📚 GCSE CCEA Science: Genetics Revision Guide | GCSE CCEA 科学：遗传 考点精讲
Genetics explains how characteristics are passed from parents to offspring. This guide covers the key concepts you need for your CCEA GCSE Science exam, including DNA structure, alleles, monohybrid crosses, inherited disorders, sex determination, and causes of variation. Each topic is explained in clear, exam-focused language to help you understand and remember the essential facts.
遗传学解释了性状如何从亲代传递给子代。本指南涵盖 CCEA GCSE 科学考试所需的关键概念，包括 DNA 结构、等位基因、单基因杂交、遗传病、性别决定以及变异的原因。每个主题都用清晰、聚焦考试的语言进行解释，帮助你理解和记忆基本事实。
1. DNA, Genes and Chromosomes | DNA、基因与染色体
DNA (deoxyribonucleic acid) is a long, double-stranded molecule found in the nucleus of almost every cell. It carries the genetic instructions for an organism’s development and functioning. DNA is organised into structures called chromosomes, which are visible under a microscope during cell division.
DNA（脱氧核糖核酸）是一种长的双链分子，几乎存在于每个细胞的细胞核中。它携带了生物体发育和功能的遗传指令。DNA 被组织成称为染色体的结构，在细胞分裂期间可在显微镜下看到。
A gene is a short section of DNA that codes for a particular protein or characteristic. Humans have around 20,000 – 25,000 genes, arranged along 23 pairs of chromosomes. One chromosome of each pair is inherited from the mother, the other from the father.
基因是 DNA 的一个短片段，它编码特定的蛋白质或性状。人类大约有 20,000 至 25,000 个基因，排列在 23 对染色体上。每对染色体中的一条来自母亲，另一条来自父亲。
2. Alleles and Variation | 等位基因与变异
An allele is a different version of the same gene. Since we inherit two copies of each gene, one from each parent, we may have two identical alleles or two different alleles for a particular characteristic. The combination of alleles determines the variation we see in traits such as eye colour, height, and blood group.
等位基因是同一基因的不同版本。由于我们从父母双方各继承一个基因拷贝，对于某一特定性状，我们可能拥有两个相同的等位基因或两个不同的等位基因。等位基因的组合决定了我们观察到的性状变异，例如眼睛颜色、身高和血型。
For example, the gene for eye colour has several alleles, leading to brown, blue, or green eyes. New alleles arise through mutations, which are changes in the DNA sequence. More than one gene often influences a single characteristic, but in GCSE Science we focus mainly on single-gene inheritance.
例如，眼睛颜色基因有多个等位基因，导致棕色、蓝色或绿色眼睛。新的等位基因通过突变（DNA 序列的改变）而产生。一个性状通常受多个基因影响，但在 GCSE 科学中，我们主要关注单基因遗传。
3. Dominant and Recessive Alleles | 显性与隐性等位基因
Alleles can be dominant or recessive. A dominant allele is always expressed in the phenotype, even if only one copy is present. A recessive allele is only expressed if two copies are present (i.e. no dominant allele is present). We use capital letters for dominant alleles and lowercase letters for recessive alleles, such as ‘B’ for brown eyes (dominant) and ‘b’ for blue eyes (recessive).
等位基因可以是显性或隐性。显性等位基因即使只存在一个拷贝也会在表型中表达。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。我们用大写字母表示显性等位基因，用小写字母表示隐性等位基因，例如用 ‘B’ 代表棕色眼睛（显性），’b’ 代表蓝色眼睛（隐性）。
If an individual has the alleles BB or Bb, they will have brown eyes because the dominant B allele masks the recessive b allele. Only the genotype bb will produce blue eyes. This principle is known as the law of dominance and is fundamental to predicting the outcome of genetic crosses.
如果一个个体拥有等位基因 BB 或 Bb，他们将拥有棕色眼睛，因为显性 B 等位基因掩盖了隐性 b 等位基因。只有基因型 bb 才会产生蓝色眼睛。这一原理被称为显性定律，是预测遗传杂交结果的基础。
4. Homozygous and Heterozygous | 纯合子与杂合子
An organism is homozygous for a trait if it has two identical alleles (e.g. BB or bb). It is heterozygous if it has two different alleles (e.g. Bb). Homozygous dominant individuals have two dominant alleles and express the dominant trait; homozygous recessive individuals have two recessive alleles and express the recessive trait.
如果生物体对某一性状拥有两个相同的等位基因（例如 BB 或 bb），则称为纯合子。如果拥有两个不同的等位基因（例如 Bb），则称为杂合子。显性纯合子个体拥有两个显性等位基因并表达显性性状；隐性纯合子个体拥有两个隐性等位基因并表达隐性性状。
A heterozygous individual carries one dominant and one recessive allele. They display the dominant characteristic but can pass the recessive allele to their offspring. Heterozygous individuals are often called ‘carriers’ when the recessive allele is associated with a genetic disorder.
杂合子个体携带一个显性等位基因和一个隐性等位基因。他们表现出显性性状，但可以将隐性等位基因传给后代。当隐性等位基因与遗传病相关时，杂合子个体通常被称为 ‘携带者’。
5. Genotype and Phenotype | 基因型与表型
Genotype refers to the specific combination of alleles an organism possesses (e.g. BB, Bb, or bb). Phenotype describes the observable physical or biochemical characteristics resulting from the genotype and its interaction with the environment. For instance, having brown eyes is a phenotype; the alleles responsible are the genotype.
基因型指生物体拥有的特定等位基因组合（例如 BB、Bb 或 bb）。表型描述的是由基因型及其与环境相互作用所产生的可观察的物理或生化特征。例如，拥有棕色眼睛是表型；而负责的等位基因组合则是基因型。
Sometimes environmental factors can influence phenotype without changing genotype. A plant with genes for tall growth may end up short if it lacks light or nutrients. In exam questions, you must distinguish clearly between genetic and environmental causes of variation.
有时环境因素可以影响表型而不改变基因型。一种具有高大生长基因的植物如果缺乏光照或营养，最终可能长得矮小。在考试题目中，你必须清楚地区分变异的遗传原因和环境原因。
6. Monohybrid Crosses and Punnett Squares | 单基因杂交与庞纳特方格
A monohybrid cross investigates the inheritance of a single characteristic controlled by one gene. A Punnett square is a grid used to predict the possible genotypes of offspring from a genetic cross. It shows all the combinations that can result when gametes from each parent fuse.
单基因杂交研究的是由单个基因控制的一种性状的遗传。庞纳特方格是一种用于预测遗传杂交后代可能基因型的网格。它显示了双亲配子融合时可能产生的所有组合。
For example, if both parents are heterozygous for the cystic fibrosis allele (Ff, where F = normal, f = cystic fibrosis), the Punnett square is:
例如，如果父母双方都是囊性纤维化等位基因的杂合子（Ff，其中 F = 正常，f = 囊性纤维化），庞纳特方格如下所示：
| F | f | |
| F | FF | Ff |
| f | Ff | ff |
The expected offspring ratio is 1 FF : 2 Ff : 1 ff. This means there is a ¾ chance of a normal phenotype and a ¼ chance of having cystic fibrosis. The offspring Ff are carriers but do not have the disorder.
预期的后代比例是 1 FF : 2 Ff : 1 ff。这意味着有 3/4 的概率表现正常，1/4 的概率患有囊性纤维化。后代 Ff 是携带者，但本身不患病。
7. Cystic Fibrosis | 囊性纤维化
Cystic fibrosis (CF) is an inherited recessive disorder caused by a faulty allele of the CFTR gene. It leads to the production of thick, sticky mucus that blocks airways and pancreatic ducts, causing breathing difficulties and digestive problems. A person must inherit two copies of the recessive allele (ff) to develop the disease.
囊性纤维化（CF）是一种由 CFTR 基因的有缺陷等位基因引起的隐性遗传病。它导致产生浓稠、黏稠的粘液，堵塞气道和胰管，引起呼吸困难和消化问题。一个人必须继承两个隐性等位基因（ff）才会患上这种疾病。
People with the genotype Ff are carriers; they do not show symptoms but can pass the faulty allele to their children. If both parents are carriers, each pregnancy has a 25% chance of producing a child with cystic fibrosis, a 50% chance of a carrier child, and a 25% chance of a child with two normal alleles.
基因型为 Ff 的人是携带者；他们没有症状，但可以将有缺陷的等位基因传给子女。如果父母双方都是携带者，每次怀孕有 25% 的几率生育一个患囊性纤维化的孩子，50% 的几率生育一个携带者孩子，以及 25% 的几率生育一个拥有两个正常等位基因的孩子。
8. Huntington’s Disease | 亨廷顿病
Huntington’s disease is a dominant genetic disorder caused by a faulty allele. Unlike recessive disorders, a person only needs one copy of the mutated allele to develop the condition. Symptoms usually appear in middle age and include involuntary movements, personality changes, and cognitive decline.
亨廷顿病是一种由有缺陷的等位基因引起的显性遗传病。与隐性遗传病不同，一个人只需要一个突变等位基因的拷贝就会发病。症状通常出现在中年，包括不自主运动、性格改变和认知能力下降。
If a parent is heterozygous (Hh, where H = Huntington’s allele, h = normal), each child has a 50% chance of inheriting the H allele and eventually developing the disease. There is no cure, and genetic testing can identify carriers before symptoms appear.
如果父母一方是杂合子（Hh，其中 H = 亨廷顿病等位基因，h = 正常），每个孩子有 50% 的几率继承 H 等位基因并最终患上该疾病。目前无法治愈，基因检测可以在症状出现前识别携带者。
9. Sex Determination | 性别决定
Human 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 Y chromosome carries the SRY gene, which triggers male development. All other chromosomes are called autosomes and are the same in both sexes.
人类的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）。Y 染色体携带 SRY 基因，该基因触发了男性发育。所有其他染色体称为常染色体，在男女两性中相同。
A Punnett square for sex inheritance shows that each pregnancy has a 50% chance of producing a boy (XY) and a 50% chance of a girl (XX), because the father can pass on either an X or a Y chromosome, while the mother always passes an X.
性别遗传的庞纳特方格显示，每次怀孕有 50% 的几率生育男孩（XY），50% 的几率生育女孩（XX），因为父亲可以传递 X 或 Y 染色体，而母亲总是传递 X 染色体。
10. Mutations | 突变
A mutation is a change in the base sequence of DNA. Mutations can occur spontaneously during DNA replication or be induced by environmental factors such as radiation or certain chemicals (mutagens). Most mutations are neutral or harmful; only rarely are they beneficial.
突变是 DNA 碱基序列的改变。突变可以在 DNA 复制过程中自发发生，也可以由环境因素如辐射或某些化学物质（诱变剂）诱发。大多数突变是中性的或有害的；只有极少数是有益的。
Some mutations alter a gene so that the protein it codes for no longer functions correctly, leading to genetic disorders. An example is the mutation causing sickle cell anaemia, where a single base change results in abnormal haemoglobin. Mutations in body cells cannot be inherited, but those in gametes can be passed to offspring.
一些突变改变了基因，使其编码的蛋白质无法正常运作，从而导致遗传病。一个例子是导致镰状细胞贫血的突变，单个碱基的变化导致异常的血红蛋白。体细胞中的突变不能遗传，但生殖细胞（配子）中的突变可以传给后代。
11. Family Pedigrees | 家族谱系图
A family pedigree chart is a diagram showing the inheritance of a trait over several generations. In such charts, squares represent males, circles represent females, and shaded symbols indicate individuals expressing the trait. A horizontal line between a square and circle represents mating, and vertical lines lead to offspring.
家族谱系图是一种显示某一性状在几代人中遗传的图表。在这种图表中，方块代表男性，圆圈代表女性，阴影符号表示表现出该性状的个体。方块和圆圈之间的水平线代表婚配，垂直线连接后代。
By analysing a pedigree, you can determine whether a trait is dominant or recessive. If the trait appears in every generation, it is likely dominant; if it skips generations, it is likely recessive. You may be asked to deduce genotypes of individuals using known allele patterns.
通过分析谱系图，你可以确定某一性状是显性还是隐性。如果该性状在每一代都出现，它很可能是显性的；如果它跳代出现，则很可能是隐性的。你可能会被要求使用已知的等位基因模式推断个体的基因型。
12. Causes of Variation | 变异的原因
Variation within a species can be caused by genetic factors, environmental factors, or a combination of both. Genetic variation arises from the different alleles inherited from parents, as well as from mutations. Environmental variation includes influences like diet, climate, and lifestyle.
物种内的变异可由遗传因素、环境因素或两者的结合引起。遗传变异来自于从父母那里继承的不同等位基因，以及来自突变。环境变异包括饮食、气候和生活方式等影响。
Continuous variation, such as height or skin colour, shows a range of phenotypes with no distinct categories and is often controlled by many genes and the environment. Discontinuous variation, such as blood group or tongue rolling, falls into clear, separate groups and is usually controlled by a single gene.
连续变异，如身高或肤色，显示一系列表型，没有明显类别，通常由许多基因和环境共同控制。不连续变异，如血型或卷舌，则分为清晰、独立的组别，通常由单个基因控制。
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