Edexcel IGCSE生物 遗传 基因 染色体 精讲
遗传学 (Genetics) 是 Edexcel IGCSE Biology 中最容易拿分、也最容易在细节上丢分的板块。它同时出现在 Paper 1B 与 Paper 2B 中，考查形式既有定义题，也有庞氏表 (Punnett square) 计算题和系谱图分析题。很多同学觉得遗传难，其实难点不在于概念抽象，而在于三个地方：第一，不清除 DNA、基因、染色体三者的层级关系；第二，混淆基因型比与表现型比；第三，概率计算没有写成分数或百分比。这篇讲解按考点顺序展开，每一节都配中英对照说明，帮助你同时应付考试的中文理解与英文答题。
Genetics is one of the highest-yield topics in Edexcel IGCSE Biology. It appears in both Paper 1B and Paper 2B, and it is tested through definitions, Punnett square calculations and pedigree analysis. Students rarely struggle because the ideas are too abstract. They lose marks for three predictable reasons: muddling the hierarchy of DNA, genes and chromosomes; confusing genotype ratios with phenotype ratios; and writing probabilities as vague words instead of fractions or percentages. This guide follows the syllabus order, with parallel Chinese and English explanations so you can revise the concept and the exam wording at the same time.
1. DNA、基因与染色体的层级关系
DNA（脱氧核糖核酸）是遗传物质，呈双螺旋结构，由四种碱基组成：腺嘌呤 (A)、胸腺嘧啶 (T)、胞嘧啶 (C) 和鸟嘌呤 (G)。碱基配对规则是 A 与 T 配对，C 与 G 配对。基因 (gene) 是 DNA 分子上的一段功能片段，它编码一种特定的蛋白质（或多肽），从而控制一个性状。染色体 (chromosome) 则是 DNA 与蛋白质结合、紧密盘绕形成的线状结构。三者是层层包含的关系：染色体包含许多基因，每个基因是一段 DNA。人类体细胞含有 46 条染色体，即 23 对同源染色体；生殖细胞（配子）只含 23 条，是单倍体。考试中最常见的失分点是把「基因」写成「染色体」，或者在问「控制某性状的单位是什么」时答成 DNA。
DNA is the genetic material and takes the form of a double helix. It is built from four bases: adenine (A), thymine (T), cytosine (C) and guanine (G). The base-pairing rule is A with T, and C with G. A gene is a length of DNA that codes for one particular protein, and that protein in turn controls a characteristic. A chromosome is a thread-like structure formed when DNA coils tightly around proteins. The relationship is hierarchical: a chromosome carries many genes, and each gene is a section of DNA. A human body cell contains 46 chromosomes arranged as 23 homologous pairs, while a gamete contains only 23 chromosomes and is described as haploid. The most common exam error is to write chromosome where the answer is gene, or to answer DNA when the question asks for the unit that codes for a characteristic.
2. 等位基因、显性与隐性
同源染色体上控制同一性状、位置相同的基因称为等位基因 (alleles)。例如控制豌豆茎高的基因有两个等位基因：T 控制高茎，t 控制矮茎。我们约定用大写字母表示显性等位基因，小写字母表示隐性等位基因。显性等位基因的特点是在杂合状态下仍然会被表现出来，也就是说只要有一个 T，植株就是高茎；隐性等位基因只有在两个拷贝同时存在（tt）时才会表现。这里有一个关键点必须记住：显性并不等于「更常见」或者「更强」，它只表示在杂合子中能够表达。很多学生把「显性」理解成「数量多」，这是概念性错误，考试中会直接扣分。
Alleles are alternative versions of the same gene, located at the same position on homologous chromosomes. For pea plant height, there are two alleles: T for tall and t for short. By convention a capital letter represents the dominant allele and a lower-case letter represents the recessive allele. A dominant allele is expressed even when only one copy is present, so a plant with one T is tall. A recessive allele is expressed only when both copies are the same recessive version (tt). One point is worth memorising: dominant does not mean more common or stronger. It simply means the allele is expressed in a heterozygote. Treating dominant as a synonym for common is a conceptual mistake that loses marks directly.
3. 基因型、表现型与纯合杂合
基因型 (genotype) 指一个生物体所携带的等位基因组合，例如 TT、Tt 或 tt；表现型 (phenotype) 指可以观察到的性状，例如高茎或矮茎。携带两个相同等位基因的个体称为纯合子 (homozygous)，例如 TT 和 tt；携带两个不同等位基因的个体称为杂合子 (heterozygous)，例如 Tt。这里必须补一句容易被忽略的内容：表现型是基因型与环境共同作用的结果。同一基因型在不同环境下可能表现出不同性状，例如同一种植物在光照充足与光照不足时株高差异明显，人类的身高也受营养影响。考卷上如果问「为什么两个基因型相同的个体表现型不同」，标准答案就是环境因素的作用，而不是「发生了突变」。
The genotype is the combination of alleles an organism carries, for example TT, Tt or tt. The phenotype is the observable characteristic, such as tall or short. An organism with two identical alleles is homozygous, for example TT or tt, while an organism with two different alleles is heterozygous, for example Tt. Add one point that is easy to overlook: the phenotype is the product of the genotype interacting with the environment. The same genotype can produce different phenotypes under different conditions. A plant with a given genotype grows taller in full sunlight than in shade, and human height is influenced by nutrition. If a question asks why two individuals with the same genotype show different phenotypes, the expected answer is an environmental effect, not a new mutation.
4. 单基因杂交与庞氏表计算
单基因杂交 (monohybrid cross) 是遗传计算题的核心。建议固定使用四步法，保证不会漏项：第一步，写出亲本的基因型；第二步，写出每个亲本产生的配子（配子只带一个等位基因）；第三步，把配子填入庞氏表进行组合；第四步，从表格中统计表现型与基因型的比例。以 Tt 与 Tt 杂交为例：两个亲本各产生 T 和 t 两种配子，庞氏表得到 TT、Tt、Tt、tt，因此基因型比为 1 : 2 : 1，表现型比为 3 高茎 : 1 矮茎。如果题目问「后代中出现矮茎的概率」，答案应写成 1/4 或 25%，而不是「比较小」。如果亲本是 Tt 与 tt（测交），后代为 Tt、Tt、tt、tt，表现型比是 1 : 1。掌握这两种标准组合，绝大多数题目都能解决。
A monohybrid cross is the core of genetics calculation questions. Use a fixed four-step method so nothing is missed. First, write the genotypes of the parents. Second, write the gametes each parent produces, remembering that a gamete carries only one allele. Third, place the gametes into a Punnett square and combine them. Fourth, read the phenotype and genotype ratios from the square. Take a cross between Tt and Tt. Each parent produces T and t gametes, giving TT, Tt, Tt and tt in the square. The genotype ratio is therefore 1 : 2 : 1 and the phenotype ratio is 3 tall to 1 short. If the question asks for the probability of a short offspring, the answer must be written as 1/4 or 25%, not as a vague phrase such as quite unlikely. If the cross is Tt with tt, a test cross, the offspring are Tt, Tt, tt and tt, giving a 1 : 1 phenotype ratio. These two standard crosses cover the great majority of questions.
5. 共显性与不完全显性
并非所有等位基因都是简单的显隐性关系。共显性 (codominance) 指两个等位基因在杂合子中都得到表达，最经典的例子是 ABO 血型系统和 MN 血型。人类 ABO 血型由三个等位基因决定：IA、IB 和 i。IA 与 IB 互为共显性，因此基因型 IAIB 表现为 AB 型血，A 抗原与 B 抗原同时出现；而 i 是隐性，只有 ii 才表现为 O 型。不完全显性 (incomplete dominance) 则指杂合子表现为两个纯合亲本的中间型，例如红花亲本与白花亲本杂交，杂合后代开粉色花。两者的区别必须在答案中明确写出：共显性是两种性状同时出现，不完全显性是出现中间性状。这是历年评卷中最常被混淆的一对概念。
Not all alleles show a simple dominant and recessive relationship. Codominance means both alleles are expressed in the heterozygote. The classic example is the ABO blood group system, which is controlled by three alleles: IA, IB and i. Because IA and IB are codominant, the genotype IAIB gives blood group AB, with both A and B antigens present on the red blood cells. The allele i is recessive, so group O requires the genotype ii. Incomplete dominance means the heterozygote shows an intermediate phenotype. A red-flowered parent crossed with a white-flowered parent produces pink offspring. The distinction must be stated precisely in an answer: codominance shows both characteristics at the same time, whereas incomplete dominance shows a blended or intermediate characteristic. Examiners report this pair as one of the most frequently confused ideas in the paper.
6. 性别决定与伴性遗传
人类的性别由性染色体决定。女性为 XX，男性为 XY。母亲只能提供 X 染色体，父亲可以提供 X 或 Y，因此后代的性别由父亲的精子决定，理论上男女比例约为 1 : 1。伴性遗传 (sex linkage) 指基因位于性染色体上，通常指 X 染色体。由于男性只有一条 X 染色体，只要这一条上带有隐性等位基因就会发病，所以红绿色盲和血友病在男性中更常见。以红绿色盲为例，用 XB 表示正常等位基因、Xb 表示致病等位基因：男性 XBY 正常，XbY 患病；女性 XBXB 正常，XBXb 是携带者（表现正常但可以传递），XbXb 患病。系谱图题目如果出现「患病男性多于患病女性」的特征，优先考虑伴 X 隐性遗传。
Human sex is determined by the sex chromosomes. Females are XX and males are XY. A mother can only contribute an X chromosome, while a father can contribute either X or Y, so the sex of the offspring is determined by the father's sperm and the expected ratio is about 1 : 1. Sex linkage refers to genes carried on a sex chromosome, usually the X chromosome. Because a male has only one X chromosome, a single recessive allele on that chromosome causes the condition, which is why red-green colour blindness and haemophilia are more common in males. For colour blindness, writing XB for the normal allele and Xb for the affected allele: a male with XBY is unaffected and XbY is affected; a female with XBXB is unaffected, XBXb is a carrier who is unaffected but can pass the allele on, and XbXb is affected. When a pedigree shows more affected males than females, recessive X-linked inheritance should be your first hypothesis.
7. 变异、系谱图与答题技巧
遗传变异的来源有三个：基因突变 (mutation) 产生新的等位基因；减数分裂中的独立分配与交换导致配子基因组合不同；受精过程中精卵结合具有随机性。这三项经常以「解释为什么兄弟姐妹之间性状不同」的形式出现，答案必须同时提到减数分裂与随机受精。答题技巧方面，请记住四条：第一，遗传图一律先标注亲本基因型再填表，不要直接写比例；第二，概率必须写成分数、小数或百分比，并说明是「概率」而不是「比例」；第三，区分基因型比与表现型比，1 : 2 : 1 是基因型比，3 : 1 是表现型比，写错方向会丢掉整题分数；第四，凡是涉及血型或伴性遗传的题目，先写出所有可能的基因型清单，再逐个排除。考前把测交、共显性、伴 X 隐性这三类标准题型各做两遍，绝大多数遗传题的分数就稳了。
Genetic variation comes from three sources: mutation, which creates new alleles; meiosis, where independent assortment and crossing over produce gametes with different allele combinations; and random fertilisation, because any sperm may fuse with any egg. Questions that ask why siblings differ in appearance expect all three ideas, so mention meiosis and random fertilisation explicitly. For exam technique, follow four rules. First, always write the parental genotypes before drawing a genetic diagram, rather than jumping straight to a ratio. Second, express probabilities as a fraction, decimal or percentage, and make clear that you are giving a probability. Third, keep genotype ratios and phenotype ratios apart: 1 : 2 : 1 is a genotype ratio, while 3 : 1 is a phenotype ratio, and reversing them loses the marks for the whole question. Fourth, for blood group or sex linkage questions, list every possible genotype first and then eliminate them one by one. Before the exam, work through two examples each of the test cross, codominance and X-linked recessive patterns and the genetics marks become predictable.
8. 典型考题演练与标准答案
题目：在人类中，卷舌能力由显性等位基因 T 控制，不能卷舌为隐性纯合 tt。一对夫妇中，丈夫能卷舌但基因型未知，妻子不能卷舌。他们生了一个不能卷舌的孩子。请写出丈夫的基因型，并计算再生一个能卷舌孩子的概率。解题过程：孩子的基因型是 tt，其中一个 t 来自母亲，另一个 t 必然来自父亲，因此父亲的基因型必定是 Tt，而不是 TT。测交 Tt 与 tt 的后代为 Tt、Tt、tt、tt，所以能卷舌的概率是 1/2，也就是 50%。标准答案必须写出推理过程，只写结果通常只能拿到部分分数。这一道题集中考查三个考点：隐性表现型可以反推亲本基因型、测交的比例是 1 : 1、概率必须写成分数或百分比。
Worked example. In humans, the ability to roll the tongue is controlled by the dominant allele T, while the inability to roll the tongue is recessive and requires the genotype tt. A man can roll his tongue but his genotype is unknown, and his partner cannot roll her tongue. They have a child who cannot roll the tongue. Write down the genotype of the man and work out the probability that their next child will be able to roll the tongue. Reasoning: the child is tt, so one t came from the mother and the other t must have come from the father. The father must therefore be Tt rather than TT. A test cross of Tt with tt produces Tt, Tt, tt and tt, so the probability of a tongue-rolling child is 1/2, which is 50%. Always show your reasoning, because a bare answer usually earns only partial credit. This one question tests three ideas at once: a recessive phenotype can reveal a parental genotype, a test cross gives a 1 : 1 ratio, and probabilities must be written as fractions or percentages.
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