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Alzheimer's disease (AD) is a non-reversible brain disorder that develops over a number of years - AQA - A-Level Biology - Question 10 - 2017 - Paper 1
Question 10
Alzheimer's disease (AD) is a non-reversible brain disorder that develops over a number of years. At the start of 2014 the number of Americans with AD was estimated ... show full transcript
Worked Solution & Example Answer:Alzheimer's disease (AD) is a non-reversible brain disorder that develops over a number of years - AQA - A-Level Biology - Question 10 - 2017 - Paper 1
Step 1
Assuming no one with AD died in 2014, calculate the annual percentage increase in AD cases in America for 2014 (lines 2–4).
Answer
To find the annual percentage increase in AD cases, we use the following formula:
ext{Percentage Increase} = rac{{ ext{New Value} - ext{Old Value}}}{{ ext{Old Value}}} imes 100In 2013, the number of Americans with AD was 5.4 million. Assuming this figure increases to 5.4 million + 0.5 million = 5.9 million in 2014, we substitute:
ext{Percentage Increase} = rac{{5.9 - 5.4}}{{5.4}} imes 100 = 9.26 ext{%} ext{ (approx.)}Step 2
Explain how donepezil could improve communication between nerve cells (lines 7–9).
Answer
Donepezil improves communication between nerve cells by inhibiting the enzyme acetylcholinesterase, which breaks down acetylcholine. With a reduced breakdown of acetylcholine, there is increased availability of this neurotransmitter in the synaptic cleft. This boost enables better signaling between neurons, thus alleviating some symptoms associated with Alzheimer’s disease.
Step 3
Suggest and explain two reasons why there is a high frequency of the E280A mutation in Yaramul (lines 13–15).
Answer
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Isolated Population: Yaramul has a historically isolated population where genetic drift can lead to a higher frequency of certain alleles, such as E280A. Isolation reduces gene flow from other populations, permitting this mutation to become more prevalent.
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Founder Effect: The E280A mutation may have originated from a common ancestor and persisted over generations. Since the mutation was likely present in the founding population of Yaramul, its frequency increased due to limited interbreeding with outsiders.
Step 4
Explain why natural selection has not reduced the frequency of the E280A mutation in the population (lines 16–17).
Answer
Natural selection has not reduced the frequency of the E280A mutation because this mutation does not necessarily lead to reduced survival or reproductive success until later in life. Since the symptoms of Alzheimer’s often manifest after reproductive age, individuals carrying the mutation may pass it on to offspring before its effects become apparent.
Step 5
Suggest and explain one reason for this variation (lines 11–12).
Answer
Variation in the age of onset for the E280A mutation could be attributed to environmental factors or lifestyle choices. External influences such as diet, stress, and exposure to toxins can interact with genetic predispositions, leading to differences in when symptoms appear.
Step 6
Suggest explanations for the figures the scientists recorded (lines 19–21).
Answer
The discrepancy between the number of E280A mutations detected (75) and the potential AD cases observed (74) may be due to several factors:
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Penetrance: Not all individuals with the E280A mutation will develop Alzheimer's, indicating varying levels of penetrance.
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Detection Methods: Some individuals might have the mutation but not show symptoms yet, resulting in lower observed AD cases compared to the number of mutations.
Step 7
Suggest why a DNA probe for the mutated triplet was not considered a suitable method for the detection of the E280A mutation (lines 22–23).
Answer
A DNA probe for the mutated triplet may not be suitable because it could lead to false positives. The triplet may be present in other contexts in the genome, resulting in non-specific binding. This means the probe might bind to similar sequences, leading to inaccuracies in mutation detection.