📚 IGCSE CIE Science: Genetics Key Concepts | IGCSE CIE 科学：遗传 考点精讲
Genetics is the branch of biology that explains how traits are passed from parents to offspring. Understanding the principles of inheritance not only helps you appreciate the diversity of life but also lays the foundation for topics like evolution, genetic disorders, and modern biotechnology. This article covers all the key concepts required for the IGCSE CIE Science syllabus, from DNA structure to natural selection, with clear explanations and examples.
遗传学是生物学中解释性状如何从亲代传递给子代的分支。理解遗传原理不仅能帮助你认识生命的多样性，也为进化、遗传疾病和现代生物技术等主题奠定了基础。本文涵盖了 IGCSE CIE 科学大纲要求的所有核心概念，从 DNA 结构到自然选择，提供清晰的解释和实例。
1. Introduction to Genetics | 遗传学导论
Genetics is the study of heredity and variation in living organisms. Heredity refers to the transmission of characteristics from one generation to the next, while variation describes the differences between individuals of the same species. The fundamental unit of heredity is the gene, a segment of DNA that codes for a specific protein or trait.
遗传学是研究生物体遗传与变异的学科。遗传是指性状从一代传到下一代的过程，而变异则描述同一物种个体之间的差异。遗传的基本单位是基因，即一段编码特定蛋白质或性状的 DNA 片段。
2. DNA and Chromosomes | DNA 与染色体
Deoxyribonucleic acid (DNA) is a long molecule made up of two strands twisted into a double helix. Each strand consists of a sugar-phosphate backbone and nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G). Base pairing rules state that A pairs with T, and C pairs with G through hydrogen bonds. In the nucleus of a cell, DNA is tightly coiled around proteins called histones to form chromosomes. Humans have 46 chromosomes arranged in 23 pairs, with one set inherited from each parent.
脱氧核糖核酸 (DNA) 是一种由两条链缠绕成双螺旋结构的长分子。每条链由糖-磷酸骨架和含氮碱基组成：腺嘌呤 (A)、胸腺嘧啶 (T)、胞嘧啶 (C) 和鸟嘌呤 (G)。碱基互补配对规则是 A 与 T 配对，C 与 G 配对，通过氢键连接。在细胞核中，DNA 紧紧缠绕在组蛋白上形成染色体。人类有 46 条染色体，组成 23 对，每套分别来自父母一方。
3. Genes and Alleles | 基因与等位基因
A gene is a specific sequence of DNA bases that determines a particular characteristic by coding for a polypeptide. Genes occupy fixed positions on chromosomes called loci. Different forms of the same gene are called alleles. For example, the gene for eye colour may have an allele for brown eyes and another for blue eyes. An individual inherits two alleles for each gene, one from each parent.
基因是 DNA 碱基的特定序列，通过编码多肽来决定某一特征。基因在染色体上占据固定位置，称为基因座。同一基因的不同形式称为等位基因。例如，控制眼睛颜色的基因可能有褐色眼的等位基因和蓝色眼的等位基因。每个基因个体从父母各继承一个等位基因，共两个。
4. Dominant and Recessive Alleles | 显性与隐性等位基因
An allele is described as dominant if it is always expressed in the phenotype even when only one copy is present. A recessive allele is only expressed when two copies are present (i.e., no dominant allele is present). Dominant alleles are represented by a capital letter (e.g., T for tall stem in pea plants), while recessive alleles are shown with the corresponding lowercase letter (t for short stem). If an organism has the genotype Tt, the dominant T allele masks the effect of the recessive t allele, so the plant will be tall.
如果一个等位基因即使在只有一个拷贝存在时也能在表现型中表达，就被称为显性。隐性等位基因只有在存在两个拷贝（即没有显性等位基因）时才会表达。显性等位基因用大写字母表示（例如，豌豆高茎用 T 表示），隐性等位基因用相应的小写字母表示（矮茎用 t 表示）。如果某生物体的基因型为 Tt，显性 T 等位基因会掩盖隐性 t 等位基因的作用，因此植株表现为高茎。
5. Homozygous and Heterozygous | 纯合子与杂合子
An organism with two identical alleles for a trait is said to be homozygous (e.g., TT or tt). A homozygous dominant individual (TT) expresses the dominant trait, while a homozygous recessive individual (tt) expresses the recessive trait. An organism with two different alleles (e.g., Tt) is heterozygous. In a heterozygote, the dominant allele determines the phenotype. The terms true-breeding or pure-breeding refer to individuals that are homozygous for a particular trait.
具有两个相同等位基因的生物体称为纯合子（例如 TT 或 tt）。纯合显性个体 (TT) 表达显性性状，纯合隐性个体 (tt) 表达隐性性状。具有两个不同等位基因（例如 Tt）的生物体称为杂合子。在杂合子中，显性等位基因决定表现型。术语“纯种”或“纯系”指在某一性状上为纯合子的个体。
6. Monohybrid Crosses and Punnett Squares | 单基因杂交与庞纳特方格
A monohybrid cross studies the inheritance of a single gene controlled by two alleles. The possible genotypes of offspring can be predicted using a Punnett square. For instance, crossing two heterozygous tall pea plants (Tt × Tt) yields a genotypic ratio of 1 TT : 2 Tt : 1 tt and a phenotypic ratio of 3 tall : 1 short. This demonstrates Mendel’s law of segregation, which states that allele pairs separate during gamete formation, and each gamete carries only one allele for each gene.
单基因杂交研究由两个等位基因控制的单一基因的遗传。子代可能的基因型可以用庞纳特方格预测。例如，将两株杂合高茎豌豆 (Tt × Tt) 杂交，得到的基因型比例为 1 TT : 2 Tt : 1 tt，表现型比例为 3 高茎 : 1 矮茎。这证明了孟德尔的分离定律：等位基因在配子形成时分离，每个配子只携带每个基因的一个等位基因。
Example Punnett square for Tt × Tt:
| T | t | |
| T | TT | Tt |
| t | Tt | tt |
Phenotypic ratio: 3 tall : 1 short
Tt × Tt 庞纳特方格示例：
表现型比例：3 高茎 : 1 矮茎
7. Sex Determination | 性别决定
In humans, 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). During meiosis, all egg cells carry a single X chromosome, while sperm cells carry either an X or a Y chromosome. If an X-bearing sperm fertilises the egg, the offspring is female (XX); if a Y-bearing sperm fertilises the egg, the offspring is male (XY). This produces a theoretical 1:1 ratio of males to females in each generation.
人类的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)。在减数分裂过程中，所有卵细胞携带一条 X 染色体，而精子细胞携带一条 X 或一条 Y 染色体。如果携带 X 的精子与卵子结合，后代为女性 (XX)；如果携带 Y 的精子与卵子结合，后代为男性 (XY)。因此理论上每一代男女比例约为 1:1。
8. Inherited Diseases | 遗传性疾病
Some genetic disorders are caused by recessive alleles located on autosomes (non-sex chromosomes). Cystic fibrosis is a recessive condition caused by a mutation in the CFTR gene. Affected individuals produce thick, sticky mucus that blocks airways and ducts. To have the disease, a person must inherit two copies of the defective allele (ff). Heterozygous carriers (Ff) do not show symptoms but can pass the allele to their children. Sickle cell anaemia is another example of a recessive disorder, in which red blood cells become crescent-shaped, reducing oxygen transport. In contrast, Huntington’s disease is caused by a dominant allele, so only one copy is needed to develop the condition, though symptoms usually appear in adulthood.
某些遗传疾病由常染色体（非性染色体）上的隐性等位基因引起。囊性纤维化是一种由 CFTR 基因突变引起的隐性疾病。患者体内产生黏稠的黏液，堵塞呼吸道和导管。要患上此病，个体必须遗传两个缺陷等位基因 (ff)。杂合携带者 (Ff) 不表现症状，但可将等位基因传给子女。镰刀型细胞贫血是另一种隐性遗传病，红细胞变成镰刀形，降低氧气输送能力。相反，亨廷顿病由显性等位基因引起，只需一个拷贝即可发病，但症状通常成年后才显现。
9. Variation: Continuous and Discontinuous | 变异：连续变异与不连续变异
Variation among individuals can be classified as continuous or discontinuous. Continuous variation shows a full range of intermediate phenotypes between two extremes, such as height or body mass. It is usually controlled by many genes (polygenic) and is influenced by the environment. Discontinuous variation falls into distinct categories with no intermediates, such as blood groups (A, B, AB, O) or tongue rolling ability. This type is typically controlled by a single gene and is less affected by the environment. Displaying data as frequency histograms or bar charts helps distinguish between the two: continuous traits form a bell-shaped curve, while discontinuous traits appear as separate columns.
个体之间的变异可分为连续变异和不连续变异。连续变异在两个极端之间存在完整的中间表型范围，例如身高或体重。它通常由多基因控制，并受环境影响。不连续变异分为明显不同的类别，没有中间类型，例如血型 (A、B、AB、O) 或卷舌能力。这类变异通常由单基因控制，受环境影响较小。用频率直方图或条形图展示数据有助于区分二者：连续性状形成钟形曲线，不连续性状则表现为分离的柱形。
10. Mutation | 突变
A mutation is a change in the base sequence of DNA. Mutations can occur spontaneously during DNA replication or be induced by mutagens such as chemicals, UV radiation, and ionising radiation. Some mutations are harmful and lead to genetic disorders, while a few may be beneficial and drive evolution. Mutations that occur in gametes can be passed to offspring; those in body cells (somatic mutations) only affect the individual. Types of gene mutation include substitution, insertion, and deletion, all of which can alter the amino acid sequence of a protein.
突变是 DNA 碱基序列的改变。突变可能在 DNA 复制过程中自发产生，也可能由致突变因素引起，如化学物质、紫外线和电离辐射。有些突变是有害的，会导致遗传疾病，而少数可能有益并推动进化。发生在配子中的突变会传递给后代；体细胞中的突变只影响个体本身。基因突变的类型包括替换、插入和缺失，这些都能改变蛋白质的氨基酸序列。
11. Natural Selection and Evolution | 自然选择与进化
Natural selection is the process by which organisms better adapted to their environment survive and reproduce more successfully, passing their advantageous alleles to the next generation. Over many generations, this leads to changes in the genetic makeup of a population, a process known as evolution. A classic example is the evolution of antibiotic resistance in bacteria: a random mutation may give a bacterium resistance; when exposed to the antibiotic, resistant bacteria survive and multiply, while non-resistant ones die. Over time, the population becomes dominated by resistant strains. This principle also explains the development of pesticide resistance in insects and the diversity of beak shapes in Darwin’s finches.
自然选择是指更适应环境的生物体能够更好地生存和繁殖，并将有利等位基因传递给后代的过程。经过多代之后，这会导致种群基因组成的改变，即进化。一个经典例子是细菌抗生素耐药性的进化：随机突变可能使某个细菌产生耐药性；当接触抗生素时，耐药细菌存活并繁殖，而非耐药细菌死亡。久而久之，种群中被耐药菌株占据主导。这一原理也解释了害虫对杀虫剂抗性的产生以及达尔文雀鸟喙形状的多样性。
12. Key Terms Summary | 核心术语总结
The following table recaps essential genetic terminology that frequently appears in IGCSE Science questions. Learning these definitions precisely is crucial for exam success.
下表总结了 IGCSE 科学试题中频繁出现的核心遗传学术语。精确掌握这些定义对于考试成功至关重要。
| Term / 术语 | Definition / 定义 |
| Gene / 基因 | A length of DNA that codes for a protein / 编码蛋白质的一段 DNA |
| Allele / 等位基因 | An alternative form of a gene / 基因的另一种形式 |
| Genotype / 基因型 | The genetic makeup of an organism / 生物体的基因组成 |
| Phenotype / 表现型 | The observable characteristics of an organism / 生物体可观察到的特征 |
| Dominant / 显性 | An allele that is expressed in the phenotype even if only one is present / 只要存在一个就会在表现型中表达的等位基因 |
| Recessive / 隐性 | An allele that is only expressed if two copies are present / 只有两个拷贝都存在时才表达的等位基因 |
| Homozygous / 纯合子 | Having two identical alleles for a gene / 某个基因具有两个相同的等位基因 |
| Heterozygous / 杂合子 | Having two different alleles for a gene / 某个基因具有两个不同的等位基因 |
| Chromosome / 染色体 | A thread-like structure of DNA carrying genetic information / 携带遗传信息的线状 DNA 结构 |
| Mutation / 突变 | A change in the DNA base sequence / DNA 碱基序列的改变 |
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