Genetics: Key Concepts and Exam Tips for IB & WJEC Science | 遗传学：IB 与 WJEC 科学考点精讲

📚 Genetics: Key Concepts and Exam Tips for IB & WJEC Science | 遗传学：IB 与 WJEC 科学考点精讲

Genetics forms the backbone of modern biology, explaining how traits are passed from one generation to the next and how variation arises. For students preparing for IB Biology or WJEC Science examinations, mastering the language of genes, alleles, chromosomes and DNA is essential. This revision guide breaks down the most important topics, connects them to both syllabi, and offers clear explanations alongside exam-focused insights to help you excel.

遗传学是现代生物学的支柱，它解释了性状如何代代相传以及变异如何产生。对于备战 IB 生物学或 WJEC 科学考试的学生来说，掌握基因、等位基因、染色体和 DNA 的语言至关重要。本复习指南将最重要的主题拆解，结合两个教学大纲的要求，提供清晰的解释和面向考试的深入见解，助你取得优异成绩。

1. Mendel’s Laws and Monohybrid Crosses | 孟德尔定律与单基因杂交

Gregor Mendel’s experiments with pea plants established the foundation of inheritance. He proposed that each trait is determined by a pair of factors (now called alleles), one inherited from each parent. The Law of Segregation states that allele pairs separate during gamete formation, and the Law of Independent Assortment says that genes for different traits are inherited independently of each other. In a monohybrid cross, we track one gene with two alleles. A cross between two heterozygous individuals (Aa × Aa) produces a genotypic ratio of 1 AA : 2 Aa : 1 aa and a phenotypic ratio of 3 : 1 if the allele shows complete dominance.

格雷戈尔•孟德尔的豌豆实验奠定了遗传的基础。他提出每个性状由一对遗传因子（现在称为等位基因）决定，分别来自双亲。分离定律指出等位基因在配子形成时分离；自由组合定律说明不同性状的基因彼此独立遗传。在单基因杂交中，我们追踪一个具有两个等位基因的基因。两个杂合个体（Aa × Aa）杂交，若等位基因为完全显性，则基因型比例为 1 AA : 2 Aa : 1 aa，表型比例为 3 : 1。

In the WJEC specification, you will be expected to construct Punnett squares and interpret pedigree charts. IB students must also understand the concept of test crosses, where an individual of unknown genotype (showing the dominant trait) is crossed with a homozygous recessive individual. If any offspring show the recessive trait, the unknown parent must be heterozygous.

在 WJEC 大纲中，你需要构建庞纳特方格并解读系谱图。IB 学生还需理解测交的概念——将基因型未知（表现出显性性状）的个体与隐性纯合个体杂交。若后代出现隐性性状，则未知亲本必为杂合。

2. Dihybrid Inheritance and Gene Interaction | 双基因遗传与基因互作

When two genes on different chromosomes are considered together, a dihybrid cross between two heterozygous individuals (AaBb × AaBb) yields the classic 9 : 3 : 3 : 1 phenotypic ratio. This ratio emerges because the alleles for each gene assort independently during meiosis. However, many traits do not follow Mendel’s ratios perfectly due to gene interactions such as epistasis. In epistasis, one gene masks or modifies the expression of another. For example, in Labrador retrievers, coat colour is determined by two genes: B (black/brown pigment) and E (pigment deposition). If a dog is homozygous recessive for E (ee), it will be yellow regardless of its B alleles.

当考虑位于不同染色体上的两个基因时，两个杂合个体（AaBb × AaBb）的双基因杂交会呈现出经典的 9 : 3 : 3 : 1 表型比例。这一比例的出现是因为各基因的等位基因在减数分裂中独立分配。但由于上位效应等基因互作，许多性状并不完全遵循孟德尔比例。在上位效应中，一个基因会掩盖或修饰另一个基因的表达。例如，拉布拉多犬的毛色由两个基因决定：B（黑色/棕色色素）和 E（色素沉积）。若狗的 E 基因为隐性纯合（ee），无论 B 基因型如何，毛色都将是黄色。

IB exam questions often present unfamiliar ratios, such as 9 : 4 : 3 or 12 : 3 : 1, requiring you to deduce that epistasis is occurring. You should be able to explain how the underlying genotypes produce the observed phenotypes. WJEC students may encounter these ideas in the context of applied genetics and selective breeding, where polygenic inheritance and interaction of multiple genes are also important.

IB 考试题目常给出非典型比例，如 9 : 4 : 3 或 12 : 3 : 1，要求你推断出存在上位效应。你应能解释背后的基因型如何产生所观察到的表型。WJEC 学生则在应用遗传学和选育的背景中可能遇到这些概念，多基因遗传和多基因互作同样重要。

3. Chromosomes, Genes and Alleles | 染色体、基因与等位基因

Genes are segments of DNA that code for a specific polypeptide or functional RNA. In eukaryotes, genes are located on linear chromosomes within the nucleus. Humans have 23 pairs of homologous chromosomes, one set from each parent. Homologous chromosomes carry the same genes in the same sequence, but the alleles — the alternative versions of a gene — may differ. The locus is the specific position of a gene on a chromosome. Alleles arise by mutation and can be dominant, recessive or co‑dominant.

基因是可编码特定多肽或功能性 RNA 的 DNA 片段。在真核生物中，基因位于细胞核内的线性染色体上。人类拥有 23 对同源染色体，每对一条来自父方、一条来自母方。同源染色体携带相同顺序的相同基因，但等位基因——基因的替代版本——可能不同。基因座是基因在染色体上的特定位置。等位基因由突变产生，可以是显性、隐性或共显性。

A key concept for both IB and WJEC is that linked genes (on the same chromosome) do not assort independently. The closer two loci are, the less likely a crossover event will separate them, leading to fewer recombinant types. You might be asked to calculate recombination frequency or to explain why observed ratios deviate from Mendelian expectations due to linkage.

IB 和 WJEC 的一个关键概念是，位于同一条染色体上的连锁基因并不独立分配。两个基因座越近，交换事件将它们分开的可能性越低，导致重组型比例降低。你可能需要计算重组频率，或解释为何观测到的比例因连锁而偏离孟德尔预期。

4. DNA Structure and Replication | DNA 结构与复制

Deoxyribonucleic acid (DNA) is a double helix composed of two antiparallel strands. Each strand has a sugar‑phosphate backbone and nitrogenous bases: adenine (A), thymine (T), cytosine (C) and guanine (G). A pairs with T via two hydrogen bonds, and C pairs with G via three hydrogen bonds. The sequence of bases encodes genetic information. During DNA replication, the double helix unwinds, and each strand serves as a template for a new complementary strand, a process called semi‑conservative replication. DNA polymerase adds nucleotides in the 5′ to 3′ direction, requiring an RNA primer to start.

脱氧核糖核酸（DNA）是由两条反向平行的链构成的双螺旋。每条链有一个糖‑磷酸骨架和含氮碱基：腺嘌呤（A）、胸腺嘧啶（T）、胞嘧啶（C）和鸟嘌呤（G）。A 与 T 通过两个氢键配对，C 与 G 通过三个氢键配对。碱基序列编码遗传信息。在 DNA 复制过程中，双螺旋解旋，每条链作为合成新互补链的模板，这一过程称为半保留复制。DNA 聚合酶以 5′ 到 3′ 方向添加核苷酸，需要 RNA 引物启动。

Both IB and WJEC require you to describe the roles of helicase, DNA polymerase and ligase. IB students may also need to outline the Meselson–Stahl experiment that provided evidence for semi‑conservative replication. Understanding replication is fundamental for grasping PCR (polymerase chain reaction) and gene technologies covered later in the course.

IB 和 WJEC 均要求描述解旋酶、DNA 聚合酶和连接酶的功能。IB 学生可能还需概述为半保留复制提供证据的梅塞尔森–斯塔尔实验。理解复制是掌握后续课程中 PCR（聚合酶链式反应）和基因技术的基础。

5. Protein Synthesis: Transcription and Translation | 蛋白质合成：转录与翻译

The central dogma of molecular biology states that information flows from DNA to RNA to protein. Transcription occurs in the nucleus: an RNA polymerase binds to the promoter region of a gene, unwinds the DNA and synthesises a messenger RNA (mRNA) strand complementary to the template strand. In eukaryotes, the primary mRNA undergoes processing — a 5′ cap and poly‑A tail are added, and introns are spliced out, leaving only exons. The mature mRNA then travels to the cytoplasm for translation.

分子生物学的中心法则认为信息从 DNA 流向 RNA 再流向蛋白质。转录发生在细胞核内：RNA 聚合酶结合到基因的启动子区域，解开 DNA 并合成一条与模板链互补的信使 RNA（mRNA）链。在真核生物中，初级 mRNA 经历加工：添加 5′ 帽和 poly‑A 尾，并切掉内含子，只留下外显子。成熟 mRNA 随后进入细胞质进行翻译。

Translation occurs on ribosomes. Transfer RNA (tRNA) molecules, each carrying a specific amino acid, recognise codons on the mRNA via their anticodon. The ribosome catalyses the formation of peptide bonds between adjacent amino acids, building a polypeptide chain. The genetic code is degenerate (more than one codon can code for the same amino acid) and universal across most organisms.

翻译在核糖体上进行。每个转运 RNA（tRNA）分子携带一个特定的氨基酸，并通过其反密码子识别 mRNA 上的密码子。核糖体催化相邻氨基酸之间形成肽键，构建多肽链。遗传密码具有简并性（多个密码子可编码同一种氨基酸），并且在大多数生物中是通用的。

Exam tips: IB questions often ask you to deduce a DNA sequence from a given mRNA strand, or to predict the effect of a point mutation. WJEC may focus more on the interpretive aspect, such as explaining why a change in the DNA might alter the protein’s function.

考试技巧：IB 题目常常要求你根据给定的 mRNA 链推导 DNA 序列，或预测点突变的影响。WJEC 可能更侧重解释性方面，比如说明 DNA 的改变为何会影响蛋白质功能。

6. Mutation and Variation | 突变与变异

A mutation is a permanent change in the DNA sequence. Substitutions (point mutations) can be silent, missense (change one amino acid) or nonsense (introduce a premature stop codon). Insertions and deletions cause frameshifts, altering the entire downstream amino acid sequence. Mutations can arise spontaneously during DNA replication or be induced by mutagens such as UV light, ionising radiation and certain chemicals. While most mutations are neutral or harmful, some can be beneficial and drive evolution.

突变是 DNA 序列的永久性改变。替换（点突变）可以是沉默的、错义的（改变一个氨基酸）或无义的（引入提前终止密码子）。插入和缺失会引发移码，彻底改变下游氨基酸序列。突变可在 DNA 复制过程中自发产生，也可能由紫外线、电离辐射及某些化学物质等诱变剂诱导。大多数突变是中性或有害的，但有些可能是有益的，并推动进化。

Variation within a population arises from mutation, meiosis (crossing over and independent assortment) and sexual reproduction (random fertilisation). The continuous variation observed for traits like height in humans is typically due to polygenic inheritance and environmental influence, while discontinuous variation (e.g., blood groups) is often controlled by a single gene.

种群内的变异来源于突变、减数分裂（交换和独立分配）以及有性生殖（随机受精）。人类身高等性状表现出的连续变异通常由多基因遗传和环境因素共同导致，而血型等不连续变异则常由单基因控制。

Both syllabi stress the importance of mutation as the original source of genetic variation. In WJEC, you may need to evaluate the effects of mutations in terms of health and disease; IB assessments can include data‑interpretation on mutation rates or the study of sickle‑cell anaemia as an example of a point mutation with a clear phenotype.

两个教学大纲都强调突变是遗传变异的根本来源。WJEC 中你可能需要评估突变对健康和疾病的影响；IB 评估可能包含突变率的数据解读，或以镰刀型细胞贫血为例——一种表型明确的点突变。

7. Gene Technologies: PCR, Gel Electrophoresis and DNA Profiling | 基因技术：PCR、凝胶电泳与 DNA 分型

Polymerase chain reaction (PCR) is a technique used to amplify a specific DNA segment. It involves repeated cycles of denaturation (heating to ~95 °C), annealing of primers (~55 °C) and extension by a heat‑stable DNA polymerase (Taq polymerase) at ~72 °C. After 30 cycles, billions of copies of the target DNA are produced. PCR is used in forensics, paternity testing and disease diagnosis.

聚合酶链式反应（PCR）是一种扩增特定 DNA 片段的技术。它包含多次循环：变性（加热至 ~95 °C）、引物退火（~55 °C）以及热稳定性 DNA 聚合酶（Taq 聚合酶）在 ~72 °C 进行延伸。30 个循环后，可获得数十亿个目标 DNA 拷贝。PCR 广泛应用于法医学、亲子鉴定和疾病诊断。

Gel electrophoresis separates DNA fragments by size. DNA samples are loaded into wells on an agarose gel, and an electric current is applied. Because DNA is negatively charged, fragments migrate towards the positive electrode, with smaller fragments moving faster. The resulting banding pattern is visualised using a fluorescent dye. DNA profiling compares the pattern of short tandem repeats (STRs) between individuals — every person (except identical twins) has a unique DNA profile.

凝胶电泳根据大小分离 DNA 片段。将 DNA 样品加入琼脂糖凝胶的孔中并施加电流。由于 DNA 带负电，片段会向正极迁移，小片段移动更快。由此产生的条带模式用荧光染料显色。DNA 分型通过比较个体间短串联重复序列（STR）的模式——除同卵双胞胎外，每个人都拥有独一无二的 DNA 指纹。

IB students may be asked to interpret gel electrophoresis results to determine paternity or to match a suspect’s DNA. WJEC often integrates these techniques with discussions on genetic screening and ethical considerations. Make sure you can outline the steps in producing a DNA profile and explain why STRs are particularly useful.

IB 学生可能会被要求解读凝胶电泳结果以确定亲子关系或匹配嫌疑人 DNA。WJEC 常将这些技术结合到遗传筛查及伦理考量的讨论中。确保你能概述生成 DNA 分型的步骤，并解释为何 STR 特别有用。

8. Genetic Engineering and GMOs | 基因工程与转基因生物

Genetic engineering involves the direct manipulation of an organism’s genome using recombinant DNA technology. A desirable gene (e.g., the human insulin gene) is isolated and inserted into a vector, often a bacterial plasmid, using the same restriction enzymes to create sticky ends. The recombinant plasmid is introduced into a host bacterium, which then expresses the gene and produces the desired protein. This is how human insulin is manufactured for diabetic patients.

基因工程运用重组 DNA 技术直接操作生物体的基因组。将所需基因（例如人胰岛素基因）分离并插入载体——通常是细菌质粒，使用相同的限制性内切酶产生黏性末端。重组质粒被导入宿主细菌，随后表达该基因并产生目标蛋白质。这正是为糖尿病患者生产人胰岛素的方式。

Genetically modified organisms (GMOs) include crops engineered for herbicide resistance or enhanced nutritional content, such as Golden Rice, which produces beta‑carotene. The potential benefits — improved food security, reduced pesticide use — must be weighed against ecological concerns and ethical issues. Both IB and WJEC require you to discuss the risks and benefits, and to understand the processes of gene transfer.

转基因生物（GMO）包括为抗除草剂或增强营养价值而改造的作物，如可产生 β‑胡萝卜素的黄金大米。其潜在益处——改善粮食安全、减少农药使用——必须与生态关切和伦理问题权衡。IB 和 WJEC 均要求你讨论风险与益处，并理解基因转移的过程。

A common exam question might ask you to describe how a named GMO was created and to evaluate its impact. Be prepared to use specific examples and to adopt a balanced, evidence‑based stance.

常见考题可能要求你描述某一具体转基因生物是如何产生的，并评价其影响。准备好引用具体实例，并采取以证据为基础的平衡立场。

9. Inheritance of Sex and Sex‑Linked Traits | 性别遗传与伴性性状

In humans, sex is determined by the X and Y chromosomes: females are XX, males are XY. The Y chromosome carries the SRY gene, which triggers male development. Because males have only one X chromosome, they are more likely to express recessive sex‑linked traits, such as red‑green colour blindness and haemophilia. A female would need two recessive alleles to express the trait, whereas a male needs only one copy inherited from his mother.

人类的性别由 X 和 Y 染色体决定：女性为 XX，男性为 XY。Y 染色体携带 SRY 基因，可启动男性发育。由于男性只有一条 X 染色体，他们更易表现隐性伴性性状，如红绿色盲和血友病。女性需要两个隐性等位基因才会表现性状，而男性只需从母亲那里继承一个即可。

When solving sex‑linked inheritance problems, you must assign alleles as superscripts on the X chromosome (e.g., X^H for normal blood clotting, X^h for haemophilia). A cross between a carrier female (X^HX^h) and a normal male (X^HY) produces a 50% chance of haemophiliac sons, while daughters may be carriers but are unlikely to be affected.

在解决伴性遗传问题时，你必须将等位基因标注为 X 染色体上的上标（例如，X^H 表示正常凝血，X^h 表示血友病）。一个携带者女性（X^HX^h）与正常男性（X^HY）婚配，儿子有 50% 概率患血友病，女儿则可能为携带者但通常不患病。

Both syllabi include interpretation of pedigree diagrams that reveal sex‑linked inheritance. Look for the pattern: affected males cannot pass the trait to their sons (since they give the Y chromosome), but all their daughters will be carriers.

两个大纲都包括对揭示伴性遗传的系谱图的解读。寻找规律：患病男性不会将性状传给儿子（因为他们提供的是 Y 染色体），但所有女儿都将是携带者。

10. Population Genetics and Evolution | 群体遗传学与进化

Population genetics studies how allele frequencies change over time. The Hardy‑Weinberg principle states that under certain conditions (no mutation, random mating, no gene flow, large population, no natural selection), allele and genotype frequencies remain constant from generation to generation. The equations p + q = 1 and p² + 2pq + q² = 1 allow you to calculate the frequencies of alleles and genotypes in a population. For instance, if the frequency of a recessive phenotype (q²) is known, you can find q, then p, and predict carrier frequencies.

群体遗传学研究等位基因频率如何随时间改变。哈代‑温伯格定律指出，在特定条件下（无突变、随机交配、无基因流动、大群体、无自然选择），等位基因和基因型频率世代保持不变。方程 p + q = 1 和 p² + 2pq + q² = 1 使你可以计算群体中等位基因和基因型的频率。例如，若已知隐性表型频率（q²），可求出 q、进而求出 p，并预测携带者频率。

Evolution occurs when these conditions are violated, leading to changes in allele frequencies. Natural selection, genetic drift and gene flow drive evolution. In IB, you may need to use Hardy–Weinberg to determine whether a population is evolving; WJEC may focus more on the broader mechanisms of evolution, including the evidence from fossils, anatomy and DNA sequences.

当这些条件被打破时，进化就会发生，导致等位基因频率变化。自然选择、遗传漂变和基因流动推动进化。在 IB 中，你可能需要用哈代‑温伯格判断一个群体是否在进化；WJEC 可能更关注进化的宏观机制，包括来自化石、解剖学和 DNA 序列的证据。

11. Ethical, Social and Legal Issues in Genetics | 遗传学中的伦理、社会与法律问题

Advances in genetics raise profound ethical questions. Genetic testing can identify carriers of inherited diseases, but the results may lead to discrimination by employers or insurers. Prenatal testing and preimplantation genetic diagnosis (PGD) enable parents to select embryos free of serious disorders, stirring debates about ‘designer babies’. Gene therapy holds promise for curing genetic diseases but also poses risks of off‑target effects and raises concerns about germline modifications that affect future generations.

遗传学的进步引发了深刻的伦理问题。基因检测可以识别遗传病携带者，但结果可能导致雇主或保险公司歧视。产前检测和胚胎植入前遗传学诊断（PGD）使父母能选择无严重疾病的胚胎，这引发了关于“设计婴儿”的争议。基因疗法有望治愈遗传病，但也存在脱靶风险，并引发对影响后代的生殖系修饰的担忧。

Both IB and WJEC require you to consider these implications. You should be able to articulate arguments for and against the use of genetic information, respecting the viewpoints of different stakeholders. Examples like the use of DNA databases in criminal investigations or the regulation of GMO crops provide rich material for discussion. Remember to balance scientific understanding with empathy and ethical reasoning.

IB 和 WJEC 都要求你思考这些影响。你应能清晰地表达支持和反对使用遗传信息的论据，并尊重不同利益相关者的观点。像刑事案件中使用 DNA 数据库或转基因作物的监管等例子，为讨论提供了丰富素材。切记在科学理解与同理心、伦理推理之间取得平衡。

12. Practical Skills and Data Interpretation | 实验技能与数据解读

Genetics examinations often include data‑based questions. You might be asked to analyse results from a dihybrid cross, deduce genotypes from a pedigree, calculate recombination frequencies from test cross data, or interpret gel electrophoresis banding patterns. Essential mathematical skills include ratios, percentages, probability calculations and the Hardy‑Weinberg equations. IB internal assessment (IA) may require you to design an experiment investigating inheritance, such as a chi‑squared test on observed versus expected ratios in fruit flies (Drosophila).

遗传学考试常包含基于数据的题目。你可能需要分析双基因杂交的结果、从系谱图推断基因型、通过测交数据计算重组频率，或解读凝胶电泳条带模式。关键数学技能包括比率、百分比、概率计算以及哈代‑温伯格方程。IB 内部评估（IA）可能要求你设计一个研究遗传的实验，例如果蝇（Drosophila）观察值与预期值之间的卡方检验。

WJEC practical assessments might focus on extracting DNA from plant material, performing gel electrophoresis or modelling meiosis with beads. Practice writing clear, logical explanations and always relate your answers to the underlying genetic principles.

WJEC 的实验评估可能侧重于从植物材料中提取 DNA、进行凝胶电泳或用珠子模拟减数分裂。练习写出清晰、符合逻辑的解释，并始终将答案与背后的遗传学原理联系起来。

By integrating conceptual understanding with practical application, you will be well prepared for both the theoretical and experimental aspects of your exams. Regular revision using past papers and active recall techniques will solidify your knowledge.

通过将概念理解与实际应用相结合，你将为考试的理论与实验部分做好充分准备。利用历年真题和主动回忆技巧定期复习，将巩固你的知识。

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