📚 AQA A-Level Biology: Genetics Key Points | AQA A-Level 生物学：遗传学考点精讲
Genetics is a central topic in AQA A-Level Biology, exploring how hereditary information is stored, transmitted, and expressed. From the structure of DNA to population genetics, mastering these concepts is essential for exam success. This revision guide covers all major genetics subtopics with clear explanations and bilingual support.
遗传学是 AQA A-Level 生物学的核心主题，探讨遗传信息如何储存、传递和表达。从 DNA 的结构到群体遗传学，掌握这些概念是考试成功的关键。本复习指南以清晰的中英双语讲解覆盖所有遗传学主要考点。
1. Structure of DNA and RNA | DNA 与 RNA 的结构
DNA is a double-stranded polynucleotide formed by monomers called nucleotides. Each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base (adenine, thymine, cytosine, or guanine). The two strands run antiparallel (5′ to 3′ and 3′ to 5′) and are held together by hydrogen bonds between complementary base pairs: A pairs with T (two H-bonds), and C pairs with G (three H-bonds).
DNA 是一种双链多聚核苷酸，由称为核苷酸的单体组成。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、胸腺嘧啶、胞嘧啶或鸟嘌呤）。两条链反向平行（5′→3′和 3′→5′），并通过互补碱基对之间的氢键连接：A 与 T 配对（两个氢键），C 与 G 配对（三个氢键）。
RNA is usually single-stranded and contains ribose sugar instead of deoxyribose, and uracil (U) replaces thymine. There are three main types: mRNA (carries genetic code), tRNA (transfers amino acids), and rRNA (forms ribosomes).
RNA 通常是单链，含有核糖而非脱氧核糖，尿嘧啶（U）取代胸腺嘧啶。主要有三种类型：mRNA（携带遗传密码）、tRNA（转运氨基酸）和 rRNA（构成核糖体）。
2. DNA Replication | DNA 复制
DNA replication occurs during the S phase of the cell cycle and is semi-conservative, meaning each new DNA molecule contains one old strand and one newly synthesised strand. The enzyme DNA helicase unwinds the double helix and breaks hydrogen bonds, forming a replication fork.
DNA 复制发生在细胞周期的 S 期，是半保留复制，即每个新 DNA 分子包含一条旧链和一条新合成的链。DNA 解旋酶解开双螺旋并断裂氢键，形成复制叉。
DNA polymerase adds free nucleotides to the exposed template strands in a 5′ to 3′ direction. The leading strand is synthesised continuously; the lagging strand is synthesised in short Okazaki fragments, which are later joined by DNA ligase. RNA primers, laid down by primase, are required for DNA polymerase to start synthesis.
DNA 聚合酶以 5′→3′ 方向将游离核苷酸添加到暴露的模板链上。前导链连续合成；后随链以短的冈崎片段形式合成，随后由 DNA 连接酶连接。DNA 聚合酶启动合成需要由引物酶放置的 RNA 引物。
3. Transcription and Translation | 转录与翻译
Transcription is the synthesis of mRNA from a DNA template. RNA polymerase binds to the promoter region and unwinds the DNA. It assembles an mRNA strand complementary to the template (antisense) strand, replacing thymine with uracil. In eukaryotes, the primary mRNA undergoes splicing: introns are removed and exons are joined to form mature mRNA.
转录是以 DNA 为模板合成 mRNA 的过程。RNA 聚合酶与启动子区域结合并解开 DNA。它组装一条与模板（反义）链互补的 mRNA 链，用尿嘧啶代替胸腺嘧啶。在真核生物中，初级 mRNA 经过剪接：内含子被去除，外显子连接形成成熟 mRNA。
Translation occurs at ribosomes. mRNA codons (triplets of bases) are read, and tRNA molecules with complementary anticodons bring specific amino acids. The ribosome catalyses peptide bond formation between amino acids, building a polypeptide chain until a stop codon is reached. The genetic code is degenerate, meaning most amino acids are specified by more than one codon.
翻译在核糖体上进行。读取 mRNA 的密码子（三个碱基一组），带有互补反密码子的 tRNA 分子将特定的氨基酸带到相应位置。核糖体催化氨基酸之间形成肽键，构建多肽链，直至遇到终止密码子。遗传密码具有简并性，即大多数氨基酸由多个密码子编码。
| Feature / 特征 | Prokaryotes / 原核生物 | Eukaryotes / 真核生物 |
| Pre-mRNA splicing | Absent | Present (introns removed) |
4. Gene Mutations | 基因突变
A gene mutation is a change in the base sequence of DNA. Substitution mutations replace one base with another and may have no effect (silent mutation) due to the degenerate code, or may change a single amino acid (missense) or create a premature stop codon (nonsense).
基因突变是 DNA 碱基序列的改变。置换突变指一个碱基被另一个替换，由于密码子简并性可能无影响（沉默突变），也可能改变一个氨基酸（错义突变）或产生提前终止密码子（无义突变）。
Insertion or deletion mutations cause a frameshift, shifting the reading frame and altering all downstream amino acids, usually resulting in a non-functional protein. Mutations can arise spontaneously during DNA replication or be induced by mutagens such as UV radiation and certain chemicals.
插入或缺失突变引起移码，改变阅读框，导致下游所有氨基酸改变，通常产生无功能蛋白质。突变可在 DNA 复制过程中自发产生，或由诱变剂引起，如紫外线辐射和某些化学物质。
Mutation → altered base sequence → different mRNA codon → altered protein primary structure
突变 → 碱基序列改变 → 不同的 mRNA 密码子 → 蛋白质一级结构改变
5. Meiosis and Genetic Variation | 减数分裂与遗传变异
Meiosis produces haploid gametes from diploid cells, involving two successive divisions. In meiosis I, homologous chromosomes pair up (bivalents) and crossing over occurs at chiasmata, where sections of chromatids are exchanged, creating new allele combinations. Independent segregation of homologous chromosomes during anaphase I further generates genetic diversity.
减数分裂从二倍体细胞产生单倍体配子，涉及两次连续分裂。在减数第一次分裂中，同源染色体配对（二价体），并在交叉处发生交换，交换染色单体片段，产生新的等位基因组合。后期 I 同源染色体的独立分配进一步产生遗传多样性。
Random fertilisation also contributes to variation, as any male gamete can fuse with any female gamete. The number of possible chromosome combinations from independent assortment alone is 2ⁿ, where n is the haploid number. For humans (n = 23), this gives over 8 million possibilities, not even including crossing over.
随机受精也促成变异，因为任何雄配子都能与任何雌配子融合。仅独立分配产生的染色体组合数就为 2ⁿ，其中 n 为单倍体数。人类 n = 23，产生的组合超过 800 万种，这还不包括交叉互换。
6. Monohybrid and Dihybrid Inheritance | 单基因与双基因遗传
A monohybrid cross involves one gene with two alleles. Dominant alleles mask the effect of recessive alleles in heterozygotes. Using a Punnett square, crossing two heterozygous individuals (Tt × Tt) yields a phenotypic ratio of 3:1 (dominant : recessive). A test cross with a homozygous recessive individual can reveal an unknown genotype.
单基因杂交涉及一个基因的两个等位基因。显性等位基因在杂合子中遮盖隐性等位基因的效应。使用庞纳特方格，杂合子互交（Tt × Tt）产生的表型比为 3:1（显性 : 隐性）。与隐性纯合子的测交可以揭示未知基因型。
A dihybrid cross examines two genes on different chromosomes. The expected F₂ phenotypic ratio for heterozygous parents (AaBb × AaBb) with independent assortment is 9:3:3:1. For example, in pea plants, round (R) and yellow (Y) are dominant: 9/16 round yellow, 3/16 round green, 3/16 wrinkled yellow, 1/16 wrinkled green.
双基因杂交研究位于不同染色体上的两个基因。若独立分配，杂合亲本（AaBb × AaBb）的预期 F₂ 表型比为 9:3:3:1。例如，在豌豆中，圆粒（R）和黄色（Y）为显性：9/16 圆黄，3/16 圆绿，3/16 皱黄，1/16 皱绿。
| Genotypes / 基因型 | Phenotypic ratio / 表型比 |
| R_Y_ | 9 round, yellow |
| R_yy | 3 round, green |
| rrY_ | 3 wrinkled, yellow |
| rryy | 1 wrinkled, green |
7. Linkage and Crossing Over | 连锁与交叉互换
When two genes are located on the same chromosome, they are linked and do not assort independently. Without crossing over, parents produce only parental-type gametes. Crossing over during prophase I can separate linked alleles, producing recombinant gametes. The frequency of recombination is used to map gene loci: a recombination frequency of 1% equals 1 map unit, or centimorgan.
当两个基因位于同一条染色体上时，它们连锁且不独立分配。没有交叉互换时，亲本只产生亲本型配子。前期 I 的交叉互换可分离连锁等位基因，产生重组型配子。重组频率可用于定位基因座：1% 的重组频率相当于 1 个图距单位，或厘摩。
For example, in a test cross involving linked genes, if 12% of offspring are recombinant, the genes are 12 map units apart. Linkage reduces the number of recombinant phenotypes, and observed ratios deviate from the expected 1:1:1:1 for an unlinked test cross.
例如，在涉及连锁基因的测交中，如果 12% 的子代是重组型，则基因相距 12 个图距单位。连锁减少了重组表型的数量，观察到的比值偏离非连锁测交预期的 1:1:1:1。
Recombination frequency (%) = (Number of recombinant offspring / Total offspring) × 100
重组频率（%）=（重组子代数 / 总子代数）× 100
8. Genetic Diversity and Hardy–Weinberg | 遗传多样性与哈代–温伯格定律
Genetic diversity is the total number of different alleles in a population. It can be increased by mutations, gene flow (migration), and meiosis (independent assortment and crossing over). A reduction in genetic diversity occurs through genetic drift, population bottlenecks, and selective breeding.
遗传多样性指群体中所有不同等位基因的总数。它可通过突变、基因流动（迁移）和减数分裂（独立分配与交叉互换）增加。遗传漂变、种群瓶颈和选择性育种则会导致遗传多样性降低。
The Hardy–Weinberg principle provides a mathematical model to predict allele and genotype frequencies in a non-evolving population. For a gene with two alleles, A (frequency p) and a (frequency q):
哈代–温伯格定律提供了一种数学模型，用于预测非进化群体中的等位基因和基因型频率。对于一个有两个等位基因的基因，A（频率 p）和 a（频率 q）：
p + q = 1
p² + 2pq + q² = 1
where p² = frequency of homozygous dominant (AA), 2pq = frequency of heterozygotes (Aa), q² = frequency of homozygous recessive (aa). The Hardy–Weinberg conditions are: large population size, random mating, no mutations, no migration, and no natural selection.
其中 p² = 显性纯合子（AA）的频率，2pq = 杂合子（Aa）的频率，q² = 隐性纯合子（aa）的频率。哈代–温伯格条件包括：群体足够大、随机交配、无突变、无迁移、无自然选择。
For example, if 1 in 10,000 individuals has a recessive condition (q² = 0.0001), then q = 0.01, p = 0.99, and the heterozygous carrier frequency is 2pq = 0.0198, about 2%.
例如，如果每 10,000 个人中有 1 人患有隐性遗传病（q² = 0.0001），则 q = 0.01，p = 0.99，杂合子携带者频率为 2pq = 0.0198，约 2%。
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