📚 IGCSE WJEC Biology: Genetics Revision Guide | IGCSE WJEC 生物：遗传学 考点精讲
Genetics is the branch of biology that explains how traits are passed from parents to offspring. It unites concepts of DNA, genes, chromosomes and variation, forming the foundation for understanding inheritance, evolution and many aspects of modern biology. This revision guide targets key topics from the WJEC IGCSE Biology specification, breaking down essential ideas into clear, exam-focused explanations with both English and Chinese text for dual-language learners.
遗传学是生物学中解释性状如何从亲代传递给后代的分支。它将DNA、基因、染色体和变异等概念统一起来，为理解遗传、进化以及现代生物学的许多方面奠定了基础。这份考点精讲针对WJEC IGCSE生物大纲，将核心知识点拆解为清晰、紧扣考试的双语解释，方便中英双语学习者掌握。
1. DNA, Genes and Chromosomes | DNA、基因和染色体
Deoxyribonucleic acid (DNA) is the molecule that carries the genetic blueprint of an organism. It is a long polymer made of nucleotides, each consisting of a sugar, a phosphate group and a nitrogenous base (adenine, thymine, cytosine or guanine). The sequence of these bases encodes the information needed to build proteins.
脱氧核糖核酸（DNA）是携带生物体遗传蓝图的分子。它是一种由核苷酸组成的长链聚合物，每个核苷酸包含一个糖、一个磷酸基团和一个含氮碱基（腺嘌呤、胸腺嘧啶、胞嘧啶或鸟嘌呤）。这些碱基的序列编码了构建蛋白质所需的信息。
A gene is a specific segment of DNA that codes for a particular protein or functional RNA. Each gene occupies a fixed position, or locus, on a chromosome. Chromosomes are thread-like structures found in the nucleus of eukaryotic cells; they are made of tightly coiled DNA wrapped around histone proteins. Humans have 46 chromosomes arranged in 23 pairs.
基因是DNA中编码特定蛋白质或功能性RNA的特定片段。每个基因在染色体上占据一个固定的位置，即基因座。染色体是真核细胞细胞核中的线状结构，由紧密盘绕在组蛋白上的DNA组成。人类有46条染色体，排列成23对。
During cell division, chromosomes replicate and condense, becoming visible under a light microscope. The diploid number (2n) in humans is 46, while gametes (sperm and egg cells) have the haploid number (n) of 23 chromosomes. This halving occurs through meiosis, ensuring that fertilisation restores the diploid number.
在细胞分裂过程中，染色体会复制并浓缩，在光学显微镜下变得可见。人类的二倍体数目（2n）是46，而配子（精子和卵细胞）具有23条染色体的单倍体数目（n）。这种数目减半是通过减数分裂实现的，确保受精后恢复二倍体数目。
2. Alleles and Genetic Terminology | 等位基因与遗传术语
An allele is an alternative version of a gene. For example, a gene that controls flower colour may have an allele for purple flowers and another allele for white flowers. Individuals inherit two alleles for each gene, one from each parent. The combination of alleles is called the genotype, while the observable characteristic is the phenotype.
等位基因是同一基因的不同版本。例如，控制花色的基因可能有一个开紫花的等位基因和另一个开白花的等位基因。个体从每个亲本继承一个等位基因，即每个基因有两个等位基因。等位基因的组合称为基因型，而可观察到的特征称为表现型。
A dominant allele always expresses its trait when present, even if only one copy is inherited. It is represented by an uppercase letter (e.g. A). A recessive allele is masked by a dominant allele and only affects the phenotype when two copies are present (homozygous recessive). It is shown with a lowercase letter (e.g. a).
显性等位基因只要存在就会表达其性状，即使只遗传了一个拷贝。它用大写字母表示（例如A）。隐性等位基因会被显性等位基因掩盖，只有在两个拷贝都存在时（隐性纯合子）才会影响表现型。它用小写字母表示（例如a）。
If an individual has two identical alleles for a gene, they are homozygous (e.g. AA or aa). If the alleles are different, they are heterozygous (e.g. Aa). In heterozygotes, the dominant allele determines the phenotype. The terms pure-breeding and carrier are also used: a pure-breeding organism is homozygous, while a carrier is heterozygous for a recessive condition but does not show it.
如果一个个体某个基因的两个等位基因相同，则称为纯合子（如AA或aa）。如果等位基因不同，则称为杂合子（如Aa）。在杂合子中，显性等位基因决定表现型。纯种和携带者这两个术语也经常使用：纯种生物体是纯合的，而携带者对于隐性性状是杂合的但不表现出来。
3. Monohybrid Inheritance | 单基因遗传
Monohybrid inheritance involves the study of a single characteristic determined by one gene with two different alleles. Gregor Mendel’s classic pea plant experiments demonstrated that traits are inherited in predictable patterns. When a pure-breeding dominant parent is crossed with a pure-breeding recessive parent, all first-generation (F₁) offspring show the dominant phenotype and are heterozygous.
单基因遗传研究的是由一个基因及其两个不同等位基因决定的单一性状。孟德尔经典的豌豆实验证明了性状可以按可预测的模式遗传。当纯种显性亲本与纯种隐性亲本杂交时，所有子一代（F₁）个体都表现出显性表现型，且均为杂合子。
If two F₁ heterozygotes are crossed, the F₂ generation typically produces a phenotypic ratio of 3:1 (dominant to recessive) and a genotypic ratio of 1:2:1 (homozygous dominant : heterozygous : homozygous recessive). This is explained by the segregation of alleles during gamete formation, where each gamete receives only one allele of the pair.
如果将两个F₁杂合子杂交，F₂代通常会产生3:1的表现型比例（显性:隐性）和1:2:1的基因型比例（纯合显性:杂合:纯合隐性）。这可以通过配子形成过程中等位基因的分离来解释：每个配子只获得成对等位基因中的一个。
Example cross: Tall (T) × Dwarf (t) pea plants
Parental generation: TT × tt
F₁ all Tt (tall); F₂ cross: Tt × Tt → 3 tall : 1 dwarf
This simple dominance model works for many traits, but there are exceptions such as codominance, where both alleles are expressed equally in the heterozygote (e.g. AB blood type), and incomplete dominance, where the heterozygote shows a blended phenotype.
这一简单的显性模型适用于许多性状，但也存在例外，比如共显性（杂合子中两个等位基因同等表达，如AB血型）和不完全显性（杂合子表现出混合的表现型）。
4. Punnett Squares and Predicting Ratios | 旁氏表与比例预测
A Punnett square is a grid used to predict the possible genotypes of offspring from a genetic cross. The possible gametes of one parent are placed along the top, and the gametes of the other parent are listed down the side. The boxes are then filled with the allele combinations to show all expected offspring genotypes.
旁氏表是一种用于预测遗传杂交后代可能基因型的方格图。将一方亲本的可能配子放在顶部，另一方亲本的配子列在侧边。然后在方格中填入等位基因组合，以显示所有预期的后代基因型。
Let us use the cross between two heterozygous tall pea plants (Tt × Tt) as an example. Each plant produces gametes carrying either T or t. The Punnett square below gives a genotypic ratio of 1 TT : 2 Tt : 1 tt and a phenotypic ratio of 3 tall : 1 dwarf.
我们以两株杂合高茎豌豆（Tt × Tt）之间的杂交为例。每株植物产生携带T或t的配子。下面的旁氏表得出基因型比例为1 TT : 2 Tt : 1 tt，表现型比例为3高茎 : 1矮茎。
| T | t | |
| T | TT | Tt |
| t | Tt | tt |
When interpreting Punnett squares, it is essential to remember that the ratios are probabilities. Each offspring has an independent chance of inheriting the alleles; a 3:1 ratio is expected over large numbers, but small samples may deviate from this.
在解读旁氏表时，必须记住这些比例是概率。每个后代都有独立的等位基因遗传概率；3:1的比例是大样本下的期望值，小样本可能偏离这一比例。
You may also be asked to predict the outcomes of a test cross: crossing an organism showing the dominant phenotype (but unknown genotype) with a homozygous recessive individual. If any offspring show the recessive trait, the unknown parent must be heterozygous.
你可能还会被要求预测测交的结果：将表现出显性表现型（但基因型未知）的个体与隐性纯合个体杂交。如果任何后代表现出隐性性状，则未知亲本一定是杂合子。
5. Pedigree Analysis | 家族谱系分析
A family pedigree is a diagram that shows the inheritance of a particular trait over several generations. Standard symbols are used: squares represent males, circles represent females. Shaded symbols indicate individuals showing the trait, while unshaded symbols represent those without it. Horizontal lines connect partners, and vertical lines descend to their children.
家族谱系图是一种显示某一特定性状在几代人中遗传情况的图表。使用标准符号：正方形代表男性，圆形代表女性。阴影符号表示表现出该性状的个体，未阴影符号代表不表现该性状的个体。水平线连接伴侣，竖线向下指向他们的子女。
By analysing a pedigree, you can often determine whether a trait is dominant or recessive, and whether it is autosomal (carried on a non-sex chromosome) or sex-linked. If a trait appears in every generation and affected individuals have at least one affected parent, it is likely dominant. If it skips generations and can appear in offspring of unaffected parents, it is likely recessive.
通过分析谱系图，通常可以判断一个性状是显性还是隐性，是常染色体遗传（位于非性染色体上）还是伴性遗传。如果一个性状在每一代中都出现，且受累个体至少有一个受累亲本，则很可能为显性。如果它隔代出现，并且可能出现在未受累父母的子女中，则很可能为隐性。
For a dominant autosomal trait, heterozygous individuals (Aa) show the trait and can pass it to roughly 50% of their children if the partner is homozygous recessive. For a recessive autosomal trait, affected individuals are homozygous (aa), and both parents must at least be carriers.
对于常染色体显性性状，杂合个体（Aa）会表现出该性状，如果伴侣是隐性纯合，则可将该性状传给大约50%的子女。对于常染色体隐性性状，受累个体为纯合子（aa），且双亲至少必须是携带者。
Identifying patterns of inheritance in pedigrees is a common exam question. Always annotate genotypes where possible and use Punnett squares to confirm your conclusions.
在谱系图中识别遗传模式是常见的考试题目。尽可能标注基因型，并使用旁氏表来验证你的结论。
6. Sex Determination and Sex Chromosomes | 性别决定与性染色体
In humans and many other organisms, sex is determined by a special pair of chromosomes called the 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.
在人类和许多其他生物中，性别由一对特殊的性染色体决定。女性拥有两条X染色体（XX），男性拥有一条X和一条Y染色体（XY）。Y染色体携带SRY基因，该基因启动雄性发育。
During gamete formation, female eggs always carry a single X chromosome. Male sperm can carry either an X or a Y chromosome. Therefore, the sperm from the father determines the sex of the child. A Punnett square for sex inheritance shows a 1:1 chance of having a boy (XY) or a girl (XX).
在配子形成过程中，女性的卵子始终携带一条X染色体。男性的精子则可以携带X或Y染色体。因此，父亲的精子决定了孩子的性别。性别遗传的旁氏表显示，生男孩（XY）或女孩（XX）的概率各为1:1。
Cross: Father (XY) × Mother (XX)
Gametes: X or Y from father; all X from mother. Offspring: 50% XX female, 50% XY male.
It is important not to confuse gender determination with sex-linked inheritance, where genes located on the X chromosome (but absent from the Y) show different inheritance patterns in males and females. For example, red-green colour blindness is more common in males because they need only one recessive allele on the single X chromosome to express it.
不要将性别决定与伴性遗传混淆，伴性遗传中位于X染色体上（但Y染色体上不存在）的基因在男性和女性中表现出不同的遗传模式。例如，红绿色盲在男性中更常见，因为他们只需在唯一的X染色体上有一个隐性等位基因即可表现出来。
7. Continuous and Discontinuous Variation | 连续变异与不连续变异
Variation refers to the differences between individuals of the same species. It can be classified as continuous or discontinuous. Continuous variation produces a range of phenotypes that fall along a smooth spectrum, such as height, mass or skin colour. These traits are usually controlled by many genes (polygenic) and are strongly influenced by environmental factors.
变异是指同一物种个体之间的差异。它可以分为连续变异和不连续变异。连续变异产生一系列沿平滑谱系分布的表现型，例如身高、体重或肤色。这些性状通常由许多基因控制（多基因），并受到环境因素的强烈影响。
When you plot continuous data on a frequency graph, it shows a normal distribution (bell-shaped curve). Examples are easily found in human populations: shoe size, leaf length and milk yield in cows. Selection for these traits often relies on measuring and breeding from the best performers.
当你将连续数据绘制成频率图时，会呈现正态分布（钟形曲线）。例子在人群中很常见：鞋码、叶片长度和奶牛的产奶量。对这些性状的选择通常依赖于度量表现最佳者并进行育种。
Discontinuous variation, on the other hand, produces distinct categories with no intermediates. Traits such as blood group (A, B, AB, O), eye colour in fruit flies or the ability to roll one’s tongue are usually controlled by a single gene with clear-cut alleles. Environmental influence is minimal. When graphed, discontinuous data show a bar chart with separate groups.
另一方面，不连续变异产生界限分明的类别，没有中间类型。血型（A、B、AB、O）、果蝇的眼色或卷舌能力等性状通常由单个基因及其明确的等位基因控制，环境影响极小。绘图时，不连续数据表现为独立的柱状图。
Both types of variation provide the raw material for natural and artificial selection. Recognising the pattern helps in predicting how traits can be inherited and selected.
这两种变异类型都为自然选择和人工选择提供了原始材料。识别变异模式有助于预测性状的遗传和选择方式。
8. Mutation, Natural Selection and Selective Breeding | 突变、自然选择与选择性育种
A mutation is a permanent change in the DNA sequence. Mutations can arise spontaneously during DNA replication or be induced by mutagens such as radiation and certain chemicals. Most mutations are neutral or harmful, but occasionally a mutation can produce a beneficial new trait.
突变是DNA序列的永久性改变。突变可能在DNA复制过程中自发产生，也可能由辐射和某些化学物质等诱变剂诱导。大多数突变是中性的或有害的，但偶尔也会产生出有利的新性状。
Beneficial mutations may increase an organism’s chance of survival and reproduction. Through the process of natural selection, individuals with advantageous traits are more likely to survive and pass their alleles to the next generation. Over many generations, the frequency of favourable alleles increases in the population, leading to evolution.
有利突变可能会增加生物体生存和繁殖的机会。通过自然选择的过程，具有优势性状的个体更有可能存活下来，并将它们的等位基因传给下一代。经过许多代后，有利等位基因在种群中的频率上升，导致进化。
Antibiotic resistance in bacteria is a clear example of natural selection driven by mutation. Bacteria that randomly acquire a resistance gene survive antibiotic treatment and multiply, making the infection harder to treat. Similarly, the evolution of peppered moths during the Industrial Revolution illustrates how environmental changes shift selection pressures.
细菌的抗生素耐药性是突变驱动自然选择的一个明显例子。随机获得耐药基因的细菌在抗生素治疗中存活并繁殖，使感染更难治疗。同样，工业革命期间桦尺蛾的进化说明了环境变化如何改变选择压力。
Selective breeding (artificial selection) is the deliberate human choice of parent organisms with desirable traits to produce offspring with those traits. Over time, this leads to a population with enhanced characteristics, such as higher crop yield, disease resistance or docile behaviour in domesticated animals. It differs from natural selection because humans, not the environment, decide which individuals breed.
选择性育种（人工选择）是人类有目的地挑选具有理想性状的亲本生物进行繁殖，以产生具有这些性状的后代。随着时间的推移，这会导致种群特征得到强化，例如更高的作物产量、抗病性或家养动物温顺的行为。它与自然选择不同，因为是人类而非环境决定哪些个体进行繁殖。
However, selective breeding reduces genetic diversity and can inadvertently concentrate harmful recessive alleles, leading to inherited health problems in some purebred animals. This reminds us that genetics is a powerful tool that must be applied with care.
然而，选择性育种会降低遗传多样性，并可能在不经意间集中有害的隐性等位基因，导致某些纯种动物出现遗传性健康问题。这提醒我们，遗传学是一种强大的工具，必须谨慎应用。
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