📚 IGCSE CIE Biology: Genetics Key Points | IGCSE CIE 生物：遗传学考点精讲
Genetics lies at the heart of modern biology, explaining how characteristics are passed from one generation to the next and why individuals within a species show variation. This article distils the essential content for the IGCSE CIE Biology syllabus, covering DNA structure, cell division, monohybrid crosses, codominance, sex-linked inheritance, mutation and natural selection. A clear grasp of these key points will strengthen your exam performance and reveal the beauty of inheritance.
遗传学是现代生物学的核心，它解释了性状如何代代相传，以及同一物种的个体为何存在差异。本文浓缩了 IGCSE CIE 生物大纲的核心内容，涵盖 DNA 结构、细胞分裂、单基因杂交、共显性、性连锁遗传、突变与自然选择。清晰掌握这些考点将助力你的考试表现，让你领略遗传之美。
1. DNA, Genes and Chromosomes | DNA、基因与染色体
The genetic material is deoxyribonucleic acid (DNA), a double helix composed of nucleotide monomers. Each nucleotide consists of a deoxyribose sugar, a phosphate group and one of four nitrogenous bases: adenine (A), thymine (T), cytosine (C) and guanine (G). Base pairing follows strict rules – A pairs with T via two hydrogen bonds, and C pairs with G via three hydrogen bonds.
遗传物质是脱氧核糖核酸 (DNA)，由核苷酸单体构成的双螺旋结构。每个核苷酸包含一个脱氧核糖、一个磷酸基团及四种含氮碱基之一：腺嘌呤 (A)、胸腺嘧啶 (T)、胞嘧啶 (C) 和鸟嘌呤 (G)。碱基配对遵循严格规律：A 与 T 以两个氢键配对，C 与 G 以三个氢键配对。
A gene is a specific length of DNA that codes for a particular protein (or polypeptide). Genes are arranged along chromosomes, which are long, coiled molecules of DNA wrapped around histone proteins. In the nucleus of a typical human body cell, there are 46 chromosomes organised into 23 homologous pairs – one set inherited from each parent. This is the diploid number (2n = 46). Gametes (sperm and egg cells) are haploid (n = 23), containing only one set of chromosomes so that fertilisation restores the diploid number.
基因是一段特定长度的 DNA，编码特定的蛋白质（或多肽）。基因沿染色体排列，染色体是缠绕在组蛋白上的长链 DNA 分子。在人体细胞的细胞核中，有 46 条染色体，构成 23 对同源染色体——每对分别来自父方和母方。这是二倍体数目 (2n = 46)。配子（精子与卵细胞）为单倍体 (n = 23)，只含一套染色体，使受精后能恢复二倍体数目。
2. The Cell Cycle and Mitosis | 细胞周期与有丝分裂
Mitosis is a type of nuclear division that produces two genetically identical daughter diploid cells. It is used for growth, repair of tissues and asexual reproduction. Before mitosis, during the S phase of interphase, each chromosome is replicated to form two identical chromatids joined at the centromere. Mitosis proceeds through prophase (chromosomes condense, nuclear membrane disappears), metaphase (chromosomes line up at the equator, spindle fibres attach), anaphase (chromatids are pulled to opposite poles) and telophase (nuclear membranes reform, chromosomes decondense). Cytokinesis then divides the cytoplasm, resulting in two separate cells with the same chromosome number as the parent.
有丝分裂是一种核分裂，产生两个遗传上完全相同的子代二倍体细胞，用于生长、组织修复和无性繁殖。有丝分裂前，在间期的 S 期，每条染色体复制形成两条相同的染色单体，在着丝粒处相连。分裂过程经历前期（染色体凝缩，核膜消失）、中期（染色体排列在赤道板，纺锤丝附着）、后期（染色单体被拉向两极）和末期（核膜重建，染色体解旋）。随后细胞质分裂完成胞质分裂，形成两个染色体数目与母细胞相同的独立细胞。
3. Meiosis and Gamete Formation | 减数分裂与配子形成
Meiosis reduces the chromosome number by half and produces four genetically non-identical haploid gametes. It involves two successive divisions. In meiosis I, homologous chromosomes pair up (synapsis) and crossing over may occur, exchanging alleles between non-sister chromatids. The homologous pairs then separate, so each daughter cell receives one chromosome from each pair, already halving the number. Meiosis II resembles mitosis: the chromatids of each chromosome are separated. The final result is four haploid cells, each with a unique combination of alleles due to crossing over and independent assortment of chromosomes. These processes are the fundamental sources of genetic variation in sexually reproducing organisms.
减数分裂将染色体数目减半，产生四个遗传上不相同的单倍体配子，包括两次连续分裂。减数第一次分裂中，同源染色体配对（联会），可能发生交叉互换，在非姐妹染色单体间交换等位基因。随后同源染色体分开，每个子细胞得到每对染色体中的一条，数目已减半。减数第二次分裂类似有丝分裂：每条染色体的染色单体分离。最终生成四个单倍体细胞，每个细胞因交叉互换和染色体的独立分配而拥有独特的等位基因组合。这些过程是有性生殖生物遗传变异的根本来源。
4. Key Genetic Terms | 关键遗传术语
Precise genetic vocabulary is vital for constructing accurate explanations. A genotype is the combination of alleles an organism possesses, while the phenotype is the observable characteristic resulting from the genotype and its interaction with the environment. An allele is an alternative form of a gene. If the two alleles at a locus are identical, the organism is homozygous; if they differ, it is heterozygous. A dominant allele is always expressed in the phenotype when present, whereas a recessive allele is only expressed when two copies are present (homozygous recessive). The table below summarises these core terms.
精确的遗传学术语对构建准确解释至关重要。基因型是指生物体拥有的等位基因组合，表现型则是在基因型与环境相互作用下表现出的可观察性状。等位基因是基因的替代形式。若同一位点上的两个等位基因相同，个体为纯合子；若不同，则为杂合子。显性等位基因一旦存在即表现为相应性状，隐性等位基因仅在纯合隐性时表达。下表总结了这些核心术语。
| Term | Definition | 中文术语 | 定义 |
|---|---|---|---|
| Gene | A DNA segment coding for a protein | 基因 | 编码蛋白质的 DNA 片段 |
| Allele | Alternative form of a gene | 等位基因 | 基因的其中一个替代形式 |
| Genotype | The combination of alleles an organism has | 基因型 | 生物拥有的等位基因组合 |
| Phenotype | The observable feature resulting from genotype and environment | 表现型 | 基因型与环境共同作用下的可观察性状 |
| Homozygous | Having two identical alleles (e.g. BB or bb) | 纯合子 | 具有两个相同等位基因 (如 BB 或 bb) |
| Heterozygous | Having two different alleles (e.g. Bb) | 杂合子 | 具有两个不同等位基因 (如 Bb) |
| Dominant | Allele that is always expressed if present | 显性 | 只要存在就一定表达的等位基因 |
| Recessive | Allele only expressed when homozygous | 隐性 | 仅纯合时表达的等位基因 |
5. Monohybrid Inheritance | 单基因遗传
A monohybrid cross follows the inheritance of one gene with two alleles. When pure-breeding (homozygous) parents with contrasting traits are crossed, the F1 offspring are all heterozygous and show the dominant phenotype. Crossing two F1 individuals produces an F2 generation with a typical phenotype ratio of 3 : 1 (dominant : recessive), provided the gene is not sex-linked and dominance is complete.
单基因杂交追踪一对等位基因的遗传。当具有相对性状的纯合亲本杂交，所有 F1 子代为杂合子，表现显性性状。让两个 F1 个体杂交，F2 代出现经典表现型比例 3 : 1（显性:隐性），前提是该基因不在性染色体上且为完全显性。
Below is a Punnett square for a cross between two heterozygous pea plants for height (T = tall, t = short). The gametes combine at random, giving a 1 TT : 2 Tt : 1 tt genotype ratio.
下面是两株杂合豌豆植株（T = 高茎, t = 矮茎）杂交的庞纳特方格。配子随机结合，产生 1 TT : 2 Tt : 1 tt 的基因型比例。
| T | t | |
| T | TT (tall) | Tt (tall) |
| t | Tt (tall) | tt (short) |
Phenotype ratio: 3 Tall : 1 Short
表现型比例：3 高茎 : 1 矮茎
A test cross is used to determine an unknown genotype. An individual showing the dominant phenotype is crossed with a homozygous recessive individual. If all offspring display the dominant trait, the unknown parent is homozygous dominant; if a 1 : 1 ratio of dominant to recessive appears, the parent is heterozygous.
测交用于确定未知基因型。将表现显性性状的个体与隐性纯合子杂交。若所有子代表现显性，则该未知亲本为显性纯合；若出现显性:隐性 1 : 1，则该亲本为杂合。
6. Codominance and Multiple Alleles | 共显性与复等位基因
Codominance occurs when both alleles in a heterozygous organism are expressed equally in the phenotype, rather than one masking the other. The human ABO blood group system is an excellent example, controlled by three alleles at a single gene locus: IA, IB and i. Alleles IA and IB are codominant: a person with genotype IAIB expresses both A and B antigens on red blood cells, giving blood group AB. Allele i is recessive to both IA and IB. The possible genotypes and phenotypes are listed below.
共显性指杂合个体中两个等位基因在表现型上同等表达，没有掩盖现象。人类 ABO 血型系统就是一个绝佳例子，由单一基因位点上的三个等位基因控制：IA、IB 和 i。IA 与 IB 为共显性：基因型 IAIB 的人在红细胞上同时表达 A 和 B 抗原，表现为 AB 血型。等位基因 i 对 IA 和 IB 均为隐性。可能的基因型与表现型列举如下。
| Genotype | Blood Group (Phenotype) | 基因型 | 血型 (表现型) |
|---|---|---|---|
| IAIA or IAi | A | IAIA 或 IAi | A |
| IBIB or IBi | B | IBIB 或 IBi | B |
| IAIB | AB | IAIB | AB |
| ii | O | ii | O |
Another case of codominance in CIE IGCSE is the sickle-cell trait. The normal haemoglobin allele (HbA) and the sickle-cell allele (HbS) are codominant. Heterozygotes (HbAHbS) have some normal and some sickled red blood cells, giving resistance to malaria alongside mild symptoms.
CIE IGCSE 中另一个共显性例子是镰刀形细胞性状。正常血红蛋白等位基因 (HbA) 与镰刀形细胞等位基因 (HbS) 共显性。杂合子 (HbAHbS) 同时拥有正常与镰刀形红细胞，在疟疾流行区具有抗性，同时症状轻微。
7. Sex Determination | 性别决定
In humans and many organisms, sex is determined by a pair of sex chromosomes. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). All egg cells carry a single X chromosome. Sperm cells carry either an X or a Y chromosome with equal probability. At fertilisation, the combination of sperm and egg determines the zygote’s sex: X + X gives female (XX), X + Y gives male (XY). Thus, there is always a 1 : 1 theoretical chance of producing male or female offspring.
在人类及许多生物中，性别由一对性染色体决定。女性有两条 X 染色体 (XX)，男性有一条 X 和一条 Y 染色体 (XY)。所有卵细胞携带一条 X 染色体，精子则以等概率携带 X 或 Y 染色体。受精时，精卵组合决定了合子的性别：X + X 产生女性 (XX)，X + Y 产生男性 (XY)。因此，理论上每次生育男女的概率均为 1 : 1。
Parents: XX (female) × XY (male) → Gametes: all X and ½ X, ½ Y → Offspring: ½ XX (female), ½ XY (male)
亲本：XX (女性) × XY (男性) → 配子：全部 X 与 ½ X, ½ Y → 子代：½ XX (女性), ½ XY (男性)
8. Sex-linked Inheritance | 性连锁遗传
Genes located on the sex chromosomes show a distinct inheritance pattern called sex-linkage. The most common examination example is red-green colour blindness, caused by a recessive allele on the X chromosome. Since males have only one X chromosome, a single recessive allele (Xr) will express colour blindness. Females require two recessive alleles (XrXr) to be colour blind; heterozygous females (XRXr) are carriers with normal vision. The Y chromosome does not carry an equivalent allele. Consequently, sex-linked recessive disorders are far more common in males.
位于性染色体上的基因表现出独特的遗传模式——性连锁。最常见的考试实例是红绿色盲，由 X 染色体上的隐性等位基因引起。男性只有一条 X 染色体，一个隐性等位基因 (Xr) 即可表现色盲。女性需要两个隐性等位基因 (XrXr) 才是色盲；杂合女性 (XRXr) 为携带者，视力正常。Y 染色体无对应等位基因。因此，性连锁隐性遗传病在男性中更为常见。
Consider a cross between a carrier female (XRXr) and a normal male (XRY). The Punnett square shows that 50% of sons are colour blind, while all daughters have at least one normal allele and thus normal vision, though 50% of daughters are carriers.
考虑一个携带者女性 (XRXr) 与正常男性 (XRY) 的杂交。庞纳特方格显示，50% 的儿子为色盲，而所有女儿至少有一个正常等位基因因而视觉正常，但 50% 的女儿是携带者。
| XR | Y | |
| XR | XRXR (normal female) | XRY (normal male) |
| Xr | XRXr (carrier female) | XrY (colour-blind male) |
9. Mutation and Genetic Variation | 突变与遗传变异
A mutation is a random, permanent change in the nucleotide sequence of DNA. Gene mutations occur when a base is substituted, inserted or deleted, potentially altering the amino acid sequence of the encoded protein. Sickle-cell anaemia results from a substitution mutation in the gene for the beta chain of haemoglobin, changing the DNA triplet from GAG to GTG; this replaces glutamic acid with valine, altering the protein’s shape. Chromosome mutations can involve changes in chromosome number or structure. Mutations generate new alleles and therefore increase genetic variation.
突变是指 DNA 核苷酸序列发生的随机、永久性改变。基因突变常由碱基置换、插入或缺失引起，可能改变编码蛋白质的氨基酸序列。镰刀形细胞贫血症就是血红蛋白 β 链基因发生置换突变所致，DNA 三联体由 GAG 变为 GTG，导致谷氨酸被缬氨酸替换，改变了蛋白质形状。染色体突变可涉及染色体数目或结构的改变。突变产生新等位基因，从而增加遗传变异。
Variation within a population can be continuous (e.g. height, body mass), where phenotypes fall along a smooth range, usually influenced by many genes and the environment. Discontinuous variation (e.g. blood group, tongue rolling) shows distinct categories, controlled by a single or few genes with little environmental effect. Both types provide the raw material for natural selection.
种群内的变异可以是连续变异（如身高、体重），表现型呈平滑变化范围，通常受多基因和环境共同影响；也可以是不连续变异（如血型、卷舌），呈现出明显类别，由单个或少数基因控制，环境影响很小。两种变异都为自然选择提供了原始材料。
10. Natural Selection and Evolution | 自然选择与进化
Natural selection is the process by which organisms better adapted to their environment tend to survive and produce more offspring. It acts on existing genetic variation within a population. Antibiotic resistance in bacteria is a clear modern example. In a bacterial population exposed to an antibiotic, a few individuals may already carry a mutation that confers resistance. These survive and reproduce, while susceptible bacteria die. Over generations, the frequency of the resistance allele increases, and the population evolves.
自然选择是指更适应环境的生物个体往往存活并繁殖更多后代的过程，它作用于种群现有的遗传变异上。细菌对抗生素产生耐药性是一个清晰的现代实例。在接触抗生素的菌群中，少数个体可能早已携带耐药突变，它们
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