📚 Mendelian Genetics: Key Exam Points for IB & CIE Biology | IB CIE 生物：孟德尔遗传考点精讲
Mendelian inheritance forms the cornerstone of classical genetics and remains a major focus in both IB Biology (Topic 3.4/10.2) and CIE A Level Biology (Chapter 16). Understanding how traits are passed from parents to offspring through discrete units called genes is essential for solving genetic problems and interpreting pedigree charts. This article summarises the key exam points you need to master, from Mendel’s laws to extensions and common pitfalls.
孟德尔遗传是经典遗传学的基石，也是 IB 生物（主题3.4/10.2）和 CIE A Level 生物（第16章）的重要考点。理解性状如何通过称为基因的离散单位从亲代传递给子代，对于解决遗传学问题和分析系谱图至关重要。本文总结了你需要掌握的关键考点，从孟德尔定律到扩展内容以及常见易错点。
1. Mendel’s Experiments and Key Terminology | 孟德尔的实验与关键术语
Gregor Mendel used pea plants (Pisum sativum) to study inheritance of traits such as seed shape, colour, and plant height. He selected true-breeding lines for each trait, ensuring that offspring consistently showed the parental phenotype when self-pollinated.
格雷戈尔·孟德尔利用豌豆（Pisum sativum）研究种子形状、颜色和植株高度等性状的遗传。他针对每个性状选择了纯种品系，确保自花授粉时子代始终表现出亲本的表型。
Key terms: Gene – a heritable factor that controls a specific characteristic; Allele – an alternative form of a gene; Genotype – the combination of alleles an organism possesses; Phenotype – the observable characteristics; Homozygous – having two identical alleles for a gene; Heterozygous – having two different alleles; Dominant – an allele that masks the effect of a recessive allele in a heterozygote; Recessive – an allele whose effect is only expressed in the homozygous state.
关键术语：基因 – 控制特定特征的可遗传因子；等位基因 – 基因的另一种形式；基因型 – 生物体拥有的等位基因组合；表型 – 可观察的特征；纯合子 – 某个基因具有两个相同的等位基因；杂合子 – 具有两个不同的等位基因；显性 – 在杂合子中掩盖隐性等位基因效应的等位基因；隐性 – 仅在纯合状态时才表现的等位基因。
In experimental crosses, the parental generation is P, the first filial generation is F₁, and the second is F₂.
在实验杂交中，亲代记为 P，第一子代为 F₁，第二子代为 F₂。
2. The Principle of Segregation (Monohybrid Cross) | 分离定律（单基因杂交）
During gamete formation, the two alleles for each gene separate, so each gamete carries only one allele. This is the law of segregation. In a monohybrid cross between two heterozygotes (e.g., Aa × Aa), the expected genotypic ratio is 1 AA : 2 Aa : 1 aa, and the phenotypic ratio is 3 dominant : 1 recessive.
在配子形成过程中，每个基因的两个等位基因分离，因此每个配子只携带一个等位基因。这就是分离定律。在两个杂合子（如 Aa × Aa）的单基因杂交中，预期的基因型比例为 1 AA : 2 Aa : 1 aa，表型比例为 3 显性 : 1 隐性。
Use a Punnett square to predict the outcomes. The law of segregation is related to the separation of homologous chromosomes in anaphase I of meiosis.
使用旁氏表预测结果。分离定律与减数第一次分裂后期同源染色体的分离有关。
3. The Principle of Independent Assortment (Dihybrid Cross) | 自由组合定律（双基因杂交）
Mendel’s second law states that alleles of different genes assort independently during gamete formation, provided the genes are located on different chromosomes. A dihybrid cross between two heterozygous individuals (AaBb × AaBb) yields a phenotypic ratio of 9 : 3 : 3 : 1 for two unlinked genes with complete dominance.
孟德尔第二定律指出，不同基因的等位基因在配子形成过程中独立分配，前提是这些基因位于不同的染色体上。两个杂合子（AaBb × AaBb）的双基因杂交，对于两个不连锁且完全显性的基因，表型比例为 9 : 3 : 3 : 1。
This can be explained by the random orientation of bivalents at metaphase I of meiosis. If the genes are linked on the same chromosome, independent assortment does not occur and recombinant phenotypes appear at lower frequencies.
这可以用减数第一次分裂中期二价体的随机排列来解释。如果基因连锁在同一染色体上，则不会发生独立分配，重组表型出现的频率较低。
4. Test Cross: Determining Unknown Genotypes | 测交：确定未知基因型
A test cross involves breeding an individual 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.
测交是将表现出显性表型（但基因型未知）的个体与隐性纯合子杂交。如果任何后代表现出隐性性状，则未知亲本一定是杂合子。
For a single gene, a test cross of a heterozygote yields a 1:1 ratio. For two genes, a test cross of a double heterozygote (AaBb) with aabb gives a 1:1:1:1 ratio if the genes are unlinked.
对于单基因，杂合子的测交产生 1:1 的比例。对于双基因，双杂合子（AaBb）与 aabb 的测交如果基因不连锁，则产生 1:1:1:1 的比例。
5. Probability and Genetic Ratios | 概率与遗传比率
The rules of probability (product rule and sum rule) are essential for calculating the likelihood of particular genotypes or phenotypes in offspring. The product rule: probability of two independent events both occurring is the product of their individual probabilities. The sum rule: probability of either of two mutually exclusive events occurring is the sum of their probabilities.
概率规则（乘法定律和加法定律）对于计算后代特定基因型或表型的可能性至关重要。乘法定律：两个独立事件同时发生的概率等于各自概率的乘积。加法定律：两个互斥事件中任一事件发生的概率等于其概率之和。
Example: In a cross AaBb × AaBb, the probability of an offspring being AABB is ¼ × ¼ = 1/16. Probability of being A-B- (dominant for both) is ¾ × ¾ = 9/16.
例如：在 AaBb × AaBb 杂交中，后代为 AABB 的概率是 ¼ × ¼ = 1/16。为 A-B-（双显性）的概率是 ¾ × ¾ = 9/16。
6. Pedigree Analysis | 系谱分析
Pedigree charts are used to trace inheritance patterns in families. In exams, you must be able to deduce whether a trait is dominant or recessive, autosomal or sex-linked by analysing the pattern of affected individuals.
系谱图用于追踪家族中的遗传模式。在考试中，你需要能够通过分析患病个体的分布推断性状是显性还是隐性、常染色体还是性连锁。
Key clues: Autosomal recessive – affected individuals can be born to unaffected parents; often skips generations. Autosomal dominant – every affected individual has an affected parent; does not skip generations. X-linked recessive – more males affected; affected males cannot pass the trait to their sons.
关键线索：常染色体隐性 – 患病个体的父母可能正常；常隔代出现。常染色体显性 – 每个患病个体都有患病的亲代；不隔代。X 连锁隐性 – 男性患者更多；患病男性不会将性状传给儿子。
Always assign genotypes using letters (e.g., A/a for autosomal, Xᴬ/Xᵃ for X-linked) and show your reasoning.
始终用字母标出基因型（例如，常染色体用 A/a，X 连锁用 Xᴬ/Xᵃ），并展示推理过程。
7. Beyond Mendelian Inheritance: Co-dominance, Incomplete Dominance, and Multiple Alleles | 超越孟德尔遗传：共显性、不完全显性和多等位基因
Not all traits follow simple dominance. In co-dominance, both alleles are expressed equally in the heterozygote (e.g., human AB blood type, Iᴬ Iᴮ). In incomplete dominance, the heterozygote shows an intermediate phenotype (e.g., pink flowers from red × white snapdragons).
并非所有性状都遵循简单的显隐性关系。在共显性中，两个等位基因在杂合子中均等地表达（例如，人类 AB 血型，Iᴬ Iᴮ）。在不完全显性中，杂合子表现出中间表型（例如，红色与白色金鱼草杂交产生粉红色花朵）。
Multiple alleles: a gene may have more than two allelic forms in a population. The ABO blood group system is a classic example, with alleles Iᴬ, Iᴮ, and i. Iᴬ and Iᴮ are co-dominant, and both are dominant over i.
多等位基因：一个基因在群体中可能有两种以上的等位形式。ABO 血型系统是一个典型例子，等位基因为 Iᴬ、Iᴮ 和 i。Iᴬ 和 Iᴮ 为共显性，
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