Genetics Revision for A-Level WJEC Science | A-Level WJEC 科学：遗传考点精讲

📚 Genetics Revision for A-Level WJEC Science | A-Level WJEC 科学：遗传考点精讲

Genetics forms the backbone of modern biology, linking molecular mechanisms with inheritance patterns and evolution. For A-Level WJEC Science, a deep understanding of DNA structure, protein synthesis, genetic variation, and population genetics is essential. This bilingual revision guide provides a structured, point-by-point walkthrough of the core topics, helping you consolidate knowledge and excel in your examinations.

遗传学是现代生物学的支柱，它将分子机制与遗传模式和进化联系起来。针对A-Level WJEC科学考试，深入理解DNA结构、蛋白质合成、遗传变异和群体遗传学至关重要。这本双语复习指南提供了逐点讲解的核心主题，帮助你巩固知识，在考试中取得优异成绩。

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

Deoxyribonucleic acid (DNA) is a double-stranded polynucleotide. Each strand is composed of nucleotides containing deoxyribose sugar, a phosphate group, and a nitrogenous base (adenine, thymine, cytosine, guanine). The two strands are antiparallel (5′ → 3′ and 3′ → 5′) and form a double helix stabilized by hydrogen bonds between complementary bases: A pairs with T (two hydrogen bonds), G with C (three hydrogen bonds).

脱氧核糖核酸（DNA）是双链多核苷酸。每条链由含有脱氧核糖、磷酸基团和含氮碱基（腺嘌呤、胸腺嘧啶、胞嘧啶、鸟嘌呤）的核苷酸组成。两条链反向平行（5′ → 3′ 和 3′ → 5′）并形成双螺旋，双螺旋通过互补碱基之间的氢键保持稳定：A与T配对（两个氢键），G与C配对（三个氢键）。

DNA carries genetic information in the sequence of bases.
DNA以碱基序列携带遗传信息。
The sugar–phosphate backbone provides structural support.
糖-磷酸骨架提供结构支持。
Histone proteins package DNA into chromosomes in eukaryotes.
在真核生物中，组蛋白将DNA包装成染色体。

2. DNA Replication | DNA复制

DNA replication is semiconservative: each new DNA molecule contains one original strand and one newly synthesised strand. The enzyme DNA helicase unwinds the double helix by breaking hydrogen bonds, creating a replication fork. Primase synthesises short RNA primers. DNA polymerase III adds free nucleotides in the 5′ → 3′ direction, using the parent strand as a template. The leading strand is synthesised continuously; the lagging strand is synthesised in Okazaki fragments, later joined by DNA ligase.

DNA复制是半保留的：每个新DNA分子包含一条原始链和一条新合成的链。DNA解旋酶通过断裂氢键解开双螺旋，形成复制叉。引物酶合成短的RNA引物。DNA聚合酶III以亲本链为模板，沿5′ → 3′方向添加游离核苷酸。前导链连续合成；滞后链以冈崎片段合成，随后由DNA连接酶连接。

Proofreading by DNA polymerase corrects mismatches.
DNA聚合酶的校对功能纠正错配。
Replication occurs during the S phase of the cell cycle.
复制发生在细胞周期的S期。
Key enzymes: helicase, topoisomerase, primase, polymerase, ligase.
关键酶：解旋酶、拓扑异构酶、引物酶、聚合酶、连接酶。

3. Protein Synthesis: Transcription | 蛋白质合成：转录

Transcription is the synthesis of messenger RNA (mRNA) from a DNA template. RNA polymerase binds to the promoter region, unwinds the DNA, and assembles RNA nucleotides complementary to the template strand. U replaces T in RNA. The mRNA strand is synthesised in the 5′ → 3′ direction. In eukaryotes, the primary transcript undergoes post-transcriptional modification: addition of a 5′ cap, a poly-A tail, and splicing to remove introns, leaving exons.

转录是以DNA为模板合成信使RNA（mRNA）的过程。RNA聚合酶与启动子区域结合，解开DNA，并组装与模板链互补的RNA核苷酸。RNA中U取代T。mRNA链沿5′ → 3′方向合成。在真核生物中，初级转录本经历转录后修饰：添加5′帽、poly-A尾，并通过剪接去除内含子，留下外显子。

Only one strand of DNA (the template/antisense strand) is transcribed.
只有一条DNA链（模板链/反义链）被转录。
Transcription occurs in the nucleus (eukaryotes).
转录发生在细胞核（真核生物）。
Alternative splicing can produce different proteins from one gene.
可变剪接可以从一个基因产生不同的蛋白质。

4. Protein Synthesis: Translation | 蛋白质合成：翻译

Translation decodes the mRNA sequence into a polypeptide at the ribosome. Transfer RNA (tRNA) molecules carry specific amino acids and have anticodons complementary to mRNA codons. The ribosome (composed of rRNA and proteins) has three sites: A (aminoacyl), P (peptidyl), and E (exit). Initiation starts at the AUG start codon. Elongation adds amino acids via peptide bond formation. Termination occurs when a stop codon (UAA, UAG, UGA) enters the A site, and release factors trigger disassembly.

翻译在核糖体上将mRNA序列解码为多肽。转运RNA（tRNA）分子携带特定氨基酸，并具有与mRNA密码子互补的反密码子。核糖体（由rRNA和蛋白质组成）有三个位点：A位（氨酰位）、P位（肽基位）和E位（出口位）。起始始于AUG起始密码子。延伸通过肽键形成添加氨基酸。当终止密码子（UAA、UAG、UGA）进入A位点时，释放因子触发解体，翻译终止。

The genetic code is non-overlapping and degenerate.
遗传密码是不重叠的，并具有简并性。
Multiple ribosomes (polysomes) can translate a single mRNA simultaneously.
多个核糖体（多聚核糖体）可以同时翻译一条mRNA。

5. The Genetic Code | 遗传密码

The genetic code is a set of rules by which nucleotide triplets (codons) specify amino acids. It is read in a 5′ → 3′ direction on mRNA. Features include: degenerate (more than one codon per amino acid), universal (shared across almost all organisms), and unambiguous (each codon codes for only one amino acid). Start codon AUG codes for methionine. Three stop codons signal termination.

遗传密码是一套规定核苷酸三联体（密码子）对应特定氨基酸的规则。它在mRNA上按5′ → 3′方向读取。特点包括：简并性（同一种氨基酸有多个密码子）、通用性（几乎所有生物共享同一套密码）、无歧义性（每个密码子只编码一种氨基酸）。起始密码子AUG编码甲硫氨酸。三个终止密码子发出终止信号。

Codon示例	Amino Acid 氨基酸	Role 作用
AUG	Methionine	Start 起始
UUU, UUC	Phenylalanine	—
UAA, UAG, UGA	None 无	Stop 终止

6. Mutations and Their Effects | 突变及其影响

Mutations are permanent changes in the DNA sequence. Point mutations include substitution (silent, missense, nonsense), insertion, and deletion. Frameshift mutations (insertions/deletions) alter the reading frame, often resulting in completely different amino acid sequences. Chromosomal mutations involve large-scale changes (deletion, duplication, inversion, translocation). Mutagens such as UV light, chemicals, and radiation increase mutation rates.

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）、插入和缺失。移码突变（插入/缺失）改变阅读框，通常导致完全不同的氨基酸序列。染色体突变涉及大规模变化（缺失、重复、倒位、易位）。诱变剂如紫外线、化学物质和辐射会增加突变率。

Silent mutations do not alter the amino acid due to degeneracy.
由于密码简并性，沉默突变不会改变氨基酸。
Sickle cell anaemia results from a single substitution (Glu → Val).
镰刀型细胞贫血症由单个碱基替换引起（谷氨酸 → 缬氨酸）。
Mutations are a source of genetic variation for evolution.
突变是进化中遗传变异的来源。

7. Mendelian Genetics: Monohybrid and Dihybrid Crosses | 孟德尔遗传：单杂交与双杂交

Mendel’s laws describe how alleles segregate and assort independently. The law of segregation states that allele pairs separate during gamete formation. The law of independent assortment states that genes for different traits are distributed independently. Monohybrid crosses follow one gene (e.g., Tt × Tt gives a 3:1 phenotypic ratio). Dihybrid crosses follow two unlinked genes (e.g., TtYy × TtYy gives a 9:3:3:1 ratio). Use Punnett squares for predictions.

孟德尔定律描述了等位基因如何分离和自由组合。分离定律指出等位基因对在配子形成时分离。自由组合定律指出不同性状的基因独立分配。单杂交遵循一个基因（例如Tt × Tt产生3:1的表型比率）。双杂交遵循两个非连锁基因（例如TtYy × TtYy产生9:3:3:1的比率）。使用庞纳特方格进行预测。

Dominant alleles mask recessive alleles in heterozygotes.
显性等位基因在杂合子中掩盖隐性等位基因。
Test cross: cross an unknown dominant phenotype with homozygous recessive.
测交：将未知显性表型与纯合隐性杂交。
Autosomal vs. sex-linked traits.
常染色体性状与伴性性状。

8. Sex-linked Inheritance | 伴性遗传

Sex-linked traits are associated with genes on the sex chromosomes (X or Y). In humans, most sex-linked disorders are X-linked recessive (e.g., haemophilia, red–green colour blindness). Males (XY) express the trait if they inherit one recessive allele, while females (XX) need two recessive alleles. The cross of a heterozygous female (carrier) with a normal male results in sons having a 50% chance of being affected.

伴性性状与性染色体（X或Y）上的基因相关。在人类中，大多数伴性遗传病是X连锁隐性遗传（如血友病、红绿色盲）。男性（XY）若遗传一个隐性等位基因即表现性状，而女性（XX）需要两个隐性等位基因。杂合女性（携带者）与正常男性杂交，儿子有50%的概率患病。

Y-linked traits are passed from father to all sons only.
Y连锁性状仅由父亲传给所有儿子。
Pedigree analysis reveals patterns of inheritance.
谱系分析揭示遗传模式。

9. Genetic Variation: Meiosis and Crossing Over | 遗传变异：减数分裂与交换

Meiosis produces genetically diverse haploid gametes. In prophase I, homologous chromosomes pair (synapsis) and crossing over occurs, exchanging alleles between non-sister chromatids. This creates new combinations of alleles. Independent assortment of chromosomes during metaphase I further increases variation. Random fertilisation also contributes. The resulting genetic variation is the raw material for natural selection.

减数分裂产生遗传多样的单倍体配子。在前期I，同源染色体配对（联会）并发生交换，非姐妹染色单体之间交换等位基因。这创造了新的等位基因组合。中期I染色体的独立分配进一步增加了变异。随机受精也作出贡献。产生的遗传变异是自然选择的原材料。

In humans, 2²³ possible chromosome combinations per gamete (without crossing over).
在人类中，每个配子有2²³种可能的染色体组合（不含交换）。
Chiasmata are visible points of crossing over.
交叉点是可见的交换位点。
Gene linkage reduces variety unless crossing over separates them.
基因连锁减少多样性，除非交换将它们分开。

10. Natural Selection and Evolution | 自然选择与进化

Natural selection acts on phenotypic variation. Individuals with advantageous alleles are more likely to survive and reproduce, passing those alleles to the next generation. Over time, allele frequencies in a population change, leading to adaptation and evolution. Types of selection include stabilising, directional, and disruptive. The Hardy–Weinberg principle provides a mathematical model for allele frequencies in a non-evolving population:

自然选择作用于表型变异。具有有利等位基因的个体更有可能生存和繁殖，将这些等位基因传递给下一代。随着时间的推移，群体中的等位基因频率发生变化，导致适应和进化。选择类型包括稳定选择、方向选择和分裂选择。哈代-温伯格原理为非进化群体中的等位基因频率提供了数学模型：

(p + q)² = p² + 2pq + q² = 1

where p = frequency of dominant allele, q = frequency of recessive allele.

其中p = 显性等位基因的频率，q = 隐性等位基因的频率。

Conditions for Hardy–Weinberg: no mutation, random mating, no gene flow, large population, no selection.
哈代-温伯格条件：无突变、随机交配、无基因流动、大群体、无选择。
Speciation occurs when populations become reproductively isolated.
当群体生殖隔离时，物种形成就发生了。
Allopatric (geographic) vs. sympatric speciation.
异域（地理）物种形成与同域物种形成。

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

Genetic engineering involves direct manipulation of an organism’s genome. Recombinant DNA technology uses restriction enzymes to cut DNA at specific sites, and ligase to join fragments. Vectors (e.g., plasmids) introduce foreign DNA into host cells. Applications include producing human insulin in bacteria, gene therapy, and genetically modified (GM) crops with pest resistance or enhanced nutrition. PCR (polymerase chain reaction) amplifies DNA in vitro using Taq polymerase and thermal cycling.

基因工程涉及对生物体基因组的直接操作。重组DNA技术使用限制性内切酶在特定位点切割DNA，用连接酶连接片段。载体（如质粒）将外源DNA引入宿主细胞。应用包括在细菌中生产人胰岛素、基因治疗以及具有抗虫性或提高营养的转基因作物。PCR（聚合酶链式反应）利用Taq聚合酶和热循环在体外扩增DNA。

Sticky ends allow complementary pairing of DNA fragments.
粘性末端允许DNA片段的互补配对。
Electrophoresis separates DNA fragments by size.
电泳根据大小分离DNA片段。
Ethical considerations: GMO labelling, biodiversity impact.
伦理考量：转基因标签、生物多样性影响。

12. Exam Tips for WJEC Genetics | WJEC遗传学考试技巧

WJEC A-Level questions often test application of knowledge to unfamiliar contexts. Practise interpreting pedigree charts, calculating Hardy–Weinberg problems, and predicting outcomes of genetic crosses. Pay attention to command words: ‘describe’ (state facts), ‘explain’ (give reasons), ‘suggest’ (apply reasoning). Use correct terminology: allele, locus, homozygote, heterozygote, phenotype, genotype. In essays, link genetic concepts to broader themes like evolution and disease.

WJEC A-Level问题经常考查将知识应用于不熟悉情境的能力。练习解读谱系图、计算哈代-温伯格问题以及预测遗传杂交的结果。注意指令词：’describe’（陈述事实）、’explain’（给出原因）、’suggest’（应用推理）。使用正确的术语：等位基因、基因座、纯合子、杂合子、表型、基因型。在论文中，将遗传学概念与进化、疾病等更广泛的主题联系起来。

Draw clear Punnett squares with gametes indicated.
绘制清晰的庞纳特方格，并注明配子。
Label DNA diagrams accurately (3′ and 5′ ends, sugars, bases).
准确标注DNA示意图（3′和5′端、糖、碱基）。
Manage time: spend proportionally to mark allocation.
时间管理：根据分值合理分配时间。
Review practical techniques: gel electrophoresis, PCR, bacterial transformation.
复习实验技术：凝胶电泳、PCR、细菌转化。

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Genetics Revision for A-Level WJEC Science | A-Level WJEC 科学：遗传 考点精讲