📚 IGCSE OCR Science: Genetics Key Points | IGCSE OCR 科学：遗传 考点精讲
Genetics is the study of how traits are passed from parents to offspring through genes. In the IGCSE OCR Science syllabus, this topic covers DNA structure, cell division, inheritance patterns, variation and mutation. Understanding these concepts is essential for explaining how characteristics are inherited and why individuals show differences.
遗传学是研究性状如何通过基因从亲代传递给子代的科学。在 IGCSE OCR 科学课程中，这一主题涵盖 DNA 结构、细胞分裂、遗传规律、变异和突变等内容。理解这些概念对于解释性状如何遗传以及个体为何存在差异至关重要。
1. DNA Structure and Function | DNA 的结构与功能
DNA (deoxyribonucleic acid) is a double helix molecule made up of two long strands. Each strand consists of repeating units called nucleotides. A nucleotide contains a phosphate group, a deoxyribose sugar and a nitrogenous base. The four bases are adenine (A), thymine (T), cytosine (C) and guanine (G). The two strands are held together by complementary base pairing: A always pairs with T, and C always pairs with G.
DNA（脱氧核糖核酸）是一个双螺旋分子，由两条长链组成。每条链由称为核苷酸的重复单元构成。每个核苷酸包含一个磷酸基团、一个脱氧核糖和一个含氮碱基。四种碱基是腺嘌呤（A）、胸腺嘧啶（T）、胞嘧啶（C）和鸟嘌呤（G）。两条链通过互补碱基配对结合在一起：A 总是与 T 配对，C 总是与 G 配对。
DNA carries the genetic code for building proteins. The sequence of bases determines the order of amino acids in a protein. A gene is a segment of DNA that codes for a particular protein. Proteins control most processes in the body and determine characteristics such as eye colour.
DNA 携带着构建蛋白质的遗传密码。碱基的序列决定了蛋白质中氨基酸的顺序。基因是编码特定蛋白质的一段 DNA。蛋白质控制着身体的大多数过程，并决定诸如眼睛颜色等性状。
2. Genes and Chromosomes | 基因与染色体
In the nucleus of a cell, DNA is organised into structures called chromosomes. Humans have 46 chromosomes arranged in 23 pairs. One chromosome of each pair comes from the mother and the other from the father. Each chromosome contains many genes arranged along its length.
在细胞核中，DNA 被组织成称为染色体的结构。人类有 46 条染色体，排列成 23 对。每对染色体中一条来自母亲，另一条来自父亲。每条染色体上含有许多沿其长度排列的基因。
An allele is a different version of the same gene. For example, the gene for eye colour may have an allele for brown eyes and an allele for blue eyes. The combination of alleles that an individual possesses is called the genotype. The observable characteristic that results from the genotype is the phenotype.
等位基因是同一基因的不同版本。例如，眼睛颜色的基因可能有棕色眼睛的等位基因和蓝色眼睛的等位基因。个体拥有的等位基因组合称为基因型。由基因型产生的可观察特征称为表型。
3. Mitosis and Growth | 有丝分裂与生长
Mitosis is a type of cell division that produces two genetically identical daughter cells. It is used for growth, repair and asexual reproduction. Before mitosis, DNA replicates so each chromosome consists of two identical chromatids. During mitosis, the chromatids separate into two new nuclei, and the cell divides.
有丝分裂是一种产生两个遗传上相同的子细胞的细胞分裂方式。它用于生长、修复和无性繁殖。在有丝分裂之前，DNA 复制，因此每条染色体由两个相同的染色单体组成。在有丝分裂过程中，染色单体分离进入两个新的细胞核，然后细胞分裂。
The stages of mitosis in order are prophase, metaphase, anaphase and telophase. In prophase, chromosomes condense and the nuclear membrane breaks down. In metaphase, chromosomes line up at the centre of the cell. In anaphase, chromatids are pulled to opposite poles. In telophase, new nuclear membranes form around each set of chromosomes.
有丝分裂的阶段依次为前期、中期、后期和末期。在前期，染色体浓缩，核膜破裂。在中期，染色体排列在细胞中央。在后期，染色单体被拉向相反的两极。在末期，新的核膜在每组染色体周围形成。
4. Meiosis and Gamete Formation | 减数分裂与配子形成
Meiosis is a special type of cell division that produces gametes (sex cells) with half the normal number of chromosomes. In humans, gametes have 23 chromosomes. This is important so that when fertilisation occurs, the normal chromosome number (46) is restored. Meiosis involves two divisions, producing four non-identical haploid cells.
减数分裂是一种特殊的细胞分裂方式，产生染色体数目为正常一半的配子（性细胞）。在人类中，配子含有 23 条染色体。这很重要，因为当受精发生时，正常的染色体数（46 条）得以恢复。减数分裂涉及两次分裂，产生四个不同的单倍体细胞。
During meiosis, homologous chromosomes pair up and exchange genetic material in a process called crossing over. This creates new combinations of alleles, leading to genetic variation in the offspring. The random assortment of chromosomes also contributes to variation.
在减数分裂过程中，同源染色体配对并通过一个称为交叉的过程交换遗传物质。这产生了新的等位基因组合，导致后代的遗传变异。染色体的随机分配也促进了变异。
5. Monohybrid Inheritance and Dominant/Recessive Alleles | 单基因遗传与显隐性等位基因
Monohybrid inheritance involves a single characteristic controlled by one gene with two alleles. Some alleles are dominant, meaning they are expressed even if only one copy is present. Other alleles are recessive, meaning they are only expressed if two copies are present. Dominant alleles are represented by a capital letter, and recessive alleles by a lower-case letter.
单基因遗传涉及由一个基因的两个等位基因控制的单一性状。有些等位基因是显性的，意味着即使只有一个拷贝也会表达。其他等位基因是隐性的，意味着只有两个拷贝才会表达。显性等位基因用大写字母表示，隐性等位基因用小写字母表示。
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 or Tt will be tall; only a plant with tt will be short. If two heterozygous tall plants (Tt) are crossed, the expected phenotype ratio in the offspring is 3 tall : 1 short.
例如，在豌豆植株中，高茎的等位基因（T）对矮茎的等位基因（t）为显性。基因型为 TT 或 Tt 的植株将是高茎；只有 tt 的植株是矮茎。如果两株杂合的高茎植株（Tt）杂交，后代预期的表型比例为 3 高茎 : 1 矮茎。
A Punnett square can be used to predict the genotypes and phenotypes of offspring:
旁氏表可以用来预测后代的基因型和表型：
| T | t | |
| T | TT | Tt |
| t | Tt | tt |
6. Genetic Diagrams and Family Pedigrees | 遗传图解与家系图
Genetic diagrams are used to show the inheritance of a characteristic from parents to offspring. They typically include the parental genotypes, gametes, and possible offspring genotypes and phenotypes. A family pedigree chart uses standard symbols (circles for females, squares for males) to trace the pattern of inheritance over several generations.
遗传图解用于显示性状从亲代到子代的遗传过程。它们通常包括亲代的基因型、配子以及可能的子代基因型和表型。家系图使用标准符号（圆形代表女性，方形代表男性）来追踪几代人的遗传模式。
In a pedigree, affected individuals are shaded. By analysing a pedigree, you can often determine whether a trait is dominant or recessive, and whether it is carried on an autosome or a sex chromosome. OCR questions often ask students to interpret a pedigree and calculate the probability that a particular individual is a carrier or affected.
在家系图中，患病个体被涂上阴影。通过分析家系图，你通常可以判断一个性状是显性还是隐性，以及它是由常染色体还是性染色体携带。OCR 的题目经常要求学生解读家系图，并计算某个特定个体是携带者或患病的概率。
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). The male parent determines the sex of the offspring because he can pass on either an X or a Y chromosome, whereas the female always passes on an X chromosome.
在人类中，性别由一对性染色体决定。女性有两条 X 染色体（XX），而男性有一条 X 和一条 Y 染色体（XY）。父亲决定了后代的性别，因为他可以传递 X 或 Y 染色体，而母亲总是传递 X 染色体。
A Punnett square for sex inheritance shows that there is always a 50% chance of having a boy and a 50% chance of having a girl in each pregnancy.
性别遗传的旁氏表显示，每次怀孕生男孩的概率总是 50%，生女孩的概率也是 50%。
| X | Y | |
| X | XX (female) | XY (male) |
| X | XX (female) | XY (male) |
8. Variation: Continuous and Discontinuous | 变异：连续变异与不连续变异
Variation describes the differences that exist between individuals of the same species. It can be continuous or discontinuous. Continuous variation results in a range of phenotypes between two extremes, such as height or skin colour. This type is usually influenced by many genes and environmental factors. The data shows a normal distribution curve.
变异描述同一物种个体之间存在的差异。它可以是连续的或不连续的。连续变异导致表型在两个极端之间呈现一个范围，例如身高或肤色。这种类型通常受多个基因和环境因素的影响。数据呈现正态分布曲线。
Discontinuous variation produces distinct groups with no intermediates, such as blood groups or ability to roll the tongue. It is typically controlled by a single gene and is less affected by the environment. Inheritance patterns for discontinuous traits are often easier to trace using genetic diagrams.
不连续变异产生没有中间类型的不同类别，例如血型或卷舌能力。它通常由单个基因控制，受环境影响较小。不连续性状的遗传模式通常更容易用遗传图解追踪。
9. Mutation | 突变
A mutation is a random change in the sequence of DNA bases. Mutations can occur spontaneously during DNA replication or be caused by mutagens such as radiation or certain chemicals. Most mutations are neutral or harmful, but some can be beneficial and drive evolution by natural selection.
突变是 DNA 碱基序列的随机改变。突变可以在 DNA 复制过程中自发发生，也可以由电离辐射或某些化学物质等诱变剂引起。大多数突变是中性的或有害的，但有些可能是有益的，并通过自然选择推动进化。
A gene mutation can change the protein produced, leading to a different phenotype. For example, sickle cell anaemia is caused by a single base substitution that alters haemoglobin. Mutations in gametes can be passed to offspring, while mutations in body cells affect only the individual.
基因突变可以改变所产生的蛋白质，导致不同的表型。例如，镰刀型细胞贫血症是由单个碱基替换引起的，它改变了血红蛋白。生殖细胞中的突变可以传递给后代，而体细胞中的突变仅影响个体本身。
10. Genetic Engineering and Selective Breeding | 基因工程与选择育种
Genetic engineering involves modifying the genome of an organism by inserting a gene from another species. This produces genetically modified (GM) organisms with desirable characteristics, such as bacteria that produce human insulin or crops resistant to pests. Enzymes called restriction enzymes are used to cut DNA, and ligase joins the inserted gene into the vector.
基因工程涉及通过植入来自另一物种的基因来改造生物体的基因组。这产生了具有所需特性的转基因生物，例如产生人胰岛素的细菌或抗虫害的作物。限制性内切酶用于切割 DNA，连接酶则将插入的基因连接到载体中。
Selective breeding (artificial selection) involves choosing parent organisms with desired traits and breeding them over several generations. It takes much longer than genetic engineering and can reduce genetic variation, but it has been used for centuries to improve crops and livestock. Both methods raise ethical considerations about long-term impacts.
选择育种（人工选择）涉及挑选具有所需性状的亲本生物并使其繁殖多代。它比基因工程耗时更长，且可能减少遗传变异，但它已被用于数百年以改良作物和家畜。这两种方法都引发了关于长期影响的伦理考虑。
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