📚 IB AQA Science: Genetics Key Points Review | IB AQA 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 and how differences arise. This article covers essential concepts in genetics aligned with IB and AQA science specifications, from Mendelian principles to modern biotechnology. Understanding these key points will help you master the fundamentals and apply them to problem-solving.
遗传学是研究遗传与变异的学科，阐述性状如何从亲代传递给子代以及差异如何产生。本文涵盖 IB 与 AQA 科学课程中遗传学的核心概念，从孟德尔原理到现代生物技术。掌握这些考点有助于理解基本原理并应用于解题。
1. Mendelian Genetics | 孟德尔遗传学
Gregor Mendel conducted experiments on pea plants and established the fundamental laws of inheritance. He observed that traits are determined by discrete units (now called genes) that segregate during gamete formation.
孟德尔通过豌豆实验提出了遗传的基本定律。他发现性状由离散的单位（现称基因）决定，这些单位在配子形成过程中发生分离。
His law of segregation states that each individual carries two alleles for a trait, and these alleles separate during meiosis so that each gamete receives only one allele. The law of independent assortment states that alleles for different traits are distributed to gametes independently of one another, provided the genes are on different chromosomes.
他的分离定律指出，每个个体对于某一性状携带两个等位基因，这些等位基因在减数分裂时分离，使得每个配子只获得一个等位基因。自由组合定律表明，不同性状的等位基因在配子中彼此独立分配，前提是这些基因位于不同的染色体上。
2. Alleles and Genotypes | 等位基因与基因型
An allele is a variant form of a gene. Organisms inherit two alleles for each gene, one from each parent. Dominant alleles mask the effect of recessive alleles in heterozygous individuals.
等位基因是基因的变体形式。生物体的每个基因都从亲代各继承一个等位基因。显性等位基因在杂合子个体中会掩盖隐性等位基因的效应。
The genotype is the genetic makeup of an organism (e.g., AA, Aa, aa), while the phenotype is the observable characteristic. Homozygous individuals have two identical alleles (AA or aa); heterozygous individuals have two different alleles (Aa).
基因型是生物体的遗传组成（如 AA、Aa、aa），表型则是可观察到的特征。纯合子个体有两个相同的等位基因（AA 或 aa）；杂合子个体有两个不同的等位基因（Aa）。
3. Monohybrid Crosses | 单因子杂交
A monohybrid cross investigates the inheritance of a single trait. When crossing two heterozygous parents (Aa x Aa), the expected genotypic ratio in the offspring is 1 AA : 2 Aa : 1 aa.
单因子杂交研究单一性状的遗传。当两个杂合亲本（Aa × Aa）杂交时，子代预期的基因型比例为 1 AA : 2 Aa : 1 aa。
Phenotypic ratio: 3 dominant : 1 recessive
表型比例为 3 显性 : 1 隐性。
Punnett squares are used to predict the combinations of alleles in offspring. They illustrate how segregation of alleles during gamete formation leads to these ratios.
旁氏表（Punnett square）用于预测子代中等位基因的组合。它们展示了配子形成时等位基因的分离如何导致这些比例。
4. Dihybrid Crosses & Independent Assortment | 双因子杂交与自由组合
A dihybrid cross examines the inheritance of two traits simultaneously. For a heterozygote cross (AaBb x AaBb) where the genes are unlinked, the phenotypic ratio is 9 : 3 : 3 : 1.
双因子杂交同时研究两个性状的遗传。对于无连锁的杂合子杂交（AaBb × AaBb），表型比例为 9 : 3 : 3 : 1。
9 dominant-dominant : 3 dominant-recessive : 3 recessive-dominant : 1 recessive-recessive
9 显性-显性 : 3 显性-隐性 : 3 隐性-显性 : 1 隐性-隐性。
This ratio arises because each pair of alleles assorts independently during meiosis, a principle confirmed by the random alignment of homologous chromosomes at metaphase I.
这一比例的出现是因为每对等位基因在减数分裂时独立分配，这一原理由同源染色体在中期 I 的随机排列所证实。
5. DNA Structure | DNA 结构
DNA (deoxyribonucleic acid) is a double helix composed of two antiparallel strands of nucleotides. Each nucleotide contains a phosphate group, a deoxyribose sugar, and a nitrogenous base (adenine, thymine, cytosine, or guanine).
DNA（脱氧核糖核酸）是由两条反向平行的核苷酸链构成的双螺旋。每个核苷酸含有一个磷酸基团、一个脱氧核糖和一个含氮碱基（腺嘌呤、胸腺嘧啶、胞嘧啶或鸟嘌呤）。
The strands are held together by hydrogen bonds between complementary base pairs: adenine pairs with thymine (A-T, two hydrogen bonds), and cytosine pairs with guanine (C-G, three hydrogen bonds). The sugar-phosphate backbones run in opposite directions, designated 5′ to 3′ and 3′ to 5′.
两条链通过互补碱基对之间的氢键相连：腺嘌呤与胸腺嘧啶配对（A-T，两个氢键），胞嘧啶与鸟嘌呤配对（C-G，三个氢键）。糖-磷酸骨架以相反方向排列，分别记为 5′ 至 3′ 和 3′ 至 5’。
6. DNA Replication | DNA 复制
DNA replication is semiconservative, meaning each new DNA molecule consists of one original strand and one newly synthesized strand. The enzyme helicase unwinds the double helix, and DNA polymerase adds complementary nucleotides to the template strands.
DNA 复制是半保留的，即每个新的 DNA 分子由一条原始链和一条新合成的链组成。解旋酶解开双螺旋，DNA 聚合酶将互补核苷酸添加到模板链上。
Replication occurs in the 5′ to 3′ direction. The leading strand is synthesized continuously, while the lagging strand is synthesized in short Okazaki fragments that are later joined by DNA ligase.
复制沿 5′ 至 3′ 方向进行。前导链连续合成，滞后链则以短的冈崎片段合成，随后由 DNA 连接酶连接。
7. Gene Expression: Transcription & Translation | 基因表达：转录与翻译
Gene expression converts DNA instructions into functional products. Transcription produces messenger RNA (mRNA) from a DNA template. RNA polymerase binds to the promoter region and synthesizes a single-stranded mRNA complementary to the template strand. In eukaryotes, the pre-mRNA is processed: a 5′ cap is added, introns are removed, and a poly-A tail is attached.
基因表达将 DNA 指令转化为功能性产物。转录以 DNA 为模板产生信使 RNA（mRNA）。RNA 聚合酶结合启动子区域，合成一条与模板链互补的单链 mRNA。在真核生物中，前体 mRNA 需要加工：添加 5′ 帽、切除内含子并加上多聚 A 尾。
Translation occurs on ribosomes. Transfer RNA (tRNA) molecules carry amino acids and recognize mRNA codons through their anticodons. Polypeptide chains form as ribosomes move along the mRNA, linking amino acids until a stop codon is reached.
翻译在核糖体上进行。转移 RNA（tRNA）携带氨基酸，并通过其反密码子识别 mRNA 上的密码子。随着核糖体沿 mRNA 移动，氨基酸被连接形成多肽链，直至遇到终止密码子。
8. Mutations | 突变
A mutation is a change in the DNA sequence. Point mutations include substitutions (silent, missense, nonsense) and frameshift mutations caused by insertions or deletions, which shift the reading frame and often drastically alter the protein.
突变是 DNA 序列的改变。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，后者会改变阅读框并通常严重改变蛋白质结构。
Mutations can be spontaneous or induced by mutagens such as UV radiation and certain chemicals. Some mutations are neutral, others cause genetic disorders, and occasionally they provide a selective advantage, driving evolution.
突变可以是自发的，也可以由诱变剂（如紫外线和某些化学物质）诱发。有些突变是中性的，有些导致遗传病，偶尔也能提供选择优势，推动进化。
9. Genetic Variation & Meiosis | 遗传变异与减数分裂
Genetic variation arises from several mechanisms during meiosis. Crossing over between homologous chromosomes in prophase I exchanges segments of DNA, creating new allele combinations. Independent assortment shuffles the maternal and paternal chromosomes randomly.
遗传变异来源于减数分裂中的多种机制。前期 I 同源染色体之间的交叉互换交换 DNA 片段，产生新的等位基因组合。自由组合则随机分配母本和父本染色体。
Furthermore, random fertilisation ensures each zygote has a unique genetic makeup. These processes, together with mutations, are the raw material for natural selection.
此外，随机受精确保每个合子拥有独特的遗传组成。这些过程与突变一起，为自然选择提供原材料。
10. Genetic Disorders | 遗传病
Many inherited disorders result from mutations in single genes. They follow Mendelian inheritance patterns: autosomal recessive, autosomal dominant, X-linked recessive, etc.
许多遗传病是由单基因突变引起的，遵循孟德尔遗传模式：常染色体隐性、常染色体显性、X 连锁隐性等。
| Disorder / 疾病 | Inheritance Pattern / 遗传模式 | Key Feature / 主要特征 |
|---|---|---|
| Cystic fibrosis / 囊性纤维化 | Autosomal recessive / 常染色体隐性 | Thick mucus in lungs and pancreas / 肺部与胰腺黏液黏稠 |
| Sickle cell anaemia / 镰刀型细胞贫血 | Autosomal recessive / 常染色体隐性 | Abnormal haemoglobin causes sickle-shaped red cells / 异常血红蛋白导致红细胞呈镰刀状 |
| Huntington’s disease / 亨廷顿病 | Autosomal dominant / 常染色体显性 | Progressive neurodegeneration / 渐进性神经退行性病变 |
| Haemophilia / 血友病 | X-linked recessive / X 连锁隐性 | Blood fails to clot normally / 血液无法正常凝固 |
Genetic testing and pedigree analysis can identify carriers and affected individuals, allowing for informed family planning.
基因检测与系谱分析可以识别携带者和患者，有助于进行知情生育决策。
11. Genetic Engineering & Biotechnology | 基因工程与生物技术
Genetic engineering involves modifying an organism’s genome using recombinant DNA technology. A gene of interest is cut out using restriction enzymes, inserted into a vector (e.g., plasmid), and introduced into a host cell. Bacteria can then produce the desired protein, such as human insulin.
基因工程利用重组 DNA 技术修改生物体的基因组。用限制酶切取目的基因，插入载体（如质粒），再导入宿主细胞。细菌随后可产生所需的蛋白质，例如人胰岛素。
PCR (polymerase chain reaction) amplifies specific DNA segments, while gel electrophoresis separates DNA fragments by size. Modern tools like CRISPR-Cas9 enable precise genome editing, offering potential therapies for genetic diseases.
PCR（聚合酶链式反应）可扩增特定的 DNA 片段，凝胶电泳则根据大小分离 DNA 片段。CRISPR-Cas9 等现代工具能够进行精准的基因组编辑，为遗传病的治疗提供了可能。
12. Cloning and Stem Cells | 克隆与干细胞
Cloning produces genetically identical copies of an organism. Reproductive cloning through somatic cell nuclear transfer (SCNT) involves transferring the nucleus of a body cell into an enucleated egg cell. This was used to create Dolly the sheep.
克隆产生基因完全相同的生物副本。通过体细胞核移植（SCNT）进行生殖性克隆，将体细胞的细胞核移植到去核卵细胞中。多利羊即是利用此技术诞生的。
Stem cells are unspecialised cells capable of self-renewal and differentiation. Embryonic stem cells are pluripotent and can develop into any cell type, while adult stem cells are multipotent. Stem cell therapies hold promise for repairing damaged tissues.
干细胞是能够自我更新和分化的未特化细胞。胚胎干细胞具有多能性，可发育成任何细胞类型；成体干细胞则为多潜能细胞。干细胞疗法在修复受损组织方面具有广阔前景。
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