📚 GCSE WJEC Science: Genetics Revision Guide | GCSE WJEC 科学：遗传 考点精讲
Genetics is the branch of biology that explains how characteristics are passed from parents to offspring. In the WJEC GCSE Science specification, you need to understand the structure of DNA, how genes control traits, and how inheritance patterns can be predicted using genetic diagrams. This revision guide covers all key topics, including monohybrid crosses, family pedigrees, sex determination, inherited disorders, and modern applications like genetic engineering.
遗传学是生物学中解释特征如何从亲代传递给后代的分支。在 WJEC GCSE 科学大纲中，你需要理解 DNA 的结构、基因如何控制性状，以及如何使用遗传图解预测遗传模式。本复习指南涵盖所有关键主题，包括单基因杂交、家族谱系、性别决定、遗传疾病以及基因工程等现代应用。
1. DNA and Chromosomes | DNA 与染色体
DNA (deoxyribonucleic acid) is a long, double-stranded molecule that carries the genetic code. In eukaryotic cells, DNA is found inside the nucleus, organised into structures called chromosomes. Each chromosome contains many genes. Human body cells have 46 chromosomes arranged in 23 pairs.
DNA（脱氧核糖核酸）是一种携带遗传密码的长链双螺旋分子。在真核细胞中，DNA 位于细胞核内，组织成称为染色体的结构。每条染色体含有许多基因。人体细胞有 46 条染色体，排列成 23 对。
Chromosomes are made of DNA tightly coiled around proteins called histones. This packaging allows a large amount of DNA to fit into the nucleus. During cell division, chromosomes become visible under a light microscope.
染色体由紧密缠绕在组蛋白上的 DNA 组成。这种包装使大量 DNA 能容纳在细胞核中。在细胞分裂过程中，染色体在光学显微镜下变得可见。
2. Genes and Alleles | 基因与等位基因
A gene is a short section of DNA that codes for a particular protein. Different versions of the same gene are called alleles. For example, the gene for eye colour has alleles for blue, brown, green, etc. Alleles occupy the same position (locus) on homologous chromosomes.
基因是编码特定蛋白质的 DNA 短片段。同一基因的不同版本称为等位基因。例如，眼睛颜色的基因有蓝色、棕色、绿色等等位基因。等位基因位于同源染色体上的相同位置（基因座）。
An organism inherits two alleles for each gene, one from each parent. These alleles may be the same (homozygous) or different (heterozygous).
一个生物体每个基因从父母各继承一个等位基因，共两个等位基因。这些等位基因可能相同（纯合）或不同（杂合）。
3. Dominant and Recessive Alleles | 显性与隐性等位基因
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 an individual has two copies (homozygous recessive). Dominant alleles are represented by an uppercase letter (e.g., A), while recessive alleles use the corresponding lowercase letter (e.g., a).
有些等位基因是显性的，这意味着即使只有一份拷贝，它们也会在表现型中表达出来。隐性等位基因只有当个体拥有两个拷贝（纯合隐性）时才会表达。显性等位基因用大写字母表示（如 A），隐性等位基因则使用相应的小写字母（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.
例如，在豌豆植株中，高茎等位基因（T）对矮茎等位基因（t）是显性的。基因型为 Tt 的植株将是高茎。
4. Genotype and Phenotype | 基因型与表现型
Genotype refers to the combination of alleles an organism has for a particular trait (e.g., AA, Aa, aa). Phenotype is the observable characteristic that results from the genotype and environmental factors (e.g., tall or short stem).
基因型指的是一个生物体对于某一特定性状所拥有的等位基因组合（如 AA、Aa、aa）。表现型是由基因型和环境因素共同作用下形成的可观察特征（如高茎或矮茎）。
Phenotype is not solely determined by genotype; the environment can influence how genes are expressed. For instance, nutrition affects height in humans.
表现型不仅仅由基因型决定；环境也能影响基因的表达方式。例如，营养会影响人类的身高。
5. Monohybrid Crosses and Punnett Squares | 单基因杂交与庞尼特方格
A monohybrid cross examines the inheritance of a single gene. A Punnett square is a diagram used to predict the probability of offspring genotypes and phenotypes. To construct a Punnett square:
单基因杂交研究的是单个基因的遗传。庞尼特方格是一种用来预测后代基因型和表现型概率的图表。构建庞尼特方格的步骤如下：
- Determine the parental genotypes.
- 确定亲本基因型。
- Write the possible gametes from each parent along the top and side.
- 将每个亲本可能产生的配子写在方格顶部和侧边。
- Fill in the grid by combining the alleles.
- 通过组合等位基因填写方格。
If both parents are heterozygous (Tt × Tt), the Punnett square gives a genotype ratio of 1 TT : 2 Tt : 1 tt and a phenotype ratio of 3 tall : 1 short.
如果双亲均为杂合子（Tt × Tt），庞尼特方格得出的基因型比为 1 TT : 2 Tt : 1 tt，表现型比为 3 高 : 1 矮。
6. Family Pedigrees | 家族谱系分析
A family pedigree chart is used to track the inheritance of a trait across generations. Males are represented by squares, females by circles. Affected individuals are shaded. Pedigrees can reveal whether an allele is dominant or recessive.
家族谱系图用于追踪某一性状在几代中的遗传情况。男性用方块表示，女性用圆圈表示。患病个体用阴影标记。谱系图可以揭示某等位基因是显性还是隐性。
A dominant trait will appear in every generation, while a recessive trait may skip generations if carriers (heterozygotes) remain unaffected.
显性性状会在每一代中都出现，而隐性性状如果携带者（杂合子）不受影响，则可能隔代出现。
7. Sex Determination | 性别决定
In humans, sex is determined by the 23rd pair of chromosomes: females have two X chromosomes (XX), males have one X and one Y chromosome (XY). The father determines the sex of the child because he can pass on either an X or a Y chromosome.
人类的性别由第 23 对染色体决定：女性有两条 X 染色体（XX），男性有一条 X 和一条 Y 染色体（XY）。父亲决定了孩子的性别，因为他可以传递 X 或 Y 染色体。
A Punnett square for sex determination shows a 50% chance of having a boy (XY) and a 50% chance of having a girl (XX) in each pregnancy.
性别决定的庞尼特方格显示每次怀孕有 50% 概率生男孩（XY），50% 概率生女孩（XX）。
8. Inherited Disorders | 遗传疾病
Some disorders are caused by recessive alleles, such as cystic fibrosis and sickle cell anaemia. Others are caused by dominant alleles, like Huntington’s disease. Cystic fibrosis affects the production of mucus, leading to lung and digestive problems. It occurs when an individual is homozygous recessive (ff).
有些疾病由隐性等位基因引起，如囊肿性纤维化与镰刀型细胞贫血症。有些则由显性等位基因引起，如亨廷顿舞蹈症。囊肿性纤维化影响黏液的产生，导致肺部和消化问题。当个体为隐性纯合子（ff）时患病。
Below is a summary of examples:
以下是示例总结：
| Disorder 疾病 | Allele Type 等位基因类型 | Genotype 基因型 |
|---|---|---|
| Cystic fibrosis 囊肿性纤维化 | Recessive 隐性 | ff |
| Huntington’s disease 亨廷顿舞蹈症 | Dominant 显性 | Hh or HH (Hh 或 HH) |
9. Variation: Continuous and Discontinuous | 变异：连续与不连续
Variation refers to differences between individuals of the same species. It can be continuous (e.g., height, weight) or discontinuous (e.g., blood group, tongue rolling). Continuous variation is influenced by many genes and environment, showing a normal distribution. Discontinuous variation is controlled by a single gene with distinct categories.
变异是指同一物种个体间的差异。它可以是连续的（如身高、体重）或不连续的（如血型、卷舌）。连续变异受多基因和环境的影响，呈正态分布。不连续变异由单个基因控制，具有明显分类。
Genetic variation arises from mutations, meiosis (independent assortment and crossing over), and random fertilisation. These processes create new allele combinations.
遗传变异来源于突变、减数分裂（自由组合与交叉互换）以及随机受精。这些过程产生新的等位基因组合。
10. Mutations | 突变
A mutation is a random change in the DNA sequence. Mutations can occur spontaneously or due to mutagens like radiation or chemicals. Most mutations are neutral, some are harmful, and very few may be beneficial. A mutation in a gene can produce a new allele, potentially altering the phenotype.
突变是 DNA 序列的随机变化。突变可自发发生或由诱变因素（如辐射或化学物质）引起。大多数突变是中性的，有些是有害的，极少数可能是有益的。基因的突变可产生新的等位基因，可能改变表现型。
If a mutation occurs in gametes, it can be passed on to offspring, contributing to genetic variation within a population.
如果突变发生在配子中，它可以传递给后代，从而增加种群内的遗传变异。
11. Selective Breeding | 选择性繁殖
Selective breeding (artificial selection) involves choosing parents with desirable traits to produce offspring with those traits. Over many generations, the characteristic becomes more pronounced. Examples include breeds of dogs, high-yield crops, and dairy cattle.
选择性繁殖（人工选择）是指选择具有理想性状的亲本，以产生具有这些性状的后代。经过多代之后，该特征变得更加突出。例子包括狗的品种、高产品种作物和奶牛。
Selective breeding reduces genetic diversity within the population and can lead to health problems due to inbreeding.
选择性繁殖减少了种群内的遗传多样性，并可能因近亲繁殖而导致健康问题。
12. Genetic Engineering and Cloning | 基因工程与克隆
Genetic engineering involves modifying the genome of an organism by introducing a foreign gene. This can give the organism new properties, such as bacteria producing human insulin or crops resistant to pests.
基因工程涉及通过引入外源基因来修改生物体的基因组。这可以使生物体获得新的特性，例如细菌产生人胰岛素或农作物抗虫害。
Adult cell cloning creates a genetically identical copy of an organism. The nucleus from an adult body cell is transferred into an empty egg cell, stimulated to divide, and implanted into a surrogate mother. This is how Dolly the sheep was produced.
成体细胞克隆创造出一个遗传上完全相同的生物体拷贝。将成体细胞中的细胞核转移到去核的卵细胞中，刺激
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