to the wrong chromosome; Non-disjunction: during meiosis, a chromosome pair does not separate so one daughter cell has an extra chromosome and one daughter cell has one less (aneuploidy) | 2 |
Explain how the example of the evolution of the unique hominin jaw illustrates the importance of mutations to evolution.
Reveal Answer
Mutations introduce new alleles into a population. Hominin jaw mutations must have been an advantage to survival, as the organisms are better suited to bipedal locomotion. These favourable alleles are passed onto offspring, meaning the mutation is maintained in future generations.
| Descriptor | Marks |
|---|---|
Award 1 mark for each of the following points, up to a maximum of 5 marks:
| 5 |
The table identifies the condition associated with a variety of ploidy changes.
| Chromosome number ploidy | Condition name |
|---|---|
| Monosomy 5 | Cri du chat syndrome |
| Trisomy 21 | Down syndrome |
| Trisomy 23 | Klinefelter syndrome |
| Monosomy 23 | Turner syndrome |
For a person with XXY sex chromosomes, which condition would they have?
Cri du chat syndrome
Down syndrome
Klinefelter syndrome
Turner syndrome
Reveal Answer
Cri du chat syndrome
Cri du chat syndrome is listed in the table as being caused by Monosomy 5, which involves chromosome 5 rather than the sex chromosomes.
Down syndrome
Down syndrome is identified in the table as Trisomy 21, which involves an extra copy of chromosome 21, not the sex chromosomes.
Klinefelter syndrome
The XXY genotype consists of three sex chromosomes (pair 23), representing a trisomy. The table identifies Trisomy 23 as Klinefelter syndrome.
Turner syndrome
Turner syndrome is listed as Monosomy 23, which implies a missing sex chromosome (genotype X0), whereas XXY indicates an extra chromosome.
In blood group inheritance in humans, three alleles (i, and ) determine blood type, which can be type O, A, B or AB.
It is known that:
- allele produces type A and is co-dominant with allele , which produces type B
- allele i produces type O
- alleles and are dominant over allele i.
A father who is heterozygous type A and a mother who is heterozygous type B have children. Predict the likely frequency of phenotypes for their offspring. Show your working.
Reveal Answer
The genotypes of the parents are and
Potential crosses
¼ type AB, ¼ type A, ¼ type B, ¼ type O
| Descriptor | Marks |
|---|---|
Identifies parents as and | 1 |
Shows appropriate working | 1 |
States a consequentially correct frequency for each blood type | 1 |
What can introduce new alleles into a population?
point mutations
non-disjunction
random fertilisation
independent assortment
Reveal Answer
point mutations
Point mutations involve a change in the DNA sequence of a gene; this alteration creates a new variant of the gene, which is the definition of a new allele.
non-disjunction
Non-disjunction is an error during cell division where chromosomes fail to separate, leading to an abnormal number of chromosomes (aneuploidy) rather than creating new alleles.
random fertilisation
Random fertilisation combines existing alleles from two parents to create unique genotypes in offspring, but it does not generate new genetic variants.
independent assortment
Independent assortment shuffles existing chromosomes into new combinations during meiosis, increasing genetic diversity without creating new alleles.
Outline how ONE type of electromagnetic radiation can cause a germline mutation.
Reveal Answer
X-rays are a type of electromagnetic radiation that damages the structure of DNA. If this damage occurs to the DNA in gametes (sex cells) then it is a germline mutation.
| Descriptor | Marks |
|---|---|
Outlines how a named type of electromagnetic radiation can cause a DNA mutation | 2 |
Provides some relevant information | 1 |
None of the above | 0 |
Populations with reduced genetic diversity face an increased risk of extinction because they
have fewer chromosomes.
have difficulty finding mates for reproduction.
are less likely to adapt to changing environments.
contain a larger proportion of heterozygous individuals.
Reveal Answer
have fewer chromosomes.
Genetic diversity refers to the variation of alleles within a population, not the number of chromosomes, which is generally a fixed characteristic of a species.
have difficulty finding mates for reproduction.
Difficulty finding mates is typically a consequence of low population density, whereas low genetic diversity primarily affects the genetic health and adaptability of the existing population.
are less likely to adapt to changing environments.
Genetic variation provides the raw material for natural selection; without it, a population lacks the traits necessary to survive and reproduce when faced with new diseases or environmental changes.
contain a larger proportion of heterozygous individuals.
Populations with reduced genetic diversity typically exhibit increased homozygosity due to inbreeding or genetic drift, rather than a larger proportion of heterozygous individuals.
Scientists have discovered a gene mutation that slows metabolism of glucose, giving people with the mutation a lower risk of developing diabetes.
Which statement best describes the effect of this new, favourable allele in a population?
Because most populations are resistant to change, the allele is unlikely to be passed on and it will eventually be lost from the population.
Since natural selection favours phenotypes that are better able to survive and reproduce, the new allele will increase in frequency over time.
As the new allele provides a survival advantage, it will take over in the population and any unfavourable alleles will be completely eliminated.
Since it produces a beneficial phenotype, every individual in the population will carry the allele within a few generations.
Reveal Answer
Because most populations are resistant to change, the allele is unlikely to be passed on and it will eventually be lost from the population.
Natural selection acts on advantageous traits, meaning a favorable allele is more likely to be passed on and increase in frequency rather than being lost.
Since natural selection favours phenotypes that are better able to survive and reproduce, the new allele will increase in frequency over time.
Natural selection drives the increase in frequency of beneficial alleles because individuals with these traits are more likely to survive and pass the allele to their offspring.
As the new allele provides a survival advantage, it will take over in the population and any unfavourable alleles will be completely eliminated.
While the favorable allele will increase in frequency, unfavorable alleles are rarely completely eliminated because they can remain hidden in heterozygous individuals.
Since it produces a beneficial phenotype, every individual in the population will carry the allele within a few generations.
Although the allele is beneficial, evolutionary changes in allele frequencies across an entire population typically occur over many generations, not just a few.
Which statement is true for DNA replication?
Adenine pairs with guanine.
The process occurs during metaphase I.
DNA polymerase unwinds the double helix.
New strands are synthesised in the 5' to 3' direction.
Reveal Answer
Adenine pairs with guanine.
According to complementary base-pairing rules, adenine pairs with thymine, while guanine pairs with cytosine.
The process occurs during metaphase I.
DNA replication occurs during the S phase of interphase, prior to the onset of cell division stages like metaphase.
DNA polymerase unwinds the double helix.
The enzyme helicase is responsible for unwinding the DNA double helix, whereas DNA polymerase synthesizes the new DNA strands.
New strands are synthesised in the 5' to 3' direction.
DNA polymerase can only add nucleotides to the free 3' hydroxyl group of a growing chain, meaning synthesis always proceeds in the direction.
Describe what is meant by the genotype of an organism.
Reveal Answer
Genotype refers to the combination of alleles that an organism has.
| Descriptor | Marks |
|---|---|
States combination of alleles | 1 |
Inherited mutations can affect the genotype of offspring. Describe how this occurs.
Reveal Answer
The parent of the offspring would have a mutation that is inheritable (in sex organs) and creates a new allele (variation of a gene).
The genotype of an offspring may then be affected as it would contain new alleles if the mutated gene from an egg or sperm is present in the zygote at fertilisation.
This will then affect the genotype of the offspring.
| Descriptor | Marks |
|---|---|
States a description of the mutation being a new allele of a gene | 1 |
States genotype contains new alleles if/when inherited | 1 |
What is the most likely outcome of a homeobox (HOX) gene mutation?
slower growth rate
body appendages in the incorrect location
failure of sex characteristics to fully develop
impaired ability of red blood cells to carry oxygen
Reveal Answer
slower growth rate
HOX genes control pattern formation and segment identity rather than the general rate of cell division or organismal growth.
body appendages in the incorrect location
HOX genes determine the identity of embryonic body segments along the anterior-posterior axis; mutations often cause homeotic transformations where body parts (like legs or antennae) develop in the wrong locations.
failure of sex characteristics to fully develop
The development of sex characteristics is primarily driven by sex chromosomes and hormonal signaling, not the homeobox genes responsible for the basic body plan.
impaired ability of red blood cells to carry oxygen
Impaired oxygen transport is typically caused by mutations in hemoglobin genes (e.g., sickle cell anemia), not by the master regulatory genes that define body structure.