📚 A-Level CIE Science: Genetics – Key Concepts and Exam Strategies | A-Level CIE 科学：遗传考点精讲
Genetics is a cornerstone of A-Level CIE Science, demanding both conceptual understanding and the ability to apply principles to unfamiliar scenarios. This revision guide covers the essential topics – from Mendelian inheritance and dihybrid crosses to epistasis, chi‑squared analysis, Hardy‑Weinberg equilibrium, mutations, and pedigree interpretation – ensuring you are fully equipped for your exams.
遗传学是A-Level CIE科学的基石，需要既理解概念又能将原理应用于陌生情境。本复习指南涵盖所有重要主题——从孟德尔遗传和双基因杂交到上位效应、卡方分析、哈迪‑温伯格平衡、突变以及系谱解读——确保你为考试做好充分准备。
1. Genetic Terminology and Mendel’s Laws | 遗传学术语与孟德尔定律
A gene is a length of DNA that codes for a polypeptide. Alleles are alternative forms of a gene, occupying the same locus on homologous chromosomes. A dominant allele masks the effect of a recessive allele in a heterozygote. Homozygous individuals possess two identical alleles (e.g., AA or aa), while heterozygous individuals carry two different alleles (e.g., Aa). Genotype refers to the genetic constitution of an organism, and phenotype is the observable characteristic resulting from the genotype and environment.
基因是编码多肽的一段DNA。等位基因是基因的替代形式，位于同源染色体的相同基因座上。显性等位基因在杂合子中掩盖隐性等位基因的效应。纯合子拥有两个相同的等位基因（如AA或aa），而杂合子携带两个不同的等位基因（如Aa）。基因型指个体的遗传组成，表型是由基因型和环境共同决定的可见性状。
Mendel’s first law, the law of segregation, states that allele pairs separate during gamete formation, so each gamete receives only one allele for each gene. His second law, the law of independent assortment, applies to genes on different chromosomes: alleles of different genes are distributed to gametes independently of one another. These laws form the basis of monohybrid and dihybrid inheritance.
孟德尔第一定律——分离定律——指出在配子形成时等位基因对分离，每个配子只获得一个等位基因。他的第二定律——自由组合定律——适用于不同染色体上的基因：不同基因的等位基因独立地分配到配子中。这些定律是单基因和双基因遗传的基础。
2. Monohybrid Crosses and Punnett Squares | 单基因杂交与棋盘格
d, the expected phenotypic ratio is 3:1 (dominant : recessive), provided complete dominance. The genotypic ratio is 1 AA : 2 Aa : 1 aa. Punnett squares are used to visualise the combinations of gametes and predict offspring genotypes.
单基因杂交跟踪单个基因的遗传。当两个杂合亲本（Aa × Aa）杂交时，在完全显性的情况下预期的表型比是3:1（显性 : 隐性）。基因型比为1 AA : 2 Aa : 1 aa。棋盘格用于直观表示配子组合并预测子代基因型。
A test cross, breeding an individual of unknown genotype showing the dominant phenotype with a homozygous recessive individual, reveals the unknown genotype. If all offspring show the dominant trait, the parent was homozygous dominant; if a 1:1 ratio of dominant to recessive appears, the parent was heterozygous.
测交是将表现出显性性状但基因型未知的个体与隐性纯合子杂交，从而揭示未知基因型。若所有子代表现显性性状，亲本为纯合显性；若出现显性:隐性为1:1的比例，则亲本为杂合子。
3. Dihybrid Crosses | 双基因杂交
1 phenotypic ratio in the F₂ generation, illustrating independent assortment. Each gene’s alleles segregate independently, generating four types of gametes (AB, Ab, aB, ab) in equal proportions.
双基因杂交考虑位于不同染色体上的两个基因。将两个双杂合子（AaBb × AaBb）杂交，在F₂代产生经典的9:3:3:1表型比，体现了自由组合。每个基因的等位基因独立分离，产生四种等比例的配子（AB, Ab, aB, ab）。
You may be asked to predict the phenotype frequencies when one or both parents have linked genes, or when gene interaction modifies the ratio. Always check whether the genes assort independently; if they are linked, the gamete ratios will not be equal, and the offspring ratios deviate from 9:3:3:1.
考试可能要求你在一个或两个亲本具有连锁基因时，或当基因互作改变比例时预测表型频率。务必判断基因是否自由组合；若存在连锁，则配子比例不再相等，子代比例也会偏离9:3:3:1。
4. Codominance and Multiple Alleles (ABO Blood Groups) | 共显性与多等位基因（ABO血型）
Codominance occurs when both alleles in a heterozygote are fully expressed. The human ABO blood group system is controlled by three alleles: Iᴬ, Iᴮ, and i. Iᴬ and Iᴮ are codominant to each other, and both are dominant over i. This produces four possible phenotypes: A (IᴬIᴬ or Iᴬi), B (IᴮIᴮ or Iᴮi), AB (IᴬIᴮ), and O (ii).
共显性是指杂合子中两个等位基因都完全表达。人类ABO血型系统由三个等位基因控制：Iᴬ、Iᴮ和i。Iᴬ与Iᴮ互为共显性，并且都对i呈显性。由此产生四种可能的表型：A型（IᴬIᴬ或Iᴬi）、B型（IᴮIᴮ或Iᴮi）、AB型（IᴬIᴮ）和O型（ii）。
When solving blood‑group problems, treat Iᴬ and Iᴮ as codominant alleles, and use superscript letters to avoid confusion. For a cross between an AB parent and an O parent, all offspring will be either A or B blood group, with genotypes Iᴬi and Iᴮi in a 1:1 ratio.
解决血型问题时，将Iᴬ和Iᴮ视为共显性等位基因，并使用上标字母以避免混淆。对于AB型亲本与O型亲本的杂交，所有子代均为A型或B型，基因型Iᴬi和Iᴮi各占一半。
5. Sex Linkage | 性连锁遗传
Sex‑linked genes are carried on the X chromosome (and occasionally the Y chromosome in humans). Males (XY) have only one X chromosome, so they express the allele carried on it, whether dominant or recessive. Females (XX) can be homozygous or heterozygous for X‑linked alleles. Classic examples include red‑green colour blindness and haemophilia, both caused by recessive alleles on the X chromosome.
性连锁基因位于X染色体上（在人类中偶尔位于Y染色体）。男性（XY）只有一条X染色体，因此无论显隐性，都会表达该染色体上的等位基因。女性（XX）对于X连锁等位基因可以是纯合或杂合。经典的例子包括红绿色盲和血友病，均由X染色体上的隐性等位基因引起。
For a cross between a carrier female (XᴺXⁿ) and a normal male (XᴺY), the expected offspring are: 25% normal females, 25% carrier females, 25% normal males, and 25% affected males. Note that males cannot be carriers of X‑linked recessive traits; they are either affected or unaffected.
携带者女性（XᴺXⁿ）与正常男性（XᴺY）杂交，预期的子代比例为：25%正常女性，25%携带者女性，25%正常男性，25%患病男性。注意男性不可能是X连锁隐性性状的携带者；他们要么患病，要么正常。
6. Gene Interactions: Epistasis | 基因互作：上位效应
Epistasis occurs when the allele of one gene masks or modifies the expression of another gene at a different locus. In recessive epistasis, the homozygous recessive genotype at one locus suppresses the effect of a second gene. A well‑known example is coat colour in mice: the gene for pigment deposition (C) is epistatic to the gene for colour (B). Individuals with genotype cc are albino, irrespective of the B alleles, resulting in a 9:3:4 ratio in a dihybrid cross.
上位效应是指一个基因的等位基因掩盖或修饰另一基因座上基因的表达。在隐性上位中，一个基因座的隐性纯合基因型抑制了第二个基因的效应。一个著名的例子是小鼠的毛色：色素沉积基因（C）对颜色基因（B）呈上位。基因型为cc的个体为白化，无论B等位基因如何，导致双基因杂交中出现9:3:4的比例。
Dominant epistasis produces a 12:3:1 ratio (e.g., fruit colour in summer squash), and duplicate recessive epistasis yields a 9:7 ratio (e.g., flower colour in sweet peas). Recognising these modified ratios is a key skill for CIE exams, as they indicate that two genes interact to control a single trait.
显性上位产生12:3:1的比例（例如西葫芦的果实颜色），而重复隐性上位产生9:7的比例（例如甜豌豆的花色）。识别这些修饰比例是CIE考试的关键技能，因为它们表明两个基因相互作用控制同一个性状。
7. Using the Chi‑squared Test in Genetics | 遗传学中的卡方检验
The chi‑ s used to determine if the offspring ratios fit a particular Mendelian ratio. The formula is χ² = Σ (O – E)² / E, where O is the observed frequency and E is the expected frequency.
卡方（χ²）检验用于判断观察值与期望值之间的差异是由偶然因素还是显著因素引起的。在遗传学中，它用于判断子代比例是否符合特定的孟德尔比例。公式为 χ² = Σ (O – E)² / E，其中O为观察频次，E为期望频次。
p = 0.05 in the table. If χ² is smaller than the critical value, the null hypothesis is accepted – r, the difference is significant, suggesting the ratio does not follow the expected model.
计算出χ²值后，确定自由度（表型类别数减1）。将χ²值与表中p=0.05处的临界值比较。若χ²较小，接受零假设——偏差由偶然引起。若较大，则差异显著，表明该比例不符合预期模型。
8. Hardy‑Weinberg Equilibrium | 哈迪‑温伯格平衡
The Hardy‑ tion in the absence of evolutionary influences. For a gene with two alleles, the frequency of the dominant allele (p) and the recessive allele (q) satisfy p + q = 1. The genotype frequencies are given by p² + 2pq + q² = 1.
哈迪‑温伯格原理指出，在一个大规模、随机交配且无进化影响的种群中，等位基因频率和基因型频率代代保持恒定。对于具有两个等位基因的基因，显性等位基因频率（p）和隐性等位基因频率（q）满足 p + q = 1。基因型频率为 p² + 2pq + q² = 1。
iduals has cystic fibrosis (q² = 1/2500), then q = 1/50 = 0.02, and p = 0.98. The carrier frequency 2pq = 2 × 0.98 × 0.02 = 0.0392, or about 4%. This calculation frequently appears in CIE exam questions.
要估算隐性遗传病携带者的频率，可从患者的频率（q²）入手。例如，若每2500人中有1人患囊性纤维化（q² = 1/2500），则 q = 1/50 = 0.02，p = 0.98。携带者频率 2pq = 2 × 0.98 × 0.02 = 0.0392，约4%。这种计算经常出现在CIE考题中。
9. Mutations and Their Effects | 突变及其影响
A gene mutation is a change in the sequence of bases in DNA. Substitution mutations replace one base with another; they may be silent (no change in amino acid), missense (different amino acid), or nonsense (premature stop codon). Insertion or deletion mutations cause frameshifts, altering the entire downstream amino acid sequence and usually resulting in non‑functional proteins.
基因突变是DNA碱基序列的改变。置换突变将一个碱基替换为另一个；它们可能是沉默（氨基酸不变）、错义（不同氨基酸）或无义（提前出现终止密码子）。插入或缺失突变引起移码，改变下游全部氨基酸序列，通常导致蛋白质失去功能。
Mutations can be spontaneous or induced by mutagens such as radiation or chemicals. Sickle cell anaemia results from a single base substitution that changes glutamic acid to valine in the β‑globin chain, illustrating how a small change can cause a severe phenotype. Mutations are the ultimate source of genetic variation, essential for evolution.
突变可以自发发生或由辐射、化学物质等诱变剂诱导。镰状细胞贫血源于单个碱基置换，将β‑珠蛋白链中的谷氨酸变成缬氨酸，说明微小的改变就能引起严重的表型。突变是遗传变异的最终来源，对进化至关重要。
10. Regulation of Gene Expression | 基因表达的调控
In prokaryotes, the lac operon is a model of gene regulation. It consists of a promoter, operator, and three structural genes (lacZ, lacY, lacA) involved in lactose metabolism. In the absence of lactose, a repressor protein binds to the operator, blocking RNA polymerase and preventing transcription. When lactose is present, it binds to the repressor, inactivating it, so the operon is transcribed.
在原核生物中，乳糖操纵子是基因调控的模型。它由启动子、操纵基因和三个参与乳糖代谢的结构基因（lacZ、lacY、lacA）组成。无乳糖时，阻遏蛋白与操纵基因结合，阻止RNA聚合酶，从而阻止转录。当乳糖存在时，它与阻遏蛋白结合使其失活，于是操纵子被转录。
Eukaryotic gene regulation is more complex, involving transcription factors, enhancers, and epigenetic modifications such as DNA methylation and histone acetylation. These mechanisms determine which genes are expressed in a particular cell type, allowing differentiation.
真核生物的基因调控更为复杂，涉及转录因子、增强子以及表观遗传修饰，如DNA甲基化和组蛋白乙酰化。这些机制决定特定细胞类型中哪些基因表达，使细胞分化成为可能。
11. Interpreting Pedigree Charts | 系谱图的解读
A pedigree chart shows the inheritance of a trait over several generations. To determine the mode of inheritance, look for patterns: if the trait appears in every generation and affected individuals have an affected parent, it is likely dominant. If it skips generations and affected children can have unaffected parents, it is likely recessive.
系谱图展示了一个性状在数代中的遗传情况。要确定遗传方式，需寻找规律：若性状在每一代均出现且患者有患病的亲本，可能是显性遗传。若隔代出现且患病孩子的父母可能正常，则可能为隐性遗传。
For sex‑linked recessive traits, more males are typically affected, and an affected male cannot pass the trait to his sons. For autosomal recessive traits, both sexes are equally likely to be affected, and consanguineous marriages increase the probability of appearance. Practice assigning genotypes to individuals and calculating the probability that an unaffected individual is a carrier.
对于X连锁隐性性状，通常男性患者更多，且患病父亲不会将性状传给儿子。对于常染色体隐性性状，两性受累几率相同，且近亲婚配会增加性状出现的概率。练习为个体分配基因型，并计算表型正常个体为携带者的概率。
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