📚 KS3 CIE Genes Exam Question and Mark Scheme | KS3 CIE 基因考试题与评分标准
This revision guide explains how to answer a typical KS3 CIE Biology question on genes. It includes a sample exam question, a detailed mark scheme, and examiner tips to help you gain full marks.
本复习指南讲解如何回答典型的 KS3 CIE 生物基因考题。内容包括一道样题、详细评分标准和考官提示，帮助你取得满分。
1. What the Exam Expects | 考试要求
In the KS3 CIE Biology syllabus, the topic 'Genes' usually appears in short-answer questions or structured tasks. You need to recall key definitions, interpret genetic diagrams, and use Punnett squares to predict offspring ratios.
在 KS3 CIE 生物大纲中，基因专题通常以简答题或结构化任务出现。你需要记忆关键定义、解读遗传图解，并使用庞纳特方格预测后代比例。
Examiners award marks for correct biological terms, clear working, and a final answer that matches the question command word such as 'state', 'explain' or 'predict'.
考官根据正确的生物学术语、清晰的解题过程和符合题目指令词（如 "陈述" "解释" "预测"）的最终答案给分。
2. Key Vocabulary in Genetics | 遗传学核心词汇
Before attempting the exam question, revise the essential terms. A gene is a section of DNA that codes for a particular characteristic, such as eye colour or blood group.
在尝试考试题之前，先复习基本术语。基因是编码特定特征（如眼色或血型）的 DNA 片段。
An allele is a different version of the same gene. A dominant allele is shown with a capital letter, and a recessive allele is shown with the same lower-case letter.
等位基因是同一基因的不同形式。显性等位基因用大写字母表示，隐性等位基因用相同的小写字母表示。
Genotype describes the alleles an organism has, while phenotype describes the physical feature that results from the genotype.
基因型描述生物体拥有的等位基因，表现型描述由基因型产生的物理特征。
3. Chromosomes, DNA and Genes | 染色体、DNA 与基因
In the nucleus of most human cells there are 46 chromosomes arranged in 23 pairs. Each chromosome is made of a long molecule of DNA coiled around proteins.
在大多数人类细胞核中有 46 条染色体，成 23 对排列。每条染色体由围绕蛋白质盘绕的长 DNA 分子组成。
A gene is a short section of DNA located on a chromosome. Different genes are found at particular places on chromosomes, and one pair of chromosomes carries the same genes in the same order.
基因是位于染色体上的短 DNA 片段。不同基因位于染色体的特定位置，一对染色体以相同顺序携带相同基因。
4. Dominant and Recessive Alleles | 显性与隐性等位基因
If a person inherits one dominant allele and one recessive allele, the dominant allele usually determines the phenotype. The recessive allele is only expressed when two copies are present.
如果一个人继承一个显性等位基因和一个隐性等位基因，显性等位基因通常决定表现型。隐性等位基因只有在存在两个拷贝时才会表达。
For example, in pea plants the allele for tall stems (T) is dominant to the allele for short stems (t). A plant with genotype Tt will be tall because the dominant T masks the recessive t.
例如，在豌豆植物中，高茎等位基因 T 对矮茎等位基因 t 为显性。基因型为 Tt 的植物表现高茎，因为显性 T 掩盖了隐性 t。
5. Monohybrid Crosses and Punnett Squares | 单基因杂交与庞纳特方格
A monohybrid cross follows the inheritance of one characteristic controlled by one gene with two alleles. You can use a Punnett square to show the possible combinations of gametes from each parent.
单基因杂交追踪由一个基因和两个等位基因控制的单一特征的遗传。你可以使用庞纳特方格展示每个亲本配子的可能组合。
Start by writing the parental genotypes. Then write the gametes each parent can produce. Place one parent's gametes across the top and the other parent's gametes down the side of the square.
首先写出亲本基因型。然后写出每个亲本能产生的配子。将一个亲本的配子放在方格顶部，另一个亲本的配子放在方格侧边。
The inside boxes show the offspring genotypes. Count the boxes to find the expected phenotype ratio, for example 3 tall : 1 short.
方格内部表示后代基因型。计算方格数以找出预期的表现型比例，例如高茎 3：矮茎 1。
Expected phenotype ratio = 3 : 1
6. Sample Exam Question: Part A Knowledge | 样题 A 部分：知识
The sample question below is written in the style of a KS3 CIE structured question. Part A tests basic recall of definitions and chromosome facts.
下面的样题按照 KS3 CIE 结构化问题的风格编写。A 部分考查定义和染色体事实的基础记忆。
| Question A1 (English) Define the term gene. [1] | 问题 A1（中文） 定义术语基因。[1] |
| Question A2 (English) State the number of chromosomes in a normal human body cell. [1] | 问题 A2（中文） 说明正常人类体细胞中的染色体数目。[1] |
| Question A3 (English) Explain the difference between a dominant allele and a recessive allele. [2] | 问题 A3（中文） 解释显性等位基因与隐性等位基因的区别。[2] |
These questions are straightforward, but many students lose marks by giving a vague definition of a gene or by writing '23' instead of '46' as the total number of chromosomes.
这些题目很简单，但许多学生因为基因定义不准确，或者把染色体总数写成 23 而不是 46 而失分。
7. Sample Exam Question: Part B Application | 样题 B 部分：应用
Part B applies your knowledge to a genetic cross. You must complete a Punnett square and then interpret the result in a short written answer.
B 部分将你的知识应用于遗传杂交。你必须完成庞纳特方格，然后用简短的书面回答解释结果。
| Question B1 (English) In pea plants, the allele for purple flowers (P) is dominant to the allele for white flowers (p). A homozygous purple-flowered plant is crossed with a white-flowered plant. Complete the Punnett square to show the possible offspring genotypes. [3] | 问题 B1（中文） 在豌豆植物中，紫花等位基因 P 对白花等位基因 p 为显性。一株纯合紫花植物与一株白花植物杂交。完成庞纳特方格以展示可能的后代基因型。[3] |
| Question B2 (English) State the phenotype ratio of the offspring. [1] | 问题 B2（中文） 说明后代的表现型比例。[1] |
| Question B3 (English) Explain why all the offspring have purple flowers even though one parent was white. [2] | 问题 B3（中文） 解释为什么一个亲本为白花，但所有后代都是紫花。[2] |
Notice that B1 asks you to show the Punnett square, so your working is part of the answer. You must write the parental genotypes before drawing the square.
注意 B1 要求你展示庞纳特方格，因此解题过程就是答案的一部分。你必须在画方格之前写出亲本基因型。
8. Mark Scheme and Examiner Notes | 评分标准与考官点评
Below is a mark scheme in the style used by CIE examiners. It shows exactly where each mark is awarded.
下面是 CIE 考官使用的评分标准风格。它精确显示了每个分数在哪里给出。
| Question | Model Answer / Mark Scheme | Marks |
| A1 | A section of DNA that codes for a protein / determines a characteristic. | 1 |
| A2 | 46 (accept 23 pairs). | 1 |
| A3 | Dominant allele is expressed when only one copy is present (1); recessive allele is only expressed when two copies are present / no dominant allele present (1). | 2 |
| B1 | notypes: PP × pp. Gametes: P, P and p, p. Correct Punnett square showing all Pp offspring. | 3 |
| B2 | 100% purple / all purple (accept 4:0). | 1 |
| B3 | Each offspring inherits one dominant P allele from the purple parent and one recessive p allele from the white parent (1). The dominant P allele masks the recessive p allele, so the phenotype is purple (1). | 2 |
Examiner notes emphasise that the Punnett square mark in B1 is not given for a fully completed table alone. You must first show the parental genotypes and the gametes clearly.
考官点评强调，B1 中的庞纳特方格分数并不只给完成表格本身。你必须先清楚地写出亲本基因型和配子。
9. Common Mistakes to Avoid | 常见错误
Mistake 1: Writing a dominant allele with a lower-case letter. Always use a capital letter for a dominant allele, such as T for tall, not t.
错误一：显性等位基因用小写字母书写。显性等位基因必须用大写字母，例如高茎用 T，而不是 t。
Mistake 2: Confusing genotype with phenotype. Genotype is the pair of alleles such as Tt; phenotype is the visible feature such as tall.
错误二：混淆基因型与表现型。基因型是等位基因对如 Tt；表现型是可见特征如高茎。
Mistake 3: Writing the wrong gametes in a Punnett square. If a parent has genotype Tt, the gametes are T and t, never TT or Tt.
错误三：在庞纳特方格中写错配子。如果亲本基因型是 Tt，配子应为 T 和 t，绝不能是 TT 或 Tt。
Mistake 4: Giving an incomplete explanation in B3. You
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