Skip to content
GradeHunt

Mark Scheme Docs

Genetics, Populations and Ecosystems

Inheritance and Genetic Crosses

10
Genotype definition:

The genetic constitution of an organism and the combination of alleles it possesses.

Sources

Not from a mark scheme, but useful content to know.

Phenotype definition: (2 points)
  1. The expressed characteristics of an organism due to the combination of its genotype
  2. And its interactions with the environment.
Sources

2025 A-Level Paper 2 Q5.1 (2 marks)

Q:

MS:

2017 A-Level Paper 2 Q7.1 (2 marks)

Q:

MS:

June 2013 A-Level Unit 4 Q3(a) (2 marks)

Q:

MS:

2016 A-Level Unit 4 Q3(a) (2 marks)

Q:

MS:

For questions where they ask you to explain the genotype ratios of the offspring of two parents when autosomal linkage is involved (so two of the genotypes are much more common than the other two), what structure should your answers follow? (4 points)
  1. State the combinations of the parents and state which alleles are autosomally linked to each other. E.g. parent 1 has genotype XxYy and parent 2 has genotype xxyy. X and Y are autosomally linked and x and y are linked.
  2. State the gametes that form as a result. E.g. Parent 1 therefore produces mainly XY and xy gametes as a result.
  3. State that any unlikely gametes only form as a result of crossing over. E.g. Xy and xY gametes form in low proportions and only form due to crossing over.
  4. State that this results in a lower number of the rare genotypes. E.g. this means there are fewer Xxyy and xxYy individuals, explaining the lower number of genotypes ‘A’ and ‘B’.Note: example in ‘Sources’.
Sources

Specimen A-Level Paper 2 Q3.2 (4 marks)

Q:

MS:

8 reasons observed phenotypic ratios are not the same as expected ratios:
  1. Small sample size.
  2. Crossing over.
  3. Random fertilisation.
  4. Epigenetics.
  5. Lethal alleles.
  6. Epistasis.
  7. Sex-linkage.
  8. Autosomal linkageNote: points 6-8 assume these were not factored into the expected ratios.
Sources

2025 A-Level Paper 2 Q5.2 (2 marks)

Q:

MS:

2018 A-Level Paper 2 Q6.1 (2 marks)

Q:

MS:

If a question asks you to calculate the probability of a ‘son’ or ‘daughter’ inheriting a condition, what must you remember?

You have to remember to halve the normal probability to account for the 50/50 chance of the offspring being the right gender.

Sources

2025 A-Level Paper 2 Q4.3 (1 marks)

Q:

MS:

2022 A-Level Paper 2 Q6.3 (2 marks)

Q:

MS:

Why are boys are less likely to inherit a certain recessive condition found on the X chromosome compared to girls: (2 points)
  1. Males only have one allele for the condition, meaning they only need to inherit one recessive allele to inherit the condition.
  2. Females need two recessive alleles/ need to be homozygous recessive to inherit the condition.
Sources

2021 A-Level Paper 2 Q4.1 (2 marks)

Q:

MS:

2018 A-Level Paper 2 Q10.3 (2 marks)

Q:

MS:

Autosomal linkage definition:

When two different genes are on the same autosome.Note: you could also say ‘alleles of different genes’ but not 'alleles' unqualified - it’s important you show that you understand the genes are different.Note: an autosome is a non-sex chromosome.

Sources

2025 A-Level Paper 2 Q5.3 (3 marks)

Q:

MS:

What are the 2 reasons there would be a high frequency of a harmful allele present in a given population?
  1. The population is isolated, leading to inbreeding and a small gene pool.
  2. The allele is inherited from a common ancestor.
Sources

2017 A-Level Paper 2 Q10.3 (2 marks)

Q:

MS:

Why is a member of a small, isolated population descended from 20 people, one of which had an allele for blindness, more likely to be blind than someone in the USA? (4 points)
  1. A genetic bottleneck occurred on the island.
  2. This means the population has a smaller gene pool and less genetic diversity.
  3. Additionally, the individuals on the island breed within the group.
  4. This means there is a higher chance of inheriting the blindness allele.
Sources

2015 As Unit 2 Q7(b)(i) (3 marks)

Q:

MS:

How do you determine whether an organism showing the dominant phenotype is homozygous or heterozygous? (2 points)
  1. Cross with a homozygous recessive member of the species.
  2. The presence of one or more offspring with the recessive phenotype indicates that the organism being tested is heterozygous. The absence of any offspring with the recessive phenotype (assuming a large sample size) suggests the organism being tested is homozygous.
Sources

2024 A-Level Paper 2 Q04.1 (2 marks)

Q:

MS:

Populations and Hardy–Weinberg

3
What is the Hardy-Weinburg principle?

Allele and genotype frequencies in a population will remain constant from generation to generation in the absence of evolutionary influences.

Sources

June 2010 A-Level Unit 4 Q3(a) (3 marks)

Q:

MS:

4 assumptions the Hardy-Weinburg principle makes:
  1. There is no selection advantage for certain alleles.
  2. There is a very low rate of mutation in alleles.
  3. There is no migration in or out of the population.
  4. Random mating occurs.
Sources

2022 A-Level Paper 2 Q6.6 (2 marks)

Q:

MS:

4 reasons the Hardy-Weinburg equation may not be able to be used to find the rate of an inherited condition:
  1. The condition isn’t caused by one gene.
  2. Environmental factors are involved.
  3. Mutations occur.
  4. The populations migrateNote: ‘random mating doesn’t occur’ could be a valid answer, but only say it if you are sure it is the case, as one mark scheme it was ignored, presumably because it wasn’t certain that it wasn’t occurring.
Sources

2025 A-Level Paper 3 Q5.1 (2 marks)

Q:

MS:

Evolution and Speciation

11
Evolution definition:

The change in the allele frequencies in a population over time.Note: It can be caused by selection pressures (natural selection) or random processes (genetic drift)

Sources

Not from a mark scheme, but useful content to know.

Gene pool definition:

All of the alleles in a population.

Sources

2017 A-level Paper 2 Q9.1 (1 mark)

Q:

MS:

6 factors that can cause phenotypic diversity in a population:
  1. Mutations.
  2. Different habitats and environment.
  3. Epigenetics.
  4. Crossing over.
  5. Independent segregation.
  6. Random fertilisationNote: basically the stuff that causes genetic diversity, which makes sense.
Sources

2024 A-Level Paper 2 Q10.1 (2 marks)

Q:

MS:

What makes selection disruptive?

Selection for both extremes.

Sources

2024 A-Level Paper 2 Q10.3 (5 marks)

Q:

MS:

Why does the use of Penicillin produce a population of bacteria that are resistant to it? (3 points)
  1. Penicillin creates a selection pressure by killing non-resistant bacteria.
  2. Resistant bacteria are more likely to survive and reproduce to pass on the resistance allele.
  3. Frequency of the allele increases over many generations.
Sources

2025 As Paper 1 Q9.2 (3 marks)

Q:

MS:

2 reasons a mutation that causes resistance to a disease doesn't tend to spread across species:
  1. Different species do not interbreed (to produce fertile offspring), meaning the mutated allele cannot be passed from one species to another.
  2. Mutations are random and only the rate of mutation is affected by the environment, meaning one species getting a certain mutation doesn’t necessarily make other species in the same environment more likely to get that same mutation.
Sources

Not from a mark scheme, but useful content to know.

What is the main effect of increasing population size on survival for that species? (2 points)
  1. An increase in intraspecific competition.
  2. For mates/ food/ territory.Note: state the correct example based on the question.
Sources

2017 A-Level Paper 2 Q2.1 (2 marks)

Q:

MS:

2012 A-Level Unit 6 EMPA Q15(b) (2 marks)

Q:

MS:

Description of how allopatric speciation occurs: (6 points)
  1. The two populations are geographically isolated.
  2. This means allopatric speciation occurs due to this isolation.
  3. This is because the two populations experience different selection pressures and environment.
  4. This means they are reproductively isolated and have separate gene pools.
  5. This means when mutations cause a change in allele frequency over time in the two populations.
  6. Different species that can no longer reproduce to produce fertile offspring are produced.
Sources

2022 A-Level Paper 2 Q5.2 (5 marks)

Q:

MS:

Why do two populations being on two different islands cause them to develop different fur colours? (Example of explaining allopatric speciation in a specific scenario) (5 points)
  1. The gene pools are separate due to geographical isolation, meaning no inbreeding occurs between the populations.
  2. A mutation of an allele that causes a different fur colour occurs in one of the populations.
  3. This mutation causes a selection advantage for the animals with it in one of the populations, because they have different selection pressures.
  4. The animals with the mutation are therefore more likely to breed and pass on the allele to their offspring.
  5. This causes an increase in the frequency of this allele within only one of the populations over many generations.Note: this is more an example that incorporates both allopatric speciation (mentioning the key words ‘geographical isolation’, ‘no inbreeding between populations’ etc.) and natural selection. It’s a reminder to include all the key phrases possible.
Sources

2021 A-Level Paper 2 Q10.1 (4 marks)

Q:

MS:

Description of how sympatric speciation occurs: (6 points)
  1. Occurs in the same habitat.
  2. The two populations are reproductively isolated.
  3. Variation occurs due to a mutation of an allele.
  4. The mutation provides a selection advantage to the population it occurs in due to different selection pressures, causing an increase in the frequency of the mutated allele across many generations.
  5. A certain type of natural selection occurs (name which type if it’s a specific example)
  6. Eventually the different species cannot interbreed to produce fertile offspring.
Sources

2017 A-Level Paper 2 Q9.2 (5 marks)

Q:

MS:

Explain how stabilising selection affects a characteristic: (3 points)
  1. Organisms with the extremes of the characteristic are least likely to survive and pass on their alleles.
  2. This means the alleles that cause the extremes of the characteristic decrease in frequency in the population over many generations.
  3. This means the frequency of organisms with the extreme characteristic decreases.
Sources

2019 A-Level Paper 1 Q04.1 (3 marks)

Q:

MS:

Populations in Ecosystems

12
Population definition: (2 points)
  1. A group of organisms of the same species in the same place at a given time.
  2. That can potentially interbreed with each other.
Sources

2024 A-Level Paper 3 Q6.1 (2 marks)

Q:

MS:

Ecosystem definition:

A community and its interaction with the non-living aspects of its environment.

Sources

Not from a mark scheme, but useful content to know.

How to conduct the capture re-capture experiment: (4 points)
  1. Capture a sample, mark, then release.
  2. Ensure the marking method doesn’t affect the chances of survival of the animal.
  3. Allow time for the animal to redistribute evenly before collecting a second sample.
  4. Population = (number in first sample × number in second sample) / number of marked animals in second sample.
Sources

2021 A-Level Paper 2 Q10.4 (4 marks)

Q:

MS:

2015 A-Level Unit 4 Q8(b) (4 marks)

Q:

MS:

2017 A-Level Paper 2 Q2.2 (3 marks)

Q:

MS:

January 2012 A-Level Unit 4 Q1(b)(i) (3 marks)

Q:

MS:

3 precautions to take with the actual marking in the capture-recapture method:
  1. Make sure it isn’t toxic so it doesn’t affect survival.
  2. Make sure it isn’t visible to predators.
  3. Make sure it doesn’t rub off so recaptured animals can be identified.
Sources

2024 A-Level Paper 2 Q10.5 (3 marks)

Q:

MS:

2 reasons for conducting the re-capture after a short delay:
  1. To allow the animals to evenly distribute.
  2. To ensure minimal population changes.Note: you can’t wait too long as population members will die/ be born.
Sources

2025 A-Level Paper 3 Q3.2 (3 marks)

Q:

MS:

5 assumptions made when using the mark-release-recapture method:
  1. No migration has occurred.
  2. No losses to predation.
  3. Marking does not affect survival.
  4. Birth rate and death rate are equal.
  5. All of the organisms belong to one population.
Sources

Specimen A-Level Paper 3 Q01.2 (2 marks)

Q:

MS:

Description of how (primary) succession occurs: (6 points)
  1. A pioneer species.
  2. Causes a change in the environment by releasing nutrients into the soil when it decomposes.
  3. This enables other species to colonise and survive in the habitat by outcompeting the previous species.
  4. This causes a change in biodiversity.
  5. Stability increases and the environment becomes less hostile.
  6. A climax community formsNote: this is the exact same for secondary - just in secondary, succession had already occurred in the environment then ‘reset’ by a natural disaster. This means there may already be some nutrients in the soil.
Sources

January 2011 A-Level Unit 4 Q8(a) (5 marks)

Q:

MS:

2019 A-Level Paper 2 Q1.1 (4 marks)

Q:

MS:

2025 A-Level Paper 2 Q10.4 (4 marks)

Q:

MS:

Why is the climax community the most stable? (4 points)
  1. More species are present.
  2. This means more complex food webs form.
  3. This means a change in one species will have little effect on the others.
  4. As there are alternative food sources.
Sources

Not from a mark scheme, but useful content to know.

How does a rising index of diversity within a community provide evidence of succession? (3 points)
  1. It shows the community is becoming less similar.
  2. Showing biodiversity is increasing.Note: the amount of biodiversity changing IS succession.
  3. Due to changes in the number of species present and changes in the biotic and abiotic factors in the environment.
Sources

2024 A-Level Paper 3 Q6.5 (2 marks)

Q:

MS:

Why is there a high biodiversity of animal species in a tropical rainforest? (3 marks)
  1. There’s a large number/ variety of plant species.
  2. This means there are more different habitats/ niches.
  3. The next point is that a greater variety of food sources.
Sources

2025 A-Level Paper 2 Q10.1 (3 marks)

Q:

MS:

4 abiotic factors that allow a fast rate of nutrient recycling and how they do it:
  1. High temperature allows for optimum enzyme action.
  2. High moisture allows for hydrolysis.
  3. Optimum pH allows for optimum enzyme action.
  4. High oxygen content of soil allows for aerobic respiration in saprobionts.
Sources

2025 A-Level Paper 2 Q10.2 (2 marks)

Q:

MS:

3 reasons + explanations for why the rate of a certain genetic disease could be higher in a small, isolated population than globally:
  1. Genetic drift causes the frequency to be higher due to chance.
  2. Only inbreeding within the population (no interbreeding with other populations) causes a small gene pool so a higher chance of inheriting the allele.
  3. The disease is inherited from a common ancestor, causing a high frequency of the allele in the small population.
Sources

2024 A-Level Paper 3 Q5.3 (2 marks)

Q:

MS:

RP 12 – Species Distribution with Quadrats

1
How to estimate the number of dandelions in a field: (5 points)
  1. Create a grid of the field.
  2. Obtain random coordinates using a random number generator.
  3. Count the number of dandelions in each coordinate using a 1m2 quadrat.
  4. Have a large sample of coordinates (10+) and calculate the mean number of dandelions per quadrat (By dividing the sum of the dandelions counted by the number of squares they were counted in).
  5. Multiply this mean by the number of squares on the grid.
Sources

2024 A-Level Paper 2 Q5.1 (5 marks)

Q:

MS:

2019 A-Level Paper 2 Q9.1 (5 marks)

Q:

MS:

2014 A-Level Unit 4 Q8(c) (5 marks)

Q:

MS:

Statistics

7
Why are statistical tests done? (3 points)
  1. They determine the probability of the results being due to chance.
  2. This means the null hypothesis can be accepted or rejected.
  3. This allows scientists to determine whether the results are significant.
Sources

January 2010 A-Level Unit 4 Q7(e) (2 marks)

Q:

MS:

2 requirements for a Chi-squared test to be used:
  1. You are determining whether there is a difference between the observed and expected outcome.
  2. The data is categoric.
Sources

Specimen A-Level Paper 2 Q3.4 (2 marks)

Q:

MS:

2 requirements for a T-test to be used:
  1. You are comparing the means of two groups.
  2. The data is normally distributed.
Sources

2019 As Paper 2 Q8.4 (2 marks)

Q:

MS:

What do larger standard deviations show?

More variation in a data set.

Note: you could possibly comment on this in an evaluation question, although I haven’t seen it in any mark schemes.

Sources

2014 A-Level Unit 6 ISA-P Q14(b) (1 marks)

Q:

MS:

How does degrees of freedom work with the Chi-squared test?

It is the number of categories - 1. For example: if you are looking to see if the frequency of 4 phenotypes differed significantly from expected, you would use a degree of freedom of 3.

Sources

2022 A-Level Paper 2 Q6.5 (2 marks)

Q:

MS:

How does degrees of freedom work for the t-test?

It is the sum of the sample size for the two means - 2. For example: if sample 1 had 5 data points and sample 2 had 6 data points, you’d use 9 degrees of freedom.

Sources

Not from a mark scheme, but useful content to know.

What does a value of P > 0.5 tell you about the difference between two means? (3 points).
  1. The difference is not significant.
  2. There is greater than a 0.5 probability that the difference is due to chance.Note: They reject ‘results’ for ‘difference’. Always say ‘difference’. If the question talks about ‘increase/ decrease’, always say this.
  3. Therefore accept the null hypothesis.Note: always say whether you accept or reject the null hypothesis for these types of questions.
Sources

2022 A-Level Paper 3 Q5.2 (2 marks)

Q:

MS: