📚 IB OCR Biology: Genetics Revision Essentials | IB OCR 生物：遗传学 考点精讲
Genetics forms the backbone of modern biology, linking molecular mechanisms to inheritance patterns and the diversity of life. In the IB and OCR specifications, a deep understanding of genetic processes — from DNA structure to gene expression and evolutionary change — is essential for mastering topics in molecular biology, heredity, and biotechnology. This guide distils the core concepts, key experiments, and common exam pitfalls into a structured revision resource, helping you build confidence and precision in answering genetics questions.
遗传学是现代生物学的支柱，将分子机制与遗传模式以及生命多样性紧密相连。在 IB 和 OCR 课程大纲中，深刻理解遗传过程——从 DNA 结构到基因表达和进化变化——对于掌握分子生物学、遗传和生物技术等主题至关重要。本指南将核心概念、关键实验和常见考试易错点浓缩为结构化的复习资源，帮助你在解答遗传学问题时建立信心并提升准确性。
1. DNA Structure and Function | DNA结构与功能
The DNA double helix consists of two antiparallel strands of nucleotides, each containing a deoxyribose sugar, a phosphate group, and a nitrogenous base (adenine, thymine, cytosine, or guanine). Complementary base pairing — A with T (two hydrogen bonds) and C with G (three hydrogen bonds) — ensures accurate replication and storage of genetic information. The sequence of bases along a strand encodes the instructions for protein synthesis, with the sugar‑phosphate backbone providing structural stability.
DNA 双螺旋由两条反向平行的核苷酸链组成，每条链包含脱氧核糖、磷酸基团和含氮碱基（腺嘌呤、胸腺嘧啶、胞嘧啶或鸟嘌呤）。互补碱基配对——A 与 T 之间两个氢键，C 与 G 之间三个氢键——保证了遗传信息的精确复制和储存。链上碱基的序列编码蛋白质合成的指令，而糖‑磷酸骨架则提供结构稳定性。
2. DNA Replication | DNA复制
elicase unwinds the double helix, creating a replication fork. DNA polymerase III adds nucleotides in the 5′ → 3′ direction, using the parental strand as a template. The leading strand is synthesised continuously, whereas the lagging strand is built in short Okazaki fragments, later joined by DNA ligase. RNA primase lays down short RNA primers to initiate synthesis.
DNA 复制是半保留的，即每个新的 DNA 分子包含一条原始链和一条新合成的链。解旋酶解旋双螺旋，形成复制叉。DNA 聚合酶 III 以母链为模板，沿 5′ → 3′ 方向添加核苷酸。前导链连续合成，而后随链则以短小的冈崎片段合成，之后由 DNA 连接酶连接。RNA 引物酶合成短的 RNA 引物以启动合成。
3. Transcription | 转录
ises a complementary mRNA strand in the 5′ → 3′ direction. Only one of the two DNA strands — the template strand — ibed. In eukaryotes, the primary mRNA transcript undergoes processing: a 5′ cap and a poly‑ dded, and introns are spliced out by spliceosomes, leaving only exons to form mature mRNA.
在转录过程中，RNA 聚合酶结合到基因的启动子区域，解旋 DNA，并以 5′ → 3′ 方向合成互补的 mRNA 链。两条 DNA 链中只有一条——模板链——被转录。在真核生物中，初级 mRNA 转录本需经过加工：添加 5′ 帽子和 poly‑A 尾巴，并由剪接体切除内含子，只留下外显子形成成熟 mRNA。
4. Translation | 翻译
Translation occurs on ribosomes, where the sequence of mRNA codons (triplets of bases) is decoded into a polypeptide chain. Transfer RNA (tRNA) molecules carry specific amino acids and possess anticodons that base‑pair with complementary mRNA codons. The ribosome moves along the mRNA, facilitating the formation of peptide bonds between adjacent amino acids. The process begins at the start codon (AUG) and terminates at a stop codon (UAA, UAG, or UGA).
翻译在核糖体上进行，mRNA 上的密码子（三个碱基为一组）序列在此被解码为多肽链。转运 RNA（tRNA）携带特定的氨基酸，其反密码子与 mRNA 上的互补密码子配对。核糖体沿 mRNA 移动，促进相邻氨基酸之间肽键的形成。这一过程从起始密码子（AUG）开始，终止于终止密码子（UAA、UAG 或 UGA）。
5. Mendelian Genetics | 孟德尔遗传学
Gregor Mendel’s laws form the foundation of classical genetics. The Law of Segregation states that each organism carries two alleles for each trait, which separate during gamete formation so that each gamete receives only one allele. The Law of Independent Assortment asserts that alleles of different genes assort independently during meiosis, provided the genes are on different chromosomes. Monohybrid and dihybrid crosses reveal phenotypic ratios of 3:1 and 9:3:3:1 respectively for completely dominant traits.
格雷戈尔·孟德尔的定律构成了经典遗传学的基础。分离定律指出，每个生物体的每个性状都携带两个等位基因，它们在配子形成时分离，使得每个配子只含有一个等位基因。自由组合定律宣称，如果基因位于不同染色体上，不同基因的等位基因在减数分裂时独立分配。单因子和双因子杂交分别显示出完全显性性状的 3:1 和 9:3:3:1 表型比例。
6. Non‑Mendelian Inheritance | 非孟德尔遗传
Not all traits follow simple Mendelian patterns. In codominance, both alleles are expressed equally in the heterozygote (e.g., AB blood type). In incomplete dominance, the heterozygote shows a blended phenotype (e.g., pink snapdragons). Sex‑linked traits, often carried on the X chromosome, show different inheritance patterns in males and females. Polygenic traits, such as skin colour, are controlled by multiple genes and exhibit continuous variation.
并非所有性状都遵循简单的孟德尔模式。在共显性中，杂合子中两个等位基因均等表达（例如 AB 血型）。在不完全显性中，杂合子表现出混合表型（例如粉色金鱼草）。性连锁性状通常位于 X 染色体上，在男性和女性中表现出不同的遗传模式。多基因性状（如肤色）由多个基因控制，并表现出连续变异。
7. Mutations and Genetic Variation | 突变与遗传变异
Mutations are permanent changes in the DNA sequence. Point mutations include substitutions (silent, missense, nonsense) and frameshift mutations caused by insertions or deletions. Chromosomal mutations involve large‑scale changes such as deletions, duplications, inversions, and translocations. Mutations can arise spontaneously during replication or be induced by mutagens such as radiation and chemicals. They are the ultimate source of genetic variation upon which natural selection acts.
突变是 DNA 序列的永久性改变。点突变包括替换（沉默、错义、无义）以及由插入或缺失引起的移码突变。染色体突变涉及大片段的改变，如缺失、重复、倒位和易位。突变可以在复制过程中自发产生，也可由辐射和化学物质等诱变剂诱发。它们是自然选择赖以作用的遗传变异的最终来源。
8. Gene Regulation and Expression | 基因调控与表达
Gene expression is tightly regulated at multiple levels. In prokaryotes, operons (e.g., the lac operon) control the transcription of functionally related genes through repressor and activator proteins. In eukaryotes, regulation involves transcription factors, enhancers, silencers, and epigenetic modifications such as DNA methylation and histone acetylation. These mechanisms ensure that genes are expressed only in the right cell type, at the right time, and in appropriate amounts.
基因表达在多个层面上受到严格调控。在原核生物中，操纵子（如乳糖操纵子）通过阻遏蛋白和激活蛋白控制功能相关基因的转录。在真核生物中，调控涉及转录因子、增强子、沉默子以及 DNA 甲基化和组蛋白乙酰化等表观遗传修饰。这些机制确保基因只在正确的细胞类型、正确的时间以适当的量表达。
9. Genetic Technology and CRISPR | 基因技术与CRISPR
Modern genetic technologies include polymerase chain reaction (PCR) for amplifying DNA, gel electrophoresis for separating DNA fragments, and DNA profiling for identification. CRISPR‑Cas9 has revolutionised gene editing: a guide RNA directs the Cas9 nuclease to a specific DNA sequence, where it creates a double‑strand break, allowing targeted gene modification. These tools are used in gene therapy, agriculture, and research.
现代基因技术包括用于扩增 DNA 的聚合酶链反应（PCR）、用于分离 DNA 片段的凝胶电泳以及用于身份鉴定的 DNA 分型。CRISPR‑Cas9 已经彻底改变了基因编辑：一段引导 RNA 将 Cas9 核酸酶导向特定的 DNA 序列，在那里制造双链断裂，从而进行靶向基因修饰。这些工具被应用于基因治疗、农业和研究中。
10. Pedigree Analysis and Genetic Counselling | 系谱分析与遗传咨询
Pedigree charts are used to trace the inheritance of traits through generations. By analysing patterns — whether the trait appears in every generation (autosomal dominant), skips generations (autosomal recessive), or affects predominantly one sex (sex‑linked) — geneticists can infer modes of inheritance. Genetic counselling uses such analysis, along with risk calculations based on Punnett squares and probability, to inform individuals about genetic disorders.
系谱图用于追踪性状在世代中的遗传。通过分析模式——性状是否在每一代都出现（常染色体显性），是否隔代出现（常染色体隐性），或是否主要影响一种性别（性连锁）——遗传学家可以推断遗传方式。遗传咨询利用这类分析，并结合基于庞纳特方格和概率的风险计算，为个体提供有关遗传病的信息。
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