AQA AS Level Biology Paper 2, November 2021: Question 6
9 marks · Medium difficulty · Extended Answer
Analyze the effects of alcohol dehydrogenase alleles ADF and ADS on fruit fly survival and allele frequency over generations in an alcohol-rich environment.
Practise this questionQuestion
Question text
06 The fruit fly is a species of small insect.
The fruit fly has a gene that codes for an enzyme called alcohol dehydrogenase (AD).
AD catalyses the breakdown of alcohol when alcohol is in the insects’ food.
The gene coding for AD has two alleles, ADF and ADS.
06.1 The enzyme encoded by the ADF allele catalyses the breakdown of alcohol faster
than the enzyme encoded by the ADS allele. Suggest why.
[3 marks]
A scientist took a random sample of adult fruit flies from a population. He measured
the frequency of the ADF allele in this sample (generation 0). He then:
• selected 100 of these insects at random and kept them in a container
• fed the insects food containing alcohol
• let the insects reproduce
• repeated these steps for 45 generations of fruit fly reproduction.
The scientist measured the frequency of the ADF allele in the 45th generation.
06.2 Suggest why the scientist took his sample from the population at random.
[1 mark]
Table 2 shows the scientist’s results.
Table 2
Generation of fruit fly F
reproduction Frequency of AD
*18* 0 0.20
45 0.74
06.3 Alcohol is toxic to fruit flies. Suggest and explain why the frequency of the ADF allele
changed during the 45 generations.
[4 marks]
06.4 Identify the type of selection investigated in the 45 generations of fruit fly reproduction.
Tick ( ) one box.
[1 mark]
No selection
Directional selection
Random selection
Stabilising selection
Mark scheme
Show the mark scheme
Question Marking Guidance Mark Comments
3. Accept converse for
06.1 1. Different primary structure/amino acid sequence; S
AD
2. Different tertiary structure/shape of active site; 3. Accept is more
3 complementary
3. Enzyme-substrate complexes more likely (with
F
enzyme from AD allele);
06.2 Avoids bias
OR 1
Results (likely to be) reliable/repeatable;
06.3 F Accept converse for
1. Flies with AD /allele have selective advantage S
(in presence of alcohol); AD
F
2. So insects (with AD more likely to) reproduce; 1. Accept description
4 of selective
3. Pass on ADF (allele/gene); advantage
4. (So) allele frequency increases;
06.4 Answer = Directional selection 1
TOTAL 9
How to answer it
Enzyme Structure and Natural Selection in Fruit Flies
What this question tests
This question assesses your understanding of protein structure (primary and tertiary), enzyme-substrate complexes, sampling methodology, natural selection mechanisms, and the identification of types of selection. You must link changes at the molecular level (alleles and proteins) to macro-level evolutionary shifts in populations.
Enzyme Structure and Function
Suggest why the enzyme encoded by the ADF allele catalyses the breakdown of alcohol faster than the enzyme encoded by the ADS allele.
✅ Correct Answer
- Different primary structure / amino acid sequence.
- Different tertiary structure / shape of the active site.
- Enzyme-substrate complexes are more likely to form (with the enzyme from the ADF allele).
💡 Key Knowledge
Different alleles contain different base sequences. This leads to a different sequence of amino acids (primary structure) during translation, altering hydrogen, ionic, and disulfide bonding to change the 3D folding (tertiary structure) of the protein.
🧠 Exam Technique
Always build your enzyme answers sequentially: Allele mutation → Amino acid change → Active site shape change → Frequency of Enzyme-Substrate (ES) complexes. Never skip intermediate steps!
❌ Common Errors
Students often lose marks by loosely stating the "enzyme shape changes" without specifying the active site, or incorrectly stating that the "active site is complementary" rather than explaining that ES complexes form more easily/frequently.
Sampling Methodology
Suggest why the scientist took his sample from the population at random.
✅ Correct Answer
- Avoids bias; OR
- Results are (likely to be) reliable / representative / repeatable.
💡 Key Knowledge
Random sampling ensures that every individual has an equal chance of being selected, which prevents human subjectivity from skewing baseline allele frequencies.
❌ Common Errors
Avoid vague terms like "to make it fair" or "to get accurate results." Examiners require precise statistical phrasing such as "avoids bias" or "ensures the sample is representative."
Mechanisms of Natural Selection
Alcohol is toxic to fruit flies. Suggest and explain why the frequency of the ADF allele changed during the 45 generations.
✅ Correct Answer
- Flies with ADF allele have a selective advantage (in the presence of alcohol);
- So insects (with ADF) are more likely to survive and reproduce;
- They pass on the ADF allele (gene) to their offspring;
- (So) allele frequency increases over generations.
🧠 Exam Technique
This is a classic 4-step natural selection narrative. Follow the template: Advantage → Differential survival/reproduction → Passing on advantageous alleles → Change in allele frequency in the gene pool.
❌ Common Errors
A major biological error is stating that "organisms adapt to their environment and pass acquired traits on." You must state that differential reproduction alters the frequency of pre-existing alleles in the population gene pool.
Types of Selection
Identify the type of selection investigated in the 45 generations of fruit fly reproduction. Tick (v) one box.
✅ Correct Answer
Directional selection (Tick the box next to "Directional selection").
💡 Key Knowledge
Directional selection occurs when an environmental change favours individuals at one extreme of a phenotypic range, causing allele frequencies to shift in a single direction over time (as seen with the rise of ADF from 0.20 to 0.74).
Topics
Biology · Practical skills · 3.4 Genetic information, variation and relationships between organisms · Experimental design
Question and mark scheme from the AQA AS Level Biology examination, Paper 2, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.