📚 Year 12 SQA Biology: Case Study Practical Drill | 案例分析实战演练
A key skill in SQA Higher Biology is applying your knowledge to unfamiliar contexts. This practical drill walks you through a realistic case study on cystic fibrosis, integrating genetics, metabolism, data handling and ethical reasoning. Use it to sharpen your problem-solving approach and prepare for exam-style scenario questions.
SQA 高等生物的关键技能之一是将知识应用于陌生的情境。本实战演练带你完成一个关于囊性纤维化的真实案例分析，融合遗传学、代谢、数据处理和伦理推理。用它来磨练你的问题解决思路，为考试中的情景题做好准备。
1. Introduction to the Case Study | 案例简介
The case concerns Emily, a 14-year-old girl who has suffered from persistent chest infections and poor weight gain since infancy. Her sweat tastes unusually salty, and a preliminary diagnosis of cystic fibrosis (CF) has been made. You are asked to review her genetic, biochemical and clinical data.
案例对象是艾米丽，一名 14 岁女孩，自婴儿期起就持续出现肺部感染和体重增长不良。她的汗液味道异常咸，初步诊断为囊性纤维化 (CF)。你需要审阅她的遗传、生化及临床数据。
CF is an autosomal recessive disorder most common in populations of Northern European descent, with a carrier frequency of about 1 in 25. It is caused by mutations in the CFTR gene, which codes for a chloride ion channel protein.
CF 是一种常染色体隐性遗传病，在北欧后裔人群中最为常见，携带者频率约为 1/25。该病由 CFTR 基因突变引起，该基因编码一种氯离子通道蛋白。
2. Genetic Inheritance and Pedigree Analysis | 遗传方式与系谱分析
Emily’s family pedigree shows that her older brother also has CF, while both parents are phenotypically healthy. There is no previous family history of the condition. This pattern is consistent with autosomal recessive inheritance, where both parents are obligate carriers.
艾米丽的家系谱显示她的哥哥也患有 CF，而父母表型均健康。家族中无既往病史。这一模式符合常染色体隐性遗传，父母均为必然携带者。
Using a Punnett square, each pregnancy of two carriers (Cc x Cc) has a 25% chance of producing an affected child (cc), a 50% chance of a carrier (Cc), and a 25% chance of a homozygous unaffected child (CC).
使用庞纳特方格，两个携带者 (Cc x Cc) 的每次妊娠有 25% 的概率生出患病孩子 (cc)、50% 的概率生出携带者 (Cc)、25% 的概率生出纯合正常孩子 (CC)。
3. Molecular Basis: CFTR Gene and Protein | 分子基础：CFTR 基因与蛋白质
Genetic testing reveals that Emily is homozygous for the ΔF508 mutation, a deletion of three nucleotides that removes phenylalanine at position 508 of the CFTR polypeptide. This is the most common CF-causing mutation, accounting for around 70% of CF alleles in the UK.
基因检测显示艾米丽是 ΔF508 突变的纯合子，该突变缺失三个核苷酸，导致 CFTR 多肽链第 508 位的苯丙氨酸被移除。这是最常见的 CF 致病突变，约占英国 CF 等位基因的 70%。
The ΔF508 protein fails to fold correctly in the endoplasmic reticulum and is degraded, so very little functional CFTR reaches the cell membrane. Without the chloride channel, epithelial cells cannot transport Cl⁻ ions properly.
ΔF508 蛋白在内质网中无法正确折叠并被降解，因此极少量有功能的 CFTR 能到达细胞膜。缺乏氯离子通道，上皮细胞无法正常转运 Cl⁻ 离子。
4. Metabolic Consequences of CFTR Dysfunction | CFTR 功能障碍的代谢后果
In the lungs, defective Cl⁻ transport reduces the outward movement of chloride and water, causing the mucus layer to become thick and dehydrated. This traps bacteria, leading to recurrent infections and inflammation, which progressively damages lung tissue.
在肺部，Cl⁻ 转运缺陷减少了氯离子和水分的向外移动，导致黏液层变得粘稠脱水。这会困住细菌，引发反复感染和炎症，逐渐损伤肺组织。
In the pancreas, blocked ducts prevent digestive enzymes from reaching the small intestine. This impairs the breakdown of lipids, proteins and carbohydrates, resulting in malnutrition and steatorrhoea (fatty stools). Emily’s low body mass index reflects this malabsorption.
在胰腺中，堵塞的导管阻止消化酶进入小肠，这影响了脂质、蛋白质和碳水化合物的分解，导致营养不良和脂肪泻。艾米丽较低的体重指数正反映了这种吸收不良。
Sweat glands show a different effect: CFTR normally reabsorbs Cl⁻ from the sweat duct back into cells, with Na⁺ following. In CF, Cl⁻ remains in the duct, producing the salty sweat that first raised clinical suspicion.
汗腺则表现出不同的效应：正常 CFTR 会将汗液导管中的 Cl⁻ 重吸收回细胞，Na⁺ 随之重吸收。在 CF 中，Cl⁻ 留在导管内，产生了引起最初临床怀疑的咸汗。
5. Interpreting Clinical Data | 解读临床数据
Emily’s sweat chloride test results are shown below. The standard diagnostic threshold for CF is a sweat chloride concentration ≥ 60 mmol/L.
艾米丽的汗液氯离子测试结果如下。CF 的标准诊断阈值为汗液氯离子浓度 >= 60 mmol/L。
| Sample | Cl⁻ concentration (mmol/L) |
|---|---|
| 1 | 85 |
| 2 | 92 |
| 3 | 88 |
All three readings exceed 60 mmol/L, confirming the diagnosis of cystic fibrosis. The high concentration indicates that CFTR-mediated chloride reabsorption is severely impaired.
三次读数均超过 60 mmol/L，确认了囊性纤维化的诊断。高浓度表明 CFTR 介导的氯离子重吸收严重受损。
Her lung function data show a forced expiratory volume in 1 second (FEV₁) that is only 62% of the predicted value for her age, height and sex. This reflects significant obstructive lung disease from mucus plugging and chronic infection.
她的肺功能数据显示第一秒用力呼气容积 (FEV₁) 仅为同年龄、身高、性别预计值的 62%。这反映了因黏液堵塞和慢性感染导致的显著阻塞性肺病。
6. Designing Diagnostic Tests | 设计诊断测试
Current screening methods include the newborn blood spot test measuring immunoreactive trypsinogen (IRT). Elevated IRT suggests pancreatic duct obstruction, prompting DNA analysis for common CFTR mutations. A positive result is followed by sweat testing to confirm the diagnosis.
目前的筛查方法包括新生儿足跟血检测，测量免疫反应性胰蛋白酶原 (IRT)。IRT 升高提示胰管阻塞，随后进行 DNA 分析以检测常见的 CFTR 突变。阳性结果后需进行汗液测试确诊。
An alternative experimental design could compare sweat Cl⁻ and Na⁺ concentrations in CF patients and healthy controls. Care must be taken to standardise sweat collection using pilocarpine iontophoresis and to avoid contamination. The independent variable is the health status (CF vs. control), while the dependent variable is ion concentration.
一项替代实验设计可以比较 CF 患者与健康对照组的汗液 Cl⁻ 和 Na⁺ 浓度。需通过匹罗卡品离子导入法标准化汗液收集并避免污染。自变量为健康状况 (CF vs. 对照)，因变量为离子浓度。
7. Ethical and Social Implications | 伦理与社会影响
Preimplantation genetic diagnosis (PGD) can be offered to couples who are both carriers, allowing embryos without the condition to be selected. This raises ethical questions about embryo selection, the value of disabled lives, and access to reproductive technologies.
可为双方均为携带者的夫妇提供植入前遗传学诊断 (PGD)，从而选择不患该病的胚胎。这引发了关于胚胎选择、残障生命价值以及生殖技术可及性的伦理问题。
Genetic screening of newborns or even the wider population must balance benefits of early treatment against psychological harm, stigma, and privacy. Informed consent and genetic counselling are essential components of responsible testing programmes.
新生儿乃至更广泛人群的基因筛查必须在早期治疗的益处与心理伤害、污名化和隐私之间取得平衡。知情同意和遗传咨询是负责任的检测计划的重要组成部分。
8. Exam-Style Data Analysis Question | 考试风格数据分析题
Question: Scientists measured the mean sweat chloride concentration of healthy individuals and three groups of CF patients with different genotypes (homozygous ΔF508, compound heterozygous ΔF508/G551D, and homozygous G551D). The results are summarised in the table below. Use the data to answer parts (a)-(c).
问题： 科学家测量了健康个体和三种不同基因型 CF 患者（ΔF508 纯合、ΔF508/G551D 复合杂合、G551D 纯合）的平均汗液氯离子浓度。结果汇总于下表。使用数据回答问题 (a)-(c)。
| Group | Mean sweat Cl⁻ (mmol/L) | ± Standard deviation |
|---|---|---|
| Healthy controls | 18 | 5 |
| ΔF508/ΔF508 | 102 | 14 |
| ΔF508/G551D | 95 | 12 |
| G551D/G551D | 88 | 11 |
(a) Compare the sweat chloride concentration of healthy controls and ΔF508 homozygotes. (1 mark)
(b) The G551D mutation produces a CFTR protein that reaches the cell membrane but has reduced channel opening probability. Explain how these data support that molecular description. (2 marks)
(c) Calculate the percentage difference in mean sweat Cl⁻ between ΔF508 homozygotes and G551D homozygotes. (2 marks)
(a) 比较健康对照与 ΔF508 纯合子的汗液氯离子浓度。(1 分)
(b) G551D 突变产生的 CFTR 蛋白可到达细胞膜，但通道开放概率降低。解释这些数据如何支持该分子描述。(2 分)
(c) 计算 ΔF508 纯合子和 G551D 纯合子之间平均汗液 Cl⁻ 的百分比差异。(2 分)
9. Model Answer and Examiner Tips | 标准答案与考官提示
(a) The mean sweat chloride concentration in ΔF508 homozygotes (102 mmol/L) is far higher than in healthy controls (18 mmol/L), an increase of 84 mmol/L. This demonstrates severe impairment of chloride reabsorption in the CFTR protein. Tip: Always quote data values.
(a) ΔF508 纯合子的平均汗液氯离子浓度 (102 mmol/L) 远高于健康对照 (18 mmol/L)，增加了 84 mmol/L。这表明 CFTR 蛋白的氯离子重吸收严重受损。提示：务必引用数据值。
(b) G551D homozygotes have a high mean sweat Cl⁻ (88 mmol/L), confirming the channel is dysfunctional. However, the value is lower than that of ΔF508 homozygotes (102 mmol/L). This suggests that while some G551D CFTR reaches the membrane (hence lower sweat Cl⁻ than complete absence), its reduced opening probability still limits chloride transport compared with fully functional wild-type channels.
(b) G551D 纯合子的平均汗液 Cl⁻ 很高 (88 mmol/L)，确认通道功能失常。但该值低于 ΔF508 纯合子 (102 mmol/L)。这表明尽管一些 G551D CFTR 到达了细胞膜（因此汗液 Cl⁻ 低于完全缺失的情况），但其降低的开放概率仍然限制了氯离子转运，与完全功能的野生型通道相比仍有不足。
(c) Percentage difference = [(102 – 88) / 102] × 100 = (14/102) × 100 ≈ 13.7%. Always show your working to secure method marks even if the final answer slips. Tip: Use the larger value as denominator when the question does not specify which group is the baseline.
(c) 百分比差异 = [(102 – 88) / 102] × 100 = (14/102) × 100 ≈ 13.7%。始终展示计算过程，即使最终答案有误也能拿到方法分。提示：题目未指明基准组时，用较大值作分母。
10. Common Pitfalls and Key Takeaways | 常见错误与要点总结
Pitfall 1: Confusing genotype with phenotype. A person with two mutant alleles may still vary in disease severity due to modifier genes and environment. Do not assume that identical mutations always lead to identical symptoms.
常见错误 1： 混淆基因型与表型。带有两个突变等位基因的患者也可能因修饰基因和环境不同而表现出不同的疾病严重程度。不要假设相同突变总会导致相同症状。
Pitfall 2: Describing CF simply as a ‘lung disease’. It is a multi-system disorder affecting the respiratory, digestive and reproductive systems. In open-response questions, acknowledge at least two organ systems.
常见错误 2： 将 CF 简单描述为“肺部疾病”。它是一种累及呼吸、消化和生殖系统的多系统疾病。在开放题中，应至少提及两个器官系统。
Pitfall 3: Misusing statistical terms. Standard deviation bars that overlap may indicate no significant difference, but a formal statistical test is needed to conclude. Comment on overlap, but do not claim significance without proper tests.
常见错误 3： 滥用统计术语。标准差误差线重合可能表示无显著差异，但需要通过正式的统计检验才能得出结论。评论重合情况，但不要未经适当检验就声称有显著性。
Key takeaways: Link molecular mechanism to whole-body symptoms. Practice drawing Punnett squares and calculating probabilities from pedigree data. Explain data using your knowledge of transport across membranes. Evaluate screening programmes by weighing up benefits and ethical concerns.
要点总结： 将分子机制与全身症状联系起来。练习绘制庞纳特方格并根据系谱数据计算概率。运用跨膜运输的知识解释数据。通过权衡益处与伦理关切来评估筛查计划。
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