IB & Edexcel Science: Genetics Key Concepts | IB 与 Edexcel 科学：遗传考点精讲

📚 IB & Edexcel Science: Genetics Key Concepts | IB 与 Edexcel 科学：遗传考点精讲

Genetics, the study of heredity and variation, forms a cornerstone of both IB Biology and Edexcel IGCSE/A Level Science curricula. This revision guide breaks down the essential concepts you must master, from DNA structure to genetic engineering, with clear bilingual explanations aligned to exam requirements.

遗传学是研究遗传与变异的学科，在 IB 生物学和 Edexcel IGCSE/A Level 科学课程中都占据核心地位。本篇考点精讲将关键概念逐一拆解，从 DNA 结构到基因工程，提供符合考试要求的清晰中英双语解析。

1. Introduction to Heredity and DNA | 遗传与 DNA 简介

Genetics explains how traits are passed from parents to offspring through genes, which are segments of DNA located on chromosomes. In both IB and Edexcel specifications, you need to understand that DNA carries the genetic code and is the molecule of inheritance.

遗传学解释性状如何通过基因从亲代传递给子代，基因是位于染色体上的 DNA 片段。在 IB 和 Edexcel 考纲中，你需要掌握 DNA 携带遗传密码，是遗传的物质基础。

2. Structure of DNA and RNA | DNA 与 RNA 的结构

DNA is a double helix composed of two antiparallel strands of nucleotides. Each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base (adenine A, thymine T, cytosine C, guanine G). Purines pair with pyrimidines: A pairs with T via two hydrogen bonds, and C pairs with G via three hydrogen bonds.

DNA 是由两条反向平行的核苷酸链组成的双螺旋。每个核苷酸包含一个脱氧核糖、一个磷酸基团和一个含氮碱基（腺嘌呤 A、胸腺嘧啶 T、胞嘧啶 C、鸟嘌呤 G）。嘌呤与嘧啶配对：A 与 T 通过两个氢键配对，C 与 G 通过三个氢键配对。

RNA is single-stranded, contains ribose sugar, and uses uracil (U) instead of thymine. There are three main types: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA), each playing a distinct role in protein synthesis.

RNA 是单链结构，含有核糖，并用尿嘧啶（U）代替胸腺嘧啶。主要有三种类型：信使 RNA（mRNA）、转运 RNA（tRNA）和核糖体 RNA（rRNA），它们在蛋白质合成中各司其职。

DNA: A = T, C ≡ G   |   RNA: A = U, C ≡ G

3. DNA Replication | DNA 复制

DNA replication is a semi-conservative process occurring during the S phase of the cell cycle. The enzyme DNA helicase unwinds the double helix, and DNA polymerase synthesises new complementary strands in the 5′ → 3′ direction using the parent strands as templates. The leading strand is synthesised continuously, while the lagging strand forms short Okazaki fragments that are later joined by DNA ligase.

DNA 复制是发生在细胞周期 S 期的半保留过程。解旋酶解开双螺旋，DNA 聚合酶以亲代链为模板，沿 5′ → 3′ 方向合成新的互补链。前导链是连续合成的，而后随链形成不连续的冈崎片段，再由 DNA 连接酶连接起来。

Experiments by Meselson and Stahl using heavy nitrogen (¹⁵N) confirmed the semi-conservative model. Students must be able to interpret density gradient centrifugation results that distinguish parental and newly synthesised DNA.

Meselson 和 Stahl 利用重氮（¹⁵N）的实验证实了半保留复制模型。学生需能够解读密度梯度离心的结果，区分亲代链和新合成链。

4. Gene Expression: Transcription and Translation | 基因表达：转录与翻译

Gene expression involves two major stages. During transcription, RNA polymerase binds to the promoter region of a gene, unwinds the DNA, and synthesises a complementary mRNA strand by base pairing (A–U, T–A, C–G, G–C). In eukaryotes, the primary transcript undergoes processing: a 5′ cap and poly-A tail are added, and introns are spliced out to form mature mRNA.

基因表达包含两大阶段。转录过程中，RNA 聚合酶结合到基因的启动子区域，解开 DNA，并根据碱基配对原则（A–U、T–A、C–G、G–C）合成互补的 mRNA 链。在真核生物中，初级转录本经过加工：添加 5′ 帽和 poly-A 尾，并切除内含子形成成熟 mRNA。

Translation takes place on ribosomes. Each codon (triplet of bases) on mRNA is recognised by a specific tRNA carrying an amino acid. The ribosome catalyses peptide bond formation, and the polypeptide chain elongates until a stop codon is reached. The sequence of amino acids determines the protein’s primary structure.

翻译在核糖体上进行。mRNA 上的每个密码子（三个碱基）由携带特定氨基酸的 tRNA 识别。核糖体催化肽键形成，多肽链不断延伸直至遇到终止密码子。氨基酸序列决定了蛋白质的一级结构。

Central Dogma: DNA → RNA → Protein

5. Mendelian Genetics and Monohybrid Cross | 孟德尔遗传与单因子杂交

Gregor Mendel’s experiments with pea plants established the principles of inheritance. Key terms include: allele (variant form of a gene), dominant (masks recessive), homozygous (two identical alleles), heterozygous (two different alleles), genotype (genetic makeup), and phenotype (observable trait). A monohybrid cross tracks one characteristic.

孟德尔用豌豆植物进行实验，奠定了遗传学原理。关键术语包括：等位基因（基因的变体形式）、显性（遮盖隐性）、纯合子（两个相同等位基因）、杂合子（两个不同等位基因）、基因型（遗传组成）和表现型（可观察性状）。单因子杂交追踪一种性状的遗传。

When a homozygous dominant (TT) is crossed with a homozygous recessive (tt), all F₁ offspring are heterozygous (Tt) and show the dominant trait. A self-cross of F₁ individuals produces an F₂ phenotypic ratio of 3 dominant : 1 recessive, and a genotypic ratio of 1 : 2 : 1 (TT : Tt : tt).

纯合显性（TT）与纯合隐性（tt）杂交时，F₁ 代全为杂合子（Tt）并表现显性性状。F₁ 个体自交产生的 F₂ 代表现型比例为 3 显性 : 1 隐性，基因型比例为 1 : 2 : 1（TT : Tt : tt）。

F₂ Phenotypic Ratio: 3 : 1

A test cross (crossing an individual of unknown genotype with a homozygous recessive) can determine whether the unknown is homozygous dominant or heterozygous. Pure breeding strains are crucial for such analyses.

测交（将未知基因型个体与纯合隐性个体杂交）能够判断该个体是纯合显性还是杂合子。纯品系在这种分析中至关重要。

6. Dihybrid Cross and Independent Assortment | 双因子杂交与自由组合

A dihybrid cross follows the inheritance of two genes located on different chromosomes. According to Mendel’s Law of Independent Assortment, alleles of different genes segregate and assort independently during gamete formation, provided the genes are unlinked.

双因子杂交追踪位于不同染色体上的两个基因的遗传。根据孟德尔的自由组合定律，只要基因不连锁，不同基因的等位基因在配子形成过程中会独立分离和组合。

Starting with pure-breeding parents (e.g., RRYY × rryy), the F₁ generation is heterozygous for both traits (RrYy). Self-crossing the F₁ yields a classic phenotypic ratio of 9 : 3 : 3 : 1 in the F₂ generation, representing all combinations of dominant and recessive traits.

从纯合亲本（如 RRYY × rryy）开始，F₁ 代对两个性状都是杂合的（RrYy）。F₁ 自交后在 F₂ 代中得到经典的表现型比例 9 : 3 : 3 : 1，涵盖了显性和隐性性状的所有组合。

Dihybrid Ratio: 9 : 3 : 3 : 1

If genes are linked (close together on the same chromosome), they tend to be inherited together and do not assort independently, altering expected ratios. Recombination frequency can be used to map gene positions.

如果基因连锁（位于同一条染色体上且距离很近），它们倾向于一起遗传，不遵循自由组合，从而改变预期的比例。重组频率可用于绘制基因图谱。

7. Sex-linked Inheritance and Pedigree Analysis | 伴性遗传与系谱分析

Sex-linked genes are located on sex chromosomes, usually the X chromosome. Since males (XY) have only one X chromosome, a single recessive allele on the X will be expressed. Females (XX) require two copies of the recessive allele to show the trait, making them carriers if heterozygous.

伴性基因位于性染色体上，通常是 X 染色体。因为男性（XY）只有一条 X 染色体，X 染色体上的单个隐性等位基因即可表达。女性（XX）需要两个隐性等位基因才会表现出性状，因此杂合子女性为携带者。

Classic examples include colour blindness and haemophilia. In pedigree charts, squares represent males, circles females; shaded symbols indicate affected individuals. You must be able to deduce genotypes and patterns of inheritance from given pedigrees.

典型例子包括红绿色盲和血友病。在系谱图中，方框代表男性，圆圈代表女性；涂黑符号表示患病个体。你必须能够从给定的系谱图中推断基因型和遗传模式。

X-linked recessive	Affected males > females; skips generations
X-linked dominant	Affected fathers pass to all daughters
Autosomal recessive	Affected individuals can have unaffected parents
Autosomal dominant	Trait appears in every generation

8. Mutations and Genetic Variation | 突变与遗传变异

Mutations are permanent changes in the DNA sequence. Point mutations involve a single nucleotide change: substitution (silent, missense, nonsense) and frameshift (insertion or deletion) that alters the reading frame. Chromosomal mutations include deletion, duplication, inversion, and translocation.

突变是 DNA 序列的永久性改变。点突变涉及单个核苷酸变化：替换（沉默、错义、无义）和移码突变（插入或缺失）会导致阅读框改变。染色体突变包括缺失、重复、倒位和易位。

Mutations can be spontaneous or induced by mutagens, such as radiation (UV, X-rays) and chemicals. They are the ultimate source of genetic variation, providing raw material for evolution. Meiosis also generates variation through crossing over and independent assortment.

突变可以是自发的，也可以由诱变剂引起，如辐射（紫外线、X 射线）和化学物质。突变是遗传变异的根本来源，为进化提供原材料。减数分裂还通过交叉互换和自由组合产生变异。

Sickle cell anaemia: Hbᴬ → Hbˢ (GAG → GTG, Glu → Val)

9. Genetic Engineering and Biotechnology | 基因工程与生物技术

Genetic engineering involves the direct manipulation of an organism’s DNA. Restriction enzymes cut DNA at specific recognition sequences, often leaving ‘sticky ends’. DNA ligase joins fragments, and plasmids serve as vectors to transfer genes into bacterial cells.

基因工程涉及对生物体 DNA 的直接操作。限制性内切酶在特定识别序列处切割 DNA，常产生“黏性末端”。DNA 连接酶连接片段，质粒作为载体将基因转入细菌细胞。

Key techniques include PCR (polymerase chain reaction) to amplify DNA, gel electrophoresis to separate fragments by size, and DNA profiling for forensic identification. Genetically modified organisms (GMOs) like insulin-producing bacteria are classic applications.

关键技术包括扩增 DNA 的 PCR（聚合酶链式反应）、按大小分离片段的凝胶电泳，以及用于法医鉴定的 DNA 图谱。转基因生物（如生产人胰岛素的细菌）是经典应用。

PCR cycles: Denaturation (95 °C) → Annealing (55–65 °C) → Extension (72 °C)

Ethical issues surrounding genetic modification, cloning, and gene therapy are often examined in both IB and Edexcel assessments, requiring balanced evaluation of benefits and risks.

涉及转基因、克隆和基因治疗的伦理问题经常出现在 IB 和 Edexcel 的考试中，需要对利弊做出平衡的评价。

10. Genetic Diseases and Screening | 遗传病与筛查

Genetic disorders result from mutations or chromosomal abnormalities. Examples include cystic fibrosis (autosomal recessive, caused by a defective CFTR gene), Huntington’s disease (autosomal dominant), and Down syndrome (trisomy 21). Screening programmes use blood tests, ultrasound, and amniocentesis to detect disorders prenatally.

遗传疾病由突变或染色体异常引起。例如囊性纤维化（常染色体隐性，由 CFTR 基因缺陷导致）、亨廷顿氏病（常染色体显性）和唐氏综合征（21 三体）。筛查项目使用血液检测、超声和羊膜穿刺进行产前诊断。

Genetic counselling helps families understand risks and make informed decisions. Understanding pedigree analysis is essential to predict the probability of inheriting a disease.

遗传咨询帮助家庭了解风险并做出知情决定。熟练掌握系谱分析对预测疾病遗传概率至关重要。

Punnett square: Probability of carrier × carrier for recessive disease → ¼ affected, ½ carriers, ¼ normal

11. Epigenetics and Environmental Influence | 表观遗传与环境影响

Epigenetics studies heritable changes in gene expression that do not involve alterations in DNA sequence. Mechanisms include DNA methylation and histone modification. These changes can be influenced by environmental factors such as diet, stress, and toxins, and may be reversible.

表观遗传学研究不涉及 DNA 序列改变的基因表达的可遗传变化。机制包括 DNA 甲基化和组蛋白修饰。这些变化会受到饮食、压力和毒素等环境因素的影响，并且可能是可逆的。

This concept blurs the line between nature and nurture, showing that phenotype is not solely dictated by genotype. For example, identical twins may differ in disease susceptibility due to epigenetic differences accumulated over a lifetime.

这一概念模糊了先天与后天的界限，表明表现型并非仅由基因型决定。例如，同卵双胞胎在疾病易感性上可能存在差异，这源于终身累积的表观遗传差异。

12. Exam Tips and Common Pitfalls | 考试技巧与常见陷阱

In IB data-based questions, always refer to specific data when explaining trends. For Edexcel core practicals, be precise about enzyme names, temperatures, and the sequences of procedures like DNA extraction or gel electrophoresis. Do not confuse transcription with translation, or codon with anticodon.

在 IB 的数据分析题中，解释趋势时一定要引用具体数据。对于 Edexcel 的核心实验，要准确书写酶的名称、温度以及 DNA 提取或凝胶电泳的步骤序列。切勿混淆转录与翻译，或密码子与反密码子。

When drawing Punnett squares, clearly label gametes and state phenotypes alongside genotypes. For sex-linked problems, always consider the probability for male and female offspring separately if required. Use proper notation (e.g., Xᴿ, Xʳ).

在绘制庞纳特方格时，要清楚标记配子，并将表现型与基因型一同注明。对于伴性遗传问题，必要时需分别考虑男性和女性后代的比例。使用正确的符号（如 Xᴿ、Xʳ）。

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