📚 GCSE WJEC Biology: Genetics Revision Guide | GCSE WJEC 生物：遗传学考点精讲
Genetics is a central topic in the WJEC GCSE Biology specification and regularly appears in exams through problem‑solving questions on monohybrid crosses, pedigree analysis, and genetic disorders. A solid understanding of the structure of DNA, the meaning of key terms, and the ability to construct and interpret Punnett squares are essential for success. This guide breaks down each concept into clear, bilingual explanations with worked examples.
遗传学是 WJEC GCSE 生物考试大纲的重点内容，经常以单基因杂交、家系分析和遗传病推断等应用题的形式出现。牢固掌握 DNA 的结构、准确理解专业术语，以及能列出并解读庞尼特方格，是取得高分的关键。本指南用清晰的中英双语讲解每个概念，并配有详尽的范例。
1. DNA, Genes and Chromosomes | DNA、基因与染色体
DNA is a double‑stranded molecule twisted into a helix that carries the genetic instructions for building proteins. A gene is a short section of DNA which codes for a specific protein, and thereby determines a particular characteristic, such as eye colour or blood type. In the nucleus, DNA is coiled and packaged into structures called chromosomes. Human body cells contain 46 chromosomes arranged in 23 pairs, while gametes contain 23 single chromosomes.
DNA 是一种双链螺旋分子，携带着合成蛋白质的遗传指令。基因是 DNA 分子上的一段特定序列，它为一条特定的蛋白质编码，从而决定某一性状，如眼睛颜色或血型。在细胞核中，DNA 缠绕、折叠成染色体。人类体细胞含有 46 条染色体（23 对），而配子含有 23 条单拷贝染色体。
WJEC examiners expect you to use these definitions precisely: a gene is a sequence of bases that codes for a polypeptide, and an allele is a different version of the same gene. Always link the concept to protein synthesis, not just to a ‘characteristic’.
WJEC 考官希望你精确使用定义：基因是编码多肽的碱基序列，等位基因则是同一基因的不同版本。始终将概念与蛋白质合成联系起来，而不只是说成一个“性状”。
2. Alleles: Dominant and Recessive | 等位基因：显性与隐性
For each gene, an individual inherits one allele from each parent. If the two alleles are different, one may mask the expression of the other. The allele that is always expressed in the phenotype when present is described as dominant (represented by a capital letter, e.g. T for tall). The allele that is only expressed when two copies are present is recessive (lowercase letter, e.g. t for short).
每个基因，个体从每一亲本各继承一个等位基因。如果两个等位基因不相同，其中一个可能抑制另一个的表达。那种只要存在就会在表型中表现出来的等位基因称为显性（用大写字母表示，如 T 代表高茎）。只有在两份拷贝都存在时才会表达的等位基因称为隐性（用小写字母表示，如 t 代表矮茎）。
In the WJEC exam, you must always define the symbols you use. A typical genetic diagram begins with: ‘Let T = allele for tall stem, t = allele for short stem’. Do not use letters that look similar, like C and G, when writing by hand.
在 WJEC 考试中，你必须先明确所使用符号的含义。典型的遗传图解开头是：“设 T = 高茎等位基因，t = 矮茎等位基因”。手写时切勿使用容易混淆的字母，例如 C 和 G。
3. Genotype and Phenotype | 基因型与表型
The genotype is the combination of alleles an organism possesses for a particular gene, e.g. TT, Tt, tt. The phenotype is the observable characteristic resulting from the genotype and its interaction with the environment, e.g. tall stem, short stem. A phenotype can be influenced by multiple genes (polygenic inheritance) or by factors such as nutrition, temperature, or light.
基因型是一个生物体针对某一基因所拥有的等位基因组合，如 TT、Tt、tt。表型则是由基因型及其与环境相互作用而产生的可观察性状，如高茎、矮茎。表型可受多个基因（多基因遗传）或营养、温度、光照等因素影响。
Examiners frequently ask for the distinction: ‘Include the words genotype and phenotype in your description’. For instance, if a plant has genotype Tt, its phenotype is tall because the dominant T allele is present.
考官常要求区分它们：“在描述中使用基因型和表型这两个词”。例如，若植物的基因型是 Tt，其表型是高茎，因为有显性 T 等位基因存在。
4. Homozygous and Heterozygous | 纯合与杂合
An individual with two identical alleles for a gene is homozygous (e.g. TT or tt). An individual with two different alleles is heterozygous (e.g. Tt). WJEC mark schemes often expect you to state whether a genotype is homozygous dominant, homozygous recessive, or heterozygous, as this determines the potential gametes produced.
拥有两个相同等位基因的个体是纯合的（如 TT 或 tt）。拥有两个不同等位基因的个体是杂合的（如 Tt）。WJEC 打分标准常常要求你说明某基因型是纯合显性、纯合隐性还是杂合，因为这决定了可能产生的配子种类。
A homozygous dominant parent produces only one type of gamete for that gene, carrying the dominant allele. A heterozygous parent produces two types of gametes in equal proportions: one carries the dominant allele, the other the recessive allele.
纯合显性亲本只产生该基因的一种配子，携带显性等位基因。杂合亲本则产生两种配子，且比例相等：一种携带显性等位基因，另一种携带隐性等位基因。
5. Monohybrid Inheritance and Punnett Squares | 单基因杂交与庞尼特方格
Monohybrid inheritance is the study of how a single characteristic controlled by one gene is passed from parents to offspring. A Punnett square is a grid used to predict the genotypes of offspring from a genetic cross. All WJEC exam boards require you to draw and complete Punnett squares, then calculate phenotypic ratios.
单基因遗传研究由单一基因控制的性状如何从亲本传递给后代。庞尼特方格是一种用来预测杂交后代基因型的表格。WJEC 考试要求你能画出并完成庞尼特方格，然后计算表型比例。
Example: Cross a heterozygous tall pea plant (Tt) with a short pea plant (tt). Gametes are T, t from the tall parent and t, t from the short parent.
例子：将杂合高茎豌豆（Tt）与矮茎豌豆（tt）杂交。高茎亲本的配子为 T 和 t，矮茎亲本的配子为 t 和 t。
| t | t | |
| T | Tt | Tt |
| t | tt | tt |
The offspring genotype ratio is 1 Tt : 1 tt, and the phenotype ratio is 1 tall : 1 short (or 50% tall, 50% short). Always reduce ratios to the simplest whole numbers.
后代基因型比例为 1 Tt : 1 tt，表型比例为 1 高茎 : 1 矮茎（或 50% 高茎，50% 矮茎）。务必将比例约简至最简整数比。
6. Family Pedigree Charts | 家族谱系图
A pedigree chart shows the inheritance of a trait over several generations. Females are represented by circles, males by squares. Shaded symbols indicate individuals who express the trait. Horizontal lines connect parents; vertical lines lead to offspring. WJEC expects you to deduce genotypes of family members and determine whether a trait is dominant or recessive by analysing the pattern of inheritance.
家系图展示某一性状在几代人中的传递情况。女性用圆圈表示，男性用方框表示。着色符号表示表达该性状的个体。水平线连接双亲，垂直线指向子女。WJEC 要求你能通过分析遗传模式推断家庭成员的基因型，并判断该性状是显性还是隐性。
Key rule: If two unaffected parents have an affected child, the trait is recessive, and both parents must be heterozygous carriers. If an affected child has one affected parent, the trait may be dominant. Always write the genotype next to each symbol when answering.
关键规律：如果两个不患病的父母生出一个患病的孩子，该性状是隐性遗传，双亲必定是杂合携带者。如果患病孩子有一位患病亲本，该性状可能是显性。答题时要在每个符号旁标注基因型。
7. Sex Determination | 性别决定
In humans, gender is determined by the 23rd pair of chromosomes: females have XX, males have XY. The Y chromosome carries the SRY gene that triggers male development. During gamete formation, females produce only X‑bearing eggs, while males produce sperm with either an X or a Y chromosome. Thus, the father’s sperm determines the sex of the child.
人类性别由第 23 对染色体决定：女性为 XX，男性为 XY。Y 染色体携带 SRY 基因，触发男性发育。配子形成时，女性只产生含 X 的卵细胞，男性则产生含 X 或含 Y 的精子。因此，父亲的精子决定孩子的性别。
Punnett square: XX × XY → ½ XX (female) + ½ XY (male)
庞尼特方格：XX × XY → ½ XX（女性）+ ½ XY（男性）
The ratio of males to females is statistically 1:1, though in small families it may vary. Use a genetic diagram to show gametes and offspring genotypes, always labelling the X and Y chromosomes correctly.
男女比例在统计上为 1:1，但在小家庭中可能有所偏离。要用遗传图解展示配子和后代基因型，并正确标注 X 和 Y 染色体。
8. Genetic Disorders: Cystic Fibrosis and Huntington’s Disease | 遗传病：囊性纤维化与亨廷顿病
WJEC requires knowledge of two inherited disorders. Cystic fibrosis is caused by a recessive allele of the CFTR gene, leading to thick, sticky mucus in the lungs and digestive system. A person must inherit two recessive alleles (ff) to have the disease; carriers (Ff) are healthy but can pass on the allele.
WJEC 要求掌握两种遗传病。囊性纤维化由 CFTR 基因的隐性等位基因引起，导致肺部及消化系统中黏液变得黏稠。必须继承两个隐性等位基因（ff）才会患病；携带者（Ff）表现健康但能传递该等位基因。
Huntington’s disease is caused by a dominant allele of the HTT gene. Symptoms, including uncontrolled movements and cognitive decline, usually appear in middle age. An individual only needs one copy of the dominant allele (Hh or HH) to develop the disease. A parent with the disease has a 50% chance of passing it on to each child.
亨廷顿病由 HTT 基因的显性等位基因所致。症状包括不受控制的运动以及认知能力下降，通常在中晚年出现。个体只需一个显性等位基因（Hh 或 HH）即可发病。患病父母将致病等位基因传给每个孩子的概率为 50%。
You must be able to predict the inheritance of these conditions using Punnett squares and give advice to families based on the probability, not a guarantee, of a child inheriting the disorder.
你必须能用庞尼特方格预测这些病的遗传规律，并根据孩子患病的概率（而非绝对保证）为家庭提供咨询。
9. Variation: Genetic and Environmental | 变异：遗传与环境
Differences between individuals of the same species are called variation. Genetic variation arises from mutations, the random assortment of chromosomes in meiosis, and fertilisation. Environmental factors such as diet, climate, exercise, and light also influence the phenotype. Most characteristics result from a combination of both.
同一物种个体间的差异称为变异。遗传变异源于突变、减数分裂中染色体的随机分配以及受精作用。饮食、气候、锻炼、光照等环境因素同样影响表型。大多数特征由两者共同决定。
Continuous variation (e.g. height, mass) shows a range of values and is often controlled by many genes and the environment. Discontinuous variation (e.g. blood group, tongue-rolling ability) falls into distinct categories and is usually controlled by a single gene with little environmental influence. WJEC may ask you to plot and interpret bar charts or histograms accordingly.
连续变异（如身高、体重）显示出一个数值范围，常由多个基因与环境共同控制。不连续变异（如血型、卷舌能力）属于明确类别，通常由单一基因控制，环境影响小。WJEC 可能要求你据此绘制并解读条形图或直方图。
10. Mutations | 突变
A mutation is a random, permanent change in the DNA sequence of a gene or chromosome. Mutations can be neutral, harmful, or rarely beneficial. They introduce new alleles into a population, providing the raw material for evolution. Ionising radiation (X‑rays, UV) and certain chemicals (carcinogens) increase the mutation rate.
突变是基因或染色体 DNA 序列发生的随机、永久性变化。突变可能是中性的、有害的，极少数情况下也可能是有益的。它们为种群引入新等位基因，是进化的原材料。电离辐射（X 射线、紫外线）和某些化学物质（致癌物）会增加突变率。
In cystic fibrosis, a deletion of three bases removes a single amino acid from the CFTR protein, causing it to malfunction. This example is frequently used in WJEC questions to link mutation to a change in protein structure and function. Always state that a change in the DNA base sequence leads to a different sequence of amino acids, altering the protein’s shape so it cannot perform its normal role.
在囊性纤维化病例中，三个碱基的缺失导致 CFTR 蛋白少了一个氨基酸，造成其功能失常。这个例子经常出现在 WJEC 试题中，用于联系突变与蛋白质结构和功能的改变。始终要说明：DNA 碱基序列的改变导致氨基酸序列不同，进而改变蛋白质形状，使其无法执行正常功能。
11. Mendel’s Laws and Genetic Diagrams | 孟德尔定律与遗传图解
Gregor Mendel’s experiments with pea plants led to the formulation of the Law of Segregation: during gamete formation, the two alleles for each gene separate, so each gamete receives only one allele. This explains why offspring inherit a mix of traits from both parents. You do not need to name the law in WJEC, but you must apply the principle by correctly separating alleles in genetic diagrams.
格雷戈尔·孟德尔的豌豆实验得出了分离定律：配子形成时，每个基因的两个等位基因彼此分离，每个配子只得到一个等位基因。这解释了后代为何从双亲继承混合的性状。WJEC 考试无需说出定律名称，但你必须在遗传图解中正确分离等位基因来应用这一原理。
A complete genetic diagram must include: a clear key, parental phenotypes and genotypes, gametes (with circles drawn around them), a Punnett square or cross‑fertilisation grid, and offspring genotypes and phenotypes with ratios. Many marks are lost by omitting the key or mislabelling gametes.
一份完整的遗传图解必须包含：清晰的符号说明、亲本表型与基因型、配子（画圆圈框起来）、庞尼特方格或杂交表格，以及后代基因型、表型及其比例。漏写符号说明或错误标注配子是常见的失分点。
12. Exam Tips and Common Mistakes | 考试技巧与常见错误
Always read the question carefully to identify whether the characteristic is dominant or recessive; this information is often hidden in the stem, e.g. ‘two unaffected parents have an affected child’. In pedigree questions, start by shading individuals with known phenotypes, then assign genotypes to the outward generation and work inward.
永远仔细阅读题目，判断性状是显性还是隐性；这一信息常常隐藏在题干中，例如“两个正常父母生出一个患儿”。家系图题中，先给已知表型的个体涂色，然后从最外一代向内逐步分配基因型。
Avoid writing gametes as double letters (e.g. TT) — each gamete contains only one allele per gene. Use consistent lettering and never change the case of your symbol halfway through a cross. If you are asked the probability of a child suffering from a recessive condition, express the answer as a percentage or fraction, such as 25% or ¼, not ‘1 in 4 chance’ alone.
避免将配子写成双字母（如 TT）——每个配子每个基因只含一个等位基因。字母使用要保持一致，切勿在杂交过程中途更改大小写。如果问的是孩子患隐性遗传病的概率，答案要用百分数或分数表示，如 25% 或 ¼，单独写“四分之一概率”可能不够。
Finally, check your ratios: the sum of all phenotype fractions should equal 1. If a Punnett square gives 4 offspring boxes, a ratio of 3:1 should be reported as 75% : 25%, not 3:1 without context. Practice drawing diagrams quickly and neatly, as time is limited in the examination hall.
最后，检查比例：所有表型分数的总和应为 1。如果庞尼特方格有 4 个格子，3:1 的比例应表达为 75% : 25%，不要单独写 3:1 而无上下文。考试时间有限，要多练习迅速且整洁地绘制图解。
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