IB CCEA Science: Genetics – Key Points | IB CCEA 科学：遗传 考点精讲

📚 IB CCEA Science: Genetics – Key Points | IB CCEA 科学：遗传 考点精讲

Genetics is the study of heredity and variation, explaining how traits are passed from parents to offspring. Both IB and CCEA science specifications require a solid understanding of DNA structure, gene expression, Mendelian and non-Mendelian inheritance, mutations, and modern genetic technologies. This revision guide distills the essential concepts, covering key definitions, processes, and problem-solving techniques for exam success.

遗传学是研究遗传和变异的学科，阐释性状如何从亲代传递给子代。IB 与 CCEA 科学课程都要求深入掌握 DNA 结构、基因表达、孟德尔与非孟德尔遗传、突变以及现代遗传技术。本文考点精讲浓缩了核心概念，梳理关键定义、过程与解题技巧，助你高效备考。

1. DNA Structure and Function | DNA 的结构与功能

DNA (deoxyribonucleic acid) 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) or guanine (G). Complementary base pairing (A–T via two hydrogen bonds; C–G via three hydrogen bonds) holds the strands together.

DNA（脱氧核糖核酸）是双螺旋结构，由两条反向平行的核苷酸链构成。每个核苷酸包含一分子脱氧核糖、一个磷酸基团和一种含氮碱基——腺嘌呤 (A)、胸腺嘧啶 (T)、胞嘧啶 (C) 或鸟嘌呤 (G)。碱基互补配对（A–T 通过两个氢键；C–G 通过三个氢键）将双链维系在一起。

The sequence of bases encodes genetic information. In eukaryotic cells, DNA is organised into linear chromosomes inside the nucleus, tightly wound around histone proteins to form chromatin. Prokaryotes have a single circular chromosome and plasmids.

碱基序列编码遗传信息。在真核细胞中，DNA 被组织成细胞核内的线状染色体，紧密缠绕在组蛋白上形成染色质。原核生物则拥有一个环状染色体和质粒。

2. DNA Replication | DNA 复制

DNA replication is semiconservative – each new DNA molecule consists of one original strand and one newly synthesised strand. The enzyme helicase unwinds the double helix and breaks hydrogen bonds. DNA polymerase then adds complementary nucleotides to the exposed template strands in the 5′ → 3′ direction, requiring a primer.

DNA 复制是半保留复制——每个新 DNA 分子含有一条原模板链和一条新合成链。解旋酶打开双螺旋并断裂氢键；随后 DNA 聚合酶以 5′ → 3′ 方向在暴露的模板链上添加互补核苷酸，此过程需要引物。

The leading strand is synthesised continuously, while the lagging strand is formed in short Okazaki fragments, later joined by DNA ligase. Proofreading by DNA polymerase ensures high fidelity, correcting most mismatches.

前导链连续合成，后随链则形成不连续的冈崎片段，最后由 DNA 连接酶连接。DNA 聚合酶的校对功能确保高保真度，能纠正多数错配碱基。

3. The Genetic Code and Protein Synthesis | 遗传密码与蛋白质合成

The genetic code is triplet-based: each codon (three bases) specifies one amino acid. The code is degenerate (multiple codons can code for the same amino acid), universal across almost all organisms, and non-overlapping. Transcription copies a gene’s DNA sequence into messenger RNA (mRNA) in the nucleus, catalysed by RNA polymerase.

遗传密码以三联体为基础：每个密码子（三个碱基）对应一种氨基酸。密码子具有简并性（多个密码子可编码同一种氨基酸）、通用性和不重叠性。转录过程在细胞核中由 RNA 聚合酶催化，将基因的 DNA 序列拷贝为信使 RNA (mRNA)。

Translation occurs at ribosomes: transfer RNAs (tRNAs) carry anticodons complementary to mRNA codons and deliver the corresponding amino acids. Peptide bonds form between amino acids, creating a polypeptide chain that folds into a functional protein.

翻译在核糖体上进行：转运 RNA (tRNA) 携带着与 mRNA 密码子互补的反密码子，并递送相应氨基酸。氨基酸之间形成肽键，生成多肽链，进而折叠成功能蛋白质。

4. Mendelian Inheritance | 孟德尔遗传

Mendel’s laws form the foundation of classical genetics. The law of segregation states that each individual possesses two alleles for a trait, which separate during gamete formation so that each gamete carries only one allele. The law of independent assortment applies to genes on different chromosomes: alleles of different genes are distributed into gametes independently.

孟德尔定律奠定了经典遗传学的基础。分离定律指出，个体每个性状具有两个等位基因，它们在配子形成时分离，使每个配子只携带一个等位基因。自由组合定律适用于不同染色体上的基因：不同基因的等位基因独立地分配入配子中。

Monohybrid crosses yield genotypic ratios of 1:2:1 for homozygous dominant, heterozygous, and homozygous recessive offspring when both parents are heterozygous. A test cross (heterozygote × homozygous recessive) reveals the genotype of an individual showing the dominant phenotype.

单基因杂交中，当双亲均为杂合时，子代基因型比为 1:2:1（显性纯合 : 杂合 : 隐性纯合）。测交（杂合体 × 隐性纯合）可用于鉴定表现显性性状个体的基因型。

Codominance (both alleles expressed equally, e.g., AB blood type) and incomplete dominance (blending, e.g., pink snapdragons) are variations of dominance that still follow Mendelian segregation.

共显性（两个等位基因同等表达，如 AB 血型）和不完全显性（性状融合，如粉色金鱼草）是显性关系的变异，但仍遵循孟德尔分离规律。

5. Non-Mendelian Inheritance and Linkage | 非孟德尔遗传与基因连锁

Sex-linked traits are controlled by genes on sex chromosomes, most often the X chromosome. In humans, colour blindness and haemophilia are X-linked recessive disorders, meaning they appear more frequently in males who have only one X chromosome.

伴性遗传性状由性染色体上的基因控制，多为 X 染色体。人类的色盲和血友病属于 X 连锁隐性遗传病，因此在只有一条 X 染色体的男性中发病率更高。

Linked genes are located on the same chromosome and tend to be inherited together, violating the law of independent assortment. The recombination frequency between linked genes, calculated from test cross data, indicates their relative distance; 1% recombination equals one map unit.

连锁基因位于同一条染色体上，倾向于共同遗传，打破了自由组合定律。通过测交数据计算的重组率可反映连锁基因间的相对距离，1% 重组率相当于一个图距单位。

6. Mutations | 突变

Gene mutations are changes in the nucleotide sequence. Point mutations include substitutions (silent, missense, or nonsense), while frameshift mutations result from insertions or deletions of bases, shifting the reading frame and often producing a nonfunctional protein.

基因突变是核苷酸序列的改变。点突变包括替换（沉默、错义或无义突变），而移码突变由碱基的插入或缺失引起，导致阅读框改变，通常生成无功能的蛋白质。

Chromosomal mutations involve large-scale changes: deletions, duplications, inversions, and translocations. Non-disjunction during meiosis can cause aneuploidy, such as trisomy 21 (Down syndrome). Mutagens like UV radiation, chemicals, and viruses increase mutation rates, though many mutations are spontaneous.

染色体突变涉及更大范围的改变：缺失、重复、倒位和易位。减数分裂中的不分离可导致非整倍性，如 21 三体综合征（唐氏综合征）。紫外线、化学物质和病毒等诱变剂会提高突变率，但许多突变是自发产生的。

7. Genetic Variation and Meiosis | 遗传变异与减数分裂

Meiosis produces haploid gametes and generates genetic variation through two key mechanisms: independent assortment of homologous chromosomes (2²³ possible combinations in humans) and crossing over between non-sister chromatids during prophase I. Random fertilisation further increases diversity.

减数分裂产生单倍体配子，并通过两个关键机制制造遗传变异：同源染色体的自由组合（人类可有 2²³ 种组合方式）以及前期 I 中非姐妹染色单体之间的交叉互换。随机受精进一步增加了多样性。

The stages of meiosis I (prophase I with synapsis and chiasmata, metaphase I, anaphase I, telophase I) and meiosis II resemble mitosis but without DNA replication between divisions. Errors in sister chromatid separation or non-disjunction can lead to gametes with abnormal chromosome numbers.

减数第一次分裂（前期 I 出现联会和交叉，中期 I、后期 I、末期 I）和减数第二次分裂与有丝分裂相似，但分裂间期无 DNA 复制。姐妹染色单体分离错误或不分离会导致配子染色体数目异常。

8. Genetic Engineering and CRISPR | 基因工程与 CRISPR 技术

Recombinant DNA technology involves isolating a gene of interest, inserting it into a vector (often a bacterial plasmid), and introducing the recombinant molecule into host cells. Restriction enzymes cut DNA at specific recognition sites, and DNA ligase seals the sugar-phosphate backbone. Insulin production and GM crops are common applications.

重组 DNA 技术包括分离目的基因、将其插入载体（常为细菌质粒）、再将重组分子导入宿主细胞。限制性内切酶在特定位点切割 DNA，DNA 连接酶封合糖-磷酸骨架。胰岛素生产和转基因作物是其常见应用。

CRISPR-Cas9 is a precise genome-editing tool: a guide RNA directs the Cas9 nuclease to a target DNA sequence, where it creates a double-strand break. The cell’s repair machinery can then introduce modifications, allowing gene knockouts or corrections.

CRISPR-Cas9 是一种精准的基因组编辑工具：向导 RNA 将 Cas9 核酸酶指引至目标 DNA 序列，在此处制造双链断裂。细胞的修复机制随后可引入修饰，实现基因敲除或修正。

Ethical considerations include ‘designer babies’, environmental impact of GMOs, and the accessibility of gene therapies. Both IB and CCEA syllabi expect students to discuss these societal implications.

伦理考量包括“设计婴儿”、转基因生物的环境影响以及基因疗法的可及性。IB 和 CCEA 课程均要求学生讨论这些社会意义。

9. Pedigree Analysis | 系谱分析

Pedigree charts trace the inheritance of traits through generations. Squares represent males, circles females; shaded symbols indicate the trait of interest. Analysing patterns helps determine whether a trait is autosomal dominant, autosomal recessive, X-linked recessive, or X-linked dominant.

系谱图用于追踪性状在家族世代中的传递。方框代表男性，圆圈代表女性；涂色符号表示具有该性状。分析遗传模式可判断性状是常染色体显性、常染色体隐性、X 连锁隐性还是 X 连锁显性。

Key clues: in autosomal recessive inheritance, affected individuals can appear in offspring of unaffected parents; in X-linked recessive, more males are affected and an affected father passes the allele to all daughters but not to sons.

关键线索：常染色体隐性遗传中，患病个体可出现于表型正常的父母所生子女中；X 连锁隐性遗传中，男性患者更多，且患病父亲将等位基因传给所有女儿但不传给儿子。

10. Common Genetic Diseases and Testing | 常见遗传病与检测

Cystic fibrosis is an autosomal recessive disorder caused by a mutation in the CFTR gene, leading to thick mucus production affecting the lungs and digestive system. Huntington’s disease is autosomal dominant, resulting in progressive neurodegeneration. Sickle cell anaemia results from a single base substitution causing abnormal haemoglobin.

囊性纤维化是常染色体隐性遗传病，由 CFTR 基因突变引起，导致粘稠黏液积聚，影响肺部和消化系统。亨廷顿病为常染色体显性，引起进行性神经退行。镰刀型细胞贫血由单个碱基替换导致异常血红蛋白。

Prenatal testing includes amniocentesis and chorionic villus sampling. Preimplantation genetic diagnosis (PGD) screens embryos before implantation. Genetic counselling helps families understand risks and make informed decisions.

产前检测包括羊膜腔穿刺和绒毛膜取样。胚胎植入前遗传学诊断 (PGD) 在胚胎植入前进行筛选。遗传咨询帮助家庭理解风险并做出知情决定。

Both IB and CCEA exams may ask students to interpret DNA gel electrophoresis results for paternity or forensic analysis, or to design PCR-based detection of specific alleles.

IB 和 CCEA 考试中，都可能要求学生解读用于亲子鉴定或法医分析的 DNA 凝胶电泳结果，或设计基于 PCR 的特定等位基因检测方案。

11. Key Definitions and Exam Tips | 核心定义与考试技巧

Ensure you can precisely define: gene (a heritable factor that controls a specific characteristic), allele (alternative form of a gene), genotype, phenotype, homozygous, heterozygous, carrier, locus, genome, and proteome. Many mark schemes reward exact wording.

务必能准确定义：基因（控制特定性状的可遗传因子）、等位基因（基因的不同形式）、基因型、表现型、纯合子、杂合子、携带者、基因座、基因组和蛋白质组。评分方案常常奖励精确用词。

Practise Punnett square problems up to dihybrid crosses, including scenarios with linkage and recombination frequencies. Draw diagrams clearly and label chromosomes, alleles, and gametes.

练习直至双基因杂交的旁氏表问题，包括连锁与重组率情境。绘图要清晰，标注染色体、等位基因和配子。

When writing about protein synthesis, explicitly mention roles of enzymes, mRNA processing (splicing to remove introns in eukaryotes), and the universality of the code linking genotype to phenotype.

在回答蛋白质合成问题时，要明确提及酶的作用、mRNA 加工（真核生物中剪切除去内含子）以及密码子通用性将基因型与表现型联系起来。

12. Experimental Genetics and Data Interpretation | 实验遗传学与数据解读

Common practical tasks include extracting DNA from fruits, constructing monohybrid crosses with Drosophila or computer simulations, and analysing karyotypes to identify chromosomal abnormalities. IB internal assessment may involve designing investigations on factors affecting DNA extraction or mutation rates.

常见实验任务包括水果 DNA 提取、利用果蝇或计算机模拟进行单基因杂交，以及分析核型以识别染色体异常。IB 内部评估可能涉及设计实验探究影响 DNA 提取或突变率的因素。

Use chi-squared tests to determine if observed phenotypic ratios fit Mendelian expectations. Understand the use of gel electrophoresis in DNA profiling and gene cloning. Interpret results involving restriction fragment length polymorphisms (RFLPs).

运用卡方检验判断观察到的表现型比率是否符合孟德尔预期。理解凝胶电泳在 DNA 指纹分析和基因克隆中的应用。解读涉及限制性片段长度多态性 (RFLP) 的结果。

Review past paper questions on genetic technology, ethical dilemmas, and pedigree probability calculations. Both syllabi value the ability to apply knowledge to novel contexts.

复习关于基因技术、伦理困境和系谱概率计算的历年试题。两种课程体系都注重将知识应用于新情境的能力。

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