📚 IGCSE OCR Biology: Genetics – Key Concepts Explained | IGCSE OCR 生物：遗传学 考点精讲
Genetics is the study of how traits are passed from parents to offspring. In the OCR IGCSE Biology specification, you need to understand key ideas such as genes, alleles, dominance, monohybrid and dihybrid inheritance, sex linkage, variation, mutation, and the role of natural selection. This article breaks down each topic with clear English and Chinese paired explanations, helping you revise effectively.
遗传学研究性状如何从亲代传递给子代。在OCR IGCSE生物考纲中，你需要掌握基因、等位基因、显隐性、单基因杂交与双基因杂交、性连锁、变异、突变以及自然选择的作用等核心概念。本文以中英对照的方式逐一讲解，助你高效复习。
1. What is Genetics? | 什么是遗传学？
Genetics is the branch of biology that deals with heredity and variation. It explains how characteristics such as eye colour, height or blood type are determined by genetic information stored in DNA. Gregor Mendel, through his experiments with pea plants, laid the foundation of modern genetics by discovering the basic principles of inheritance.
遗传学是研究遗传和变异的生物学分支。它解释了眼睛颜色、身高或血型等特征是如何由储存在DNA中的遗传信息决定的。格雷戈尔·孟德尔通过豌豆实验发现了遗传的基本规律，奠定了现代遗传学的基础。
The genetic material in most organisms is organized into chromosomes, which carry genes. Each gene is a section of DNA that codes for a specific protein, thereby influencing a trait.
大多数生物的遗传物质被组织成染色体，染色体上携带着基因。每个基因是一段编码特定蛋白质的DNA，从而影响性状。
2. Genes, Alleles and Chromosomes | 基因、等位基因与染色体
A gene is a short length of DNA on a chromosome that determines a particular characteristic. Chromosomes occur in pairs in diploid cells; humans have 23 pairs. For each gene, the two copies may be identical or slightly different. These different versions of the same gene are called alleles.
基因是染色体上决定特定性状的一段DNA。染色体在二倍体细胞中成对出现；人类有23对染色体。每个基因的两个拷贝可能相同，也可能略有不同。这些同一基因的不同形式称为等位基因。
If the two alleles are the same, the organism is homozygous for that gene. If they are different, it is heterozygous. The combination of alleles forms the genotype, while the physical expression of the trait is the phenotype.
如果两个等位基因相同，该生物在这个基因上是纯合的。如果不同，则是杂合的。等位基因的组合构成基因型，而性状的物理表达则是表现型。
3. Dominant and Recessive Alleles | 显性等位基因与隐性等位基因
Some alleles are dominant, meaning they are always expressed in the phenotype even if only one copy is present. Recessive alleles are only expressed when two copies are present (homozygous recessive). In genetic diagrams, dominant alleles are shown with an uppercase letter (e.g. A) and recessive alleles with a lowercase letter (e.g. a).
有些等位基因为显性，意味着只要存在一个拷贝，就会在表现型中表达。隐性等位基因只有在两个拷贝都存在（隐性纯合）时才会表达。在遗传图解中，显性等位基因用大写字母表示（如A），隐性等位基因用小写字母表示（如a）。
For example, in pea plants, the allele for tall stems (T) is dominant over the allele for short stems (t). A plant with genotype Tt will be tall because the dominant allele masks the recessive one.
例如，在豌豆中，高茎等位基因（T）对矮茎等位基因（t）为显性。基因型为Tt的植株表现为高茎，因为显性等位基因掩盖了隐性等位基因的作用。
4. Monohybrid Inheritance | 单基因杂交
Monohybrid inheritance is the study of how a single characteristic is passed on. Using a Punnett square, you can predict the genotypes and phenotypes of offspring. For a cross between two heterozygous parents (Tt × Tt), the expected offspring ratio is 3 dominant : 1 recessive in phenotype, and 1 TT : 2 Tt : 1 tt in genotype.
单基因杂交研究单一性状的传递规律。使用庞纳特方格可以预测后代的基因型和表现型。两个杂合亲本（Tt × Tt）杂交，预期的后代表现型比例为3显性 : 1隐性，基因型比例为1 TT : 2 Tt : 1 tt。
| Gametes | T | t |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
The 3:1 ratio only appears when a large number of offspring are produced and the alleles segregate randomly during gamete formation. This follows Mendel’s law of segregation.
只有当后代数量很大且等位基因在配子形成过程中随机分离时，才会出现3:1的比例。这遵循孟德尔的分离定律。
5. Dihybrid Inheritance and Independent Assortment | 双基因杂交与独立分配
Dihybrid inheritance involves two different genes located on different chromosomes. Mendel’s law of independent assortment states that alleles of different genes are distributed to gametes independently. For example, a cross between two plants heterozygous for seed shape (R = round, r = wrinkled) and seed colour (Y = yellow, y = green), both RrYy, gives a phenotypic ratio of 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green in the offspring.
双基因杂交涉及位于不同染色体上的两个不同基因。孟德尔的独立分配定律指出，不同基因的等位基因独立地分配到配子中。例如，两个在种子形状（R = 圆，r = 皱）和种子颜色（Y = 黄，y = 绿）上均为杂合的植株（RrYy）杂交，后代表现型比例为9圆黄 : 3圆绿 : 3皱黄 : 1皱绿。
This ratio can be obtained using a 4 × 4 Punnett square showing the combination of gametes (RY, Ry, rY, ry). It demonstrates that the inheritance of one trait does not influence the other.
这一比例可通过4×4的庞纳特方格展示配子（RY、Ry、rY、ry）的组合得到。它表明一个性状的遗传不影响另一个性状。
6. Sex Chromosomes and Sex-linked Inheritance | 性染色体与性连锁遗传
In humans, sex is determined by the 23rd pair of chromosomes: XX for females and XY for males. The Y chromosome is much shorter and carries few genes. Genes located on the X chromosome but absent on the Y are called sex-linked genes. Examples include red-green colour blindness and haemophilia.
人类性别由第23对染色体决定：女性为XX，男性为XY。Y染色体短得多，携带的基因很少。位于X染色体上但Y染色体上没有的基因称为性连锁基因。例子包括红绿色盲和血友病。
Because males have only one X chromosome, a single recessive allele on the X will be expressed. Females need two recessive alleles to show the trait. Therefore, sex-linked disorders are more common in males. A carrier female (heterozygous) has a normal phenotype but can pass the recessive allele to her children.
由于男性只有一个X染色体，X染色体上的一个隐性等位基因就会表达。女性需要两个隐性等位基因才会表现出性状。因此，性连锁疾病在男性中更常见。携带者女性（杂合）表现型正常，但可能将隐性等位基因传给子女。
7. Variation: Continuous and Discontinuous | 变异：连续变异与不连续变异
Variation refers to differences between individuals of the same species. It can be continuous, such as height or skin colour, where individuals show a range of phenotypes with no distinct categories. Continuous variation is controlled by many genes (polygenic) and is influenced by the environment.
变异指同物种个体之间的差异。它可以是连续的，如身高或肤色，个体表现出一系列没有明确类别的表现型。连续变异由多基因控制，并受环境影响。
Discontinuous variation, such as blood groups or tongue rolling, falls into distinct categories with no intermediates. It is usually controlled by a single gene and is little affected by the environment. Plotting a bar chart or histogram helps distinguish the two types.
不连续变异，如血型或卷舌能力，分为截然不同的类别，没有中间类型。它通常由单基因控制，几乎不受环境影响。绘制柱形图或直方图有助于区分这两种类型。
8. Mutation | 突变
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 (UV, X-rays) or certain chemicals. They may result in an altered protein or no change at all.
突变是DNA碱基序列的随机、永久性改变。突变可在DNA复制过程中自发产生，也可由诱变剂如电离辐射（紫外线、X射线）或某些化学物质诱导。它们可能导致蛋白质改变，也可能没有任何变化。
Mutations can be harmful, neutral, or rarely beneficial. A beneficial mutation may increase an organism’s chance of survival; for instance, a mutation giving bacteria antibiotic resistance is advantageous in the presence of antibiotics. Mutations are the only source of new alleles, driving genetic variation in populations.
突变可能有害、中性，或罕见地有益。有益的突变可能增加生物的生存机会；例如，赋予细菌抗生素抗性的突变在抗生素存在时是有利的。突变是产生新等位基因的唯一来源，推动种群遗传变异。
9. Natural Selection and Evolution | 自然选择与进化
Natural selection is the process by which individuals with advantageous traits are more likely to survive and reproduce, passing those traits to the next generation. Over many generations, the frequency of these alleles increases, leading to evolution. Charles Darwin proposed this mechanism after observing variation and adaptation in finches on the Galápagos Islands.
自然选择是指具有有利性状的个体更可能生存和繁殖，并将这些性状传给下一代。经过许多世代，这些等位基因的频率增加，导致进化。查尔斯·达尔文在观察加拉帕戈斯群岛雀类的变异和适应后提出了这一机制。
For example, antibiotic-resistant bacteria survive when antibiotics are used, while non-resistant bacteria die. The resistant bacteria reproduce, so the population becomes largely resistant. This is an example of evolution by natural selection.
例如，使用抗生素时，抗药性细菌存活下来，而非抗药性细菌死亡。抗药性细菌繁殖，因此种群大部分变得具有抗药性。这是自然选择导致进化的一个例子。
Fossil records, comparative anatomy and DNA analysis provide evidence for evolution. Natural selection explains how species become better adapted to their environment over time.
化石记录、比较解剖学和DNA分析为进化提供了证据。自然选择解释了物种如何随着时间推移更适应其环境。
10. Selective Breeding and Genetic Engineering | 选择性育种与基因工程
Selective breeding (artificial selection) is the process by which humans breed plants and animals for particular desirable traits. It involves choosing parents with the best characteristics and breeding them over many generations. Examples include high-yielding crops, pedigree dogs, and dairy cattle producing more milk. However, it reduces genetic diversity and can lead to health problems due to inbreeding.
选择性育种（人工选择）是人类为了特定理想性状而培育动植物的过程。它涉及选择具有最佳特征的亲本，并经过多代繁殖。例子包括高产作物、纯种狗和产奶量更高的奶牛。然而，它会减少遗传多样性，并可能因近交导致健康问题。
Genetic engineering is a modern technique where a gene from one organism is inserted into the genome of another. The recipient organism is called a transgenic or genetically modified (GM) organism. For instance, the human insulin gene has been inserted into bacteria to produce insulin for treating diabetes.
基因工程是一种现代技术，将一种生物的基因插入另一种生物的基因组中。受体生物称为转基因生物。例如，人胰岛素基因已被插入细菌中，用于生产治疗糖尿病的胰岛素。
The process involves cutting out the desired gene using restriction enzymes, inserting it into a plasmid, and transferring the plasmid into the host cell. Genetic engineering allows for the rapid production of useful proteins and development of crops with improved traits such as pest resistance.
该过程涉及使用限制酶切下所需基因，将其插入质粒，再将质粒转入宿主细胞。基因工程能够快速生产有用蛋白质，并培育出具有抗虫等改良性状的作物。
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