IGCSE Edexcel Science: Genetics – Key Concepts | IGCSE Edexcel 科学：遗传考点精讲

📚 IGCSE Edexcel Science: Genetics – Key Concepts | IGCSE Edexcel 科学：遗传考点精讲

Genetics is a cornerstone topic in the IGCSE Edexcel Science specification, linking cell biology, reproduction, evolution and modern biotechnology. A solid grasp of key terms, inheritance patterns and problem-solving with Punnett squares and pedigree charts is essential for success. This revision guide breaks down every core concept into clear, bilingual explanations, packed with exam-ready tips to help you conquer both Paper 1 and Paper 2 questions.

遗传是 IGCSE Edexcel 科学大纲中的核心模块，它把细胞生物学、生殖、进化与现代生物技术串联在一起。扎实掌握关键术语、遗传规律，并能熟练运用庞纳特方格和家系图解题，是考取高分的关键。本篇考点精讲将所有核心概念拆解为中英双语清晰解析，并融入了考试抢分技巧，助你轻松应对卷一和卷二。

1. DNA, Genes and Chromosomes | DNA、基因与染色体

DNA (deoxyribonucleic acid) is a double-stranded, helical molecule that stores the genetic blueprint of an organism. A gene is a short section of DNA that codes for a specific protein, and each gene occupies a fixed position, or locus, on a chromosome. In the nucleus of diploid cells, DNA is tightly coiled around proteins to form chromosomes, which occur in pairs – one copy inherited from each parent.

DNA（脱氧核糖核酸）是一种双螺旋结构的长链分子，储存着生物体全套遗传信息。基因是 DNA 上的一段短序列，负责编码某种特定的蛋白质，每个基因在染色体上都有固定的位置（基因座）。在二倍体细胞的细胞核中，DNA 紧密缠绕在组蛋白上形成染色体，染色体总是成对出现，一条来自父方，一条来自母方。

You must remember that different species have different numbers of chromosomes. In humans, 46 chromosomes (23 pairs) are present in most body cells. Gametes (sperm and egg cells) are haploid, containing only 23 single chromosomes, and through fertilisation the diploid number is restored.

必须牢记，不同物种的染色体数目各不相同。人类的大多数体细胞含有 46 条染色体（23 对）。精子和卵细胞这样的配子是单倍体，只含 23 条不成对的染色体；通过受精，二倍体数目得以恢复。

2. Alleles and Dominance | 等位基因与显隐性

An allele is a variant form of a gene. For example, the gene for eye colour may have an allele for brown and an allele for blue. Alleles control the same characteristic but can produce different expressions. In a pair of alleles, one can be dominant and the other recessive. A dominant allele (represented by a capital letter, e.g. B) masks the effect of a recessive allele (lowercase, e.g. b) when both are present.

等位基因是同一基因的不同版本。例如，控制眼色的基因可能有‘褐色’和‘蓝色’两种等位基因。等位基因控制同一性状，但可能产生不同的表现。在成对的等位基因中，一个可能是显性，另一个是隐性。当显性等位基因（用大写字母表示，如 B）与隐性等位基因（小写字母，如 b）同时存在时，显性会掩盖隐性的效应。

It is crucial to understand that dominance is not always about being ‘stronger’ or more common; it simply means that the trait determined by the dominant allele appears in the phenotype even if only one copy is present. Co-dominance, where both alleles are expressed equally, is also tested – for instance, in ABO blood groups (alleles Iᴬ and Iᴮ).

请注意，显性并不意味着更“强壮”或更常见，它仅仅意味着只要存在一个显性等位基因，其决定的性状就会在表现型中体现出来。共显性（等位基因同时表达，如 ABO 血型中的 Iᴬ 和 Iᴮ）也是考试常见考点。

3. Key Terms: Genotype, Phenotype, Homozygous and Heterozygous | 关键术语：基因型、表现型、纯合子、杂合子

The genotype is the combination of alleles an organism possesses for a given trait (e.g. BB, Bb or bb). The phenotype is the observable characteristic (e.g. brown eyes, blue eyes). An individual with two identical alleles, such as BB or bb, is described as homozygous. Homologous chromosomes carry the same allele at the same locus. An individual with two different alleles, Bb, is heterozygous.

基因型是指生物体控制某一性状的等位基因组合（如 BB、Bb 或 bb）。表现型则是可观察到的特征（如褐色眼、蓝色眼）。拥有两个相同等位基因（如 BB 或 bb）的个体称为纯合子，此时同源染色体在相同基因座上带有相同等位基因。拥有两个不同等位基因（Bb）的个体称为杂合子。

In IGCSE exams, you must be able to draw or interpret a genetic cross using these terms. If a heterozygous tall pea plant (Tt) is crossed with a homozygous dwarf plant (tt), the possible genotypes and phenotypes of offspring are predicted using a Punnett square. Always underline key terms and relate genotype to phenotype.

在 IGCSE 考试中，你需要运用这些术语绘制或解读遗传杂交。例如，一株杂合高茎豌豆（Tt）与一株纯合矮茎豌豆（tt）杂交，后代可能的基因型和表现型要用庞纳特方格来推断。答题时务必圈出关键词，并清楚对应基因型与表现型。

4. Monohybrid Inheritance and Punnett Squares | 单基因杂交与庞纳特方格

A monohybrid cross follows the inheritance of a single pair of alleles. The classic example is Mendel’s pea plant height. When two heterozygous tall plants (Tt) are crossed, the expected genotype ratio is 1 TT : 2 Tt : 1 tt, and the phenotype ratio is 3 tall : 1 dwarf. The Punnett square is a grid that combines parental gametes to show all possible offspring genotypes.

单基因杂交追踪的是一对等位基因的遗传。经典例子是孟德尔的豌豆株高实验。当两株杂合高茎豌豆（Tt）杂交时，预期的基因型比例为 1 TT : 2 Tt : 1 tt，表现型比例为 3 高茎 : 1 矮茎。庞纳特方格是一个表格，将亲本配子组合起来，展示后代所有可能的基因型组合。

Constructing a Punnett square correctly is a key skill. Write the male parent’s gametes along the top and the female parent’s gametes down the left. Then fill in each cell by combining the alleles. For example, a cross between Bb and Bb would generate BB, Bb, Bb and bb. If the trait shows simple dominance, the phenotype ratio is 3:1.

正确绘制庞纳特方格是一项核心技能：把父本配子写在表格顶部，母本配子写在左侧列，然后组合每个格子中的等位基因。例如，Bb × Bb 可产生 BB、Bb、Bb 和 bb。如果性状表现为简单显性，表现型比例即为 3:1。

Examiners often ask for the probability or percentage of an offspring having a particular trait. For Bb × Bb, the chance of a homozygous recessive offspring (bb) is ¼ or 25%.

考官常会要求计算后代具有特定性状的概率或百分比。在 Bb × Bb 杂交中，出现纯合隐性后代（bb）的概率是 ¼，即 25%。

5. Family Pedigree Charts | 家族谱系图

Pedigree charts are diagrams that track the inheritance of a specific trait through multiple generations of a family. Squares represent males, circles represent females. A filled symbol indicates an individual expressing the trait (affected), while an unfilled symbol shows an unaffected individual. Horizontal lines between symbols denote mating, and vertical lines lead to offspring.

家系图是一种追踪家族中某特定性状多代传递的图解。正方形代表男性，圆形代表女性；实心符号表示表现出该性状的个体（受累者），空心符号表示未受累个体。符号之间的横线表示婚配，竖线则连接亲代与子代。

When analysing a pedigree, first determine if the trait is dominant or recessive. A recessive trait can skip generations; affected offspring can be born to unaffected parents if both are carriers. A dominant trait tends to appear in every generation. Use the patterns to deduce genotypes of individuals – often you will write possible genotypes beside each symbol (e.g. Aa, aa).

分析家系图时，先判断性状是显性还是隐性。隐性性状会隔代出现；如果父母都是携带者，可以生下正常但携带隐性等位基因的后代。显性性状则几乎代代出现。通过这些规律可推知各个体的基因型，通常在符号旁标注可能的基因型，如 Aa、aa 等。

6. Sex Determination | 性别决定

In humans, sex is determined by the pair of sex chromosomes, X and Y. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). The father therefore determines the sex of the child because he can pass on either an X or a Y chromosome, whereas the mother always passes on an X chromosome.

人类的性别由一对性染色体 X 和 Y 决定。女性拥有两条 X 染色体（XX），男性则有一条 X 和一条 Y 染色体（XY）。因此，父亲的精子携带 X 或 Y 染色体决定了孩子的性别，因为母亲总是提供一条 X 染色体。

A simple Punnett square shows the equal probability of having a male or female child:

	X (mother)	X (mother)
X (father)	XX ♀	XX ♀
Y (father)	XY ♂	XY ♂

This results in a 1:1 ratio of males to females on a probability basis, though natural variation may cause slight deviations in small families.

因此，从概率上说男孩与女孩的比例为 1:1，但在小样本家庭中可能有自然波动。记住，父亲提供 Y 染色体时发育为男性，提供 X 染色体时则为女性。

7. Variation: Continuous and Discontinuous | 变异：连续变异与不连续变异

Variation describes the differences between individuals of the same species. Discontinuous variation falls into distinct categories with no intermediates – for example, blood groups (A, B, AB, O) or tongue rolling (can/cannot). Such traits are usually controlled by a single gene and are little affected by the environment. Data are best represented by bar charts.

变异指的是同种生物个体间的差异。不连续变异表现为截然不同的类别，没有中间类型，例如血型（A、B、AB、O）或卷舌能力（能/不能）。这类性状通常由单个基因控制，受环境影响极小，适合用条形图呈现。

Continuous variation shows a range of intermediate values and is influenced by many genes (polygenic inheritance) as well as environmental factors. Examples include height, body mass and skin colour. When plotted as a frequency diagram, the data usually form a bell-shaped normal distribution curve.

连续变异则表现出连续的数值范围，由多个基因（多基因遗传）与环境因素共同作用。身高、体重和肤色都是典型例子。将数据绘制成频数分布图时，通常会呈现钟形的正态分布曲线。

8. Mutation and Genetic Disorders | 突变与遗传病

A mutation is a random, permanent change in the base sequence of DNA. Mutations can occur spontaneously during DNA replication or be induced by mutagens such as ionising radiation (X-rays, UV light) and certain chemicals. Some mutations are harmful and lead to genetic disorders like cystic fibrosis or sickle cell anaemia; a few may give an organism a selective advantage.

突变是 DNA 碱基序列发生的随机且永久性的改变。突变可在 DNA 复制过程中自发产生，也可能由诱变因素（如电离辐射、紫外线及某些化学物质）诱发。部分突变是有害的，会导致囊性纤维化、镰刀型细胞贫血等遗传病；少数突变则可能赋予生物选择优势。

In IGCSE Edexcel, you need to describe the cause and symptoms of one or two specific genetic disorders. Cystic fibrosis is caused by a recessive allele of the CFTR gene and results in the production of thick, sticky mucus affecting the lungs and digestive system. Sickle cell anaemia is caused by a co-dominant allele that alters the structure of haemoglobin, causing red blood cells to become sickle-shaped and block capillaries.

在 Edexcel IGCSE 考试中，你需要描述一两种特定遗传病的病因和症状。囊性纤维化由 CFTR 基因的隐性等位基因引起，导致分泌黏稠的黏液，影响肺部和消化系统。镰刀型细胞贫血则是由于共显性等位基因改变了血红蛋白结构，使红细胞呈镰刀状、堵塞毛细血管。

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

Natural selection is the process by which individuals with advantageous alleles are more likely to survive, reproduce and pass these alleles to their offspring. Over many generations, the frequency of these beneficial alleles increases in the gene pool, leading to evolution – the gradual change in heritable characteristics of a population.

自然选择是指拥有优势等位基因的个体更有可能存活、繁殖并将这些等位基因传递给后代的过程。经过许多代，有益等位基因在基因库中的频率逐渐升高，从而导致进化——即种群可遗传性状的逐渐改变。

A classic example is the evolution of antibiotic-resistant bacteria such as MRSA. In a population of bacteria, a few may possess a mutation that gives resistance. When antibiotics are used, non-resistant bacteria die, but the resistant ones survive, reproduce and soon the population becomes largely resistant. This is an excellent model to illustrate selection pressure.

经典例子是耐抗生素细菌（如 MRSA）的进化。在一个细菌群体中，少数个体携带抗性突变。使用抗生素后，非抗性细菌死亡，而抗性细菌存活并大量繁殖，很快使整个群体变得具有抗性。这极好地展示了选择压力的作用。

Remember that evolution driven by natural selection can lead to the formation of new species if populations become geographically isolated and adapt to different environments (allopatric speciation).

须知，由自然选择驱动的进化，若种群发生地理隔离并适应不同环境，可导致新物种形成（异域物种形成）。

10. Selective Breeding, Genetic Engineering and Cloning | 选择性育种、基因工程与克隆

Selective breeding (artificial selection) involves choosing parent organisms with desirable traits and breeding them together over many generations. This has produced high-yield crops, pedigree dogs and dairy cattle. However, it reduces genetic variation and can amplify harmful recessive alleles.

选择性育种（人工选择）是指挑选具有理想性状的亲本进行多代培育，从而得到高产作物、纯种犬和乳用牛等。然而，这会降低遗传变异，并可能放大有害的隐性等位基因。

Genetic engineering goes a step further by directly modifying the genome of an organism. A useful gene (e.g. the human insulin gene) is cut out using restriction enzymes, inserted into a bacterial plasmid using ligase, and the modified bacteria are cultured to produce human insulin. This is a classic IGCSE example of biotechnology.

基因工程更进一步，直接改造生物体的基因组。用限制性内切酶切下有用基因（如人胰岛素基因），用连接酶将其插入细菌质粒，再将改造后的细菌培养以生产人胰岛素。这是 IGCSE 必考的生物技术典型案例。

Cloning can be natural (asexual reproduction, identical twins) or artificial. Plant tissue culture (micropropagation) allows many identical plants to be grown from a small piece of tissue. Animal cloning, such as the production of Dolly the sheep by somatic cell nuclear transfer, is also covered. Be prepared to discuss advantages and ethical concerns of genetic modification.

克隆可以是自然的（无性繁殖、同卵双生）或人工的。植物组织培养（微繁殖）能由一小块组织培育出大量相同植株；动物克隆（如通过体细胞核移植产生的多莉羊）也在考纲之内。需准备好讨论基因改造的优势和伦理问题。

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IGCSE Edexcel Science: Genetics – Key Concepts | IGCSE Edexcel 科学：遗传 考点精讲