📚 Mendelian Genetics for IGCSE AQA Biology | IGCSE AQA 生物：孟德尔遗传考点精讲
This article provides a detailed breakdown of Mendelian genetics concepts required for the AQA IGCSE Biology exam. Grasping Mendel’s laws and their applications is critical for tackling inheritance problems, predicting offspring traits, and interpreting genetic diagrams.
本文为AQA IGCSE生物考试所需掌握的孟德尔遗传考点提供详尽梳理。理解孟德尔定律及其应用，对解答遗传问题、预测后代性状以及解读遗传图解至关重要。
1. Who Was Gregor Mendel? | 格雷戈尔·孟德尔是谁？
Gregor Mendel was an Austrian monk who conducted breeding experiments on pea plants in the mid-19th century. He is regarded as the father of modern genetics because his work established the fundamental principles of heredity, even before the discovery of genes and chromosomes.
格雷戈尔·孟德尔是19世纪中叶通过豌豆植株进行繁育实验的奥地利修道士。他被尊为现代遗传学之父，因为其工作在基因和染色体被发现之前就确立了遗传的基本原理。
Mendel chose pea plants because they have distinct contrasting traits, can self-pollinate or cross-pollinate, and produce many offspring quickly. By carefully tracking traits such as seed shape and flower colour, he deduced patterns of inheritance that were later validated at the molecular level.
孟德尔选择豌豆是因为它们具有明显的对比性状，能自花授粉或异花授粉，且快速产生大量后代。他通过仔细追踪种子形状和花色等性状，推导出后来在分子层面被证实的遗传模式。
2. Mendel’s Key Experiments | 孟德尔的关键实验
Mendel began with pure-breeding (true-breeding) lines of pea plants that consistently showed a particular trait over many generations. He then crossed two pure-breeding plants with contrasting traits — for example, tall plants with dwarf plants — to produce the first filial generation (F₁).
孟德尔从纯种（真实遗传）的豌豆品系开始，这些品系在多个世代中始终表现出特定性状。然后他将两个具有对比性状的纯种植株杂交——例如高茎与矮茎——产生第一子代（F₁）。
Surprisingly, all F₁ offspring displayed only one of the parental traits (e.g., all were tall). When Mendel allowed the F₁ plants to self-pollinate, the resulting F₂ generation showed both traits in a consistent ratio of approximately 3:1, with the ‘lost’ trait reappearing.
令人惊讶的是，所有F₁子代只表现亲本中的一个性状（如全部为高茎）。当孟德尔让F₁植株自花授粉，所得的F₂代则以大约3:1的比例同时出现了两个性状，原本‘消失’的性状重新出现。
3. Key Terminology: Alleles, Dominant & Recessive | 关键术语：等位基因、显性与隐性
An allele is a variant form of a gene. Individuals inherit two alleles for each gene, one from each parent. Mendel proposed that some alleles are dominant and mask the effect of recessive alleles when both are present in a heterozygous individual.
等位基因是基因的变体形式。每个个体从双亲各继承一个等位基因，共两个。孟德尔提出，有些等位基因是显性的，当与隐性等位基因同时存在于杂合个体时，会掩盖隐性等位基因的效应。
A dominant allele is always expressed in the phenotype even if only one copy is present, while a recessive allele is only expressed when two copies are inherited (homozygous recessive). For example, in pea plants, the allele for tall stems (T) is dominant over the allele for dwarf stems (t).
显性等位基因即使仅有一个拷贝也会在表现型中表达，而隐性等位基因只有在两个拷贝都被继承（隐性纯合）时才会表达。例如，豌豆中高茎等位基因（T）对矮茎等位基因（t）为显性。
4. Homozygous and Heterozygous Combinations | 纯合子与杂合子组合
A homozygous individual has two identical alleles for a gene (e.g., TT or tt). A heterozygous individual has two different alleles (e.g., Tt). Homologous chromosomes carry the alleles at the same locus, and these combinations determine how traits are passed on.
纯合个体拥有两个相同的等位基因（如TT或tt）。杂合个体则拥有两个不同的等位基因（如Tt）。同源染色体在相同基因座上携带这些等位基因，这些组合决定了性状如何传递。
In a monohybrid cross, the heterozygous genotype (Tt) produces the dominant phenotype because the dominant allele masks the recessive one. Understanding the difference between homozygous and heterozygous states is fundamental when constructing genetic diagrams.
在单基因杂交中，杂合基因型（Tt）产生显性表现型，因为显性等位基因掩盖了隐性等位基因。理解纯合与杂合状态的区别是构建遗传图解的基础。
5. Genotype vs Phenotype | 基因型对比表现型
The genotype is the genetic makeup of an organism — the combination of alleles it possesses. The phenotype is the observable physical or physiological characteristic resulting from the genotype and its interaction with the environment.
基因型是指生物体的遗传组成——即其所拥有的等位基因组合。表现型则是基因型与环境相互作用所产生的可观察的物理或生理特征。
For instance, a pea plant with genotype TT or Tt will show a tall phenotype, while only tt plants are dwarf. Even two organisms with the same phenotype can have different genotypes, which becomes clear when studying their offspring ratios.
例如，基因型为TT或Tt的豌豆植株表现为高茎，而只有tt植株是矮茎。即使两个表现型相同的生物也可能拥有不同的基因型，这在研究其后代比例时就会显现出来。
6. Monohybrid Crosses and Punnett Squares | 单基因杂交与庞纳特方格
A monohybrid cross tracks the inheritance of a single gene with two alleles. Using a Punnett square — a grid that predicts possible offspring allele combinations — we can visualise the cross between parents of known genotype.
单基因杂交追踪由两个等位基因决定的单个基因的遗传。使用庞纳特方格——一种预测可能子代等位基因组合的网格——可以直观呈现已知基因型亲本的杂交结果。
For example, crossing two heterozygous tall pea plants (Tt × Tt) yields a genotypic ratio of 1 TT : 2 Tt : 1 tt and a phenotypic ratio of 3 tall : 1 dwarf. The Punnett square for this cross is shown below:
例如，将两株杂合高茎豌豆杂交（Tt × Tt），其基因型比为 1 TT : 2 Tt : 1 tt，表现型比为 3高茎 : 1矮茎。该杂交的庞纳特方格如下：
| T | t | |
| T | TT | Tt |
| t | Tt | tt |
Always place one parent’s gametes along the top and the other parent’s along the side, then fill the cells. The resulting genotypes can be used to calculate expected ratios in large numbers of offspring.
始终将一方亲本的配子置于表格上方，另一方亲本的配子置于侧边，再填充单元格。所得基因型可用于计算大量子代的预期比例。
7. Predicting Offspring Ratios and Probability | 预测后代比例与概率
Mendelian crosses predict the probability of offspring inheriting certain traits, not exact numbers in every family. A 3:1 phenotypic ratio emerges from heterozygous parents only when large sample sizes are analysed and when the trait is controlled by a single gene with complete dominance.
孟德尔杂交预测的是子代继承特定性状的概率，而非每个家系中的确切数字。只有当分析大样本量且性状由单基因完全显性控制时，杂合亲本才会产生3:1的表现型比。
Using a Punnett square, the chance of a homozygous dominant (TT) offspring from a Tt × Tt cross is ¼, the chance of heterozygous (Tt) is ½, and the chance of homozygous recessive (tt) is ¼. Probability thus explains Mendel’s observed ratios mathematically.
利用庞纳特方格，Tt × Tt 杂交得到纯合显性（TT）子代的概率为¼，杂合（Tt）的概率为½，纯合隐性（tt）的概率为¼。因此概率从数学上解释了孟德尔观察到的比例。
8. The Test Cross: Determining Unknown Genotypes | 测交：确定未知基因型
A test cross is used to determine whether an organism showing a dominant trait is homozygous dominant or heterozygous. The individual is crossed with a homozygous recessive individual. If all offspring display the dominant trait, the unknown parent is likely homozygous dominant.
测交用于确定表现出显性性状的生物是显性纯合还是杂合。将该个体与隐性纯合个体杂交。如果所有子代表现显性性状，则未知亲本很可能为显性纯合。
If any offspring display the recessive trait, the unknown parent must be heterozygous because only a heterozygous parent can pass on a recessive allele to produce a recessive phenotype when paired with a recessive allele from the other parent.
若任何子代表现隐性性状，则未知亲本必定是杂合的，因为只有杂合亲本才能传递隐性等位基因，在与另一亲本的隐性等位基因组合后产生隐性表现型。
9. Codominance and Blood Group Inheritance | 共显性与血型遗传
Codominance occurs when both alleles in a heterozygote are fully expressed in the phenotype without blending. The human ABO blood group system is a classic example, governed by three alleles: Iᴬ, Iᴮ (codominant to each other), and i (recessive).
共显性是指杂合子中的两个等位基因在表现型中都完全表达而不发生混合。人类ABO血型系统是一个经典例子，由三个等位基因控制：Iᴬ、Iᴮ（彼此共显性）和i（隐性）。
The possible genotypes and phenotypes are: IᴬIᴬ or Iᴬi → blood group A; IᴮIᴮ or Iᴮi → blood group B; IᴬIᴮ → blood group AB (codominance); ii → blood group O. A cross between Iᴬi and Iᴮi can produce all four blood types in the offspring.
可能的基因型和表现型为：IᴬIᴬ 或 Iᴬi → A型血；IᴮIᴮ 或 Iᴮi → B型血；IᴬIᴮ → AB型血（共显性）；ii → O型血。Iᴬi 与 Iᴮi 的杂交可在子代中产生所有四种血型。
10. Interpreting Family Pedigrees | 解读家族谱系图
A pedigree chart is a diagram showing the inheritance of a trait through several generations of a family. Squares represent males, circles represent females, shaded symbols indicate individuals expressing the trait, and unshaded symbols show non-affected individuals.
谱系图是显示一个性状在家族几代中遗传方式的图表。方块代表男性，圆圈代表女性，填充符号表示表现出该性状的个体，未填充符号表示未受影响者。
By analysing a pedigree, we can often deduce whether a trait is dominant or recessive. For a recessive disorder, two unaffected parents can produce an affected child if both are carriers (heterozygous). A dominant trait usually appears in every generation.
通过分析谱系图，我们往往能推断某个性状是显性还是隐性。对于隐性遗传病，若双亲均为携带者（杂合），则未患病的父母可以生出患病子女。显性性状通常每一代都会出现。
11. Inherited Disorders: Cystic Fibrosis vs Polydactyly | 遗传病：囊性纤维化与多指
Cystic fibrosis is a recessive genetic disorder caused by a mutated allele of the CFTR gene. To develop the disease, a person must inherit two copies of the recessive allele (ff). Carriers (Ff) are unaffected but can pass the allele to their children.
囊性纤维化是一种由CFTR基因突变等位基因引起的隐性遗传病。患者必须遗传两个隐性等位基因（ff）。携带者（Ff）不患病，但可将该等位基因传给子女。
Polydactyly (extra fingers or toes) is a dominant disorder caused by a dominant allele (P). A person with at least one P allele will exhibit the trait. Therefore, an affected parent has a 50% chance of passing the condition to each child if the parent is heterozygous.
多指（额外的手指或脚趾）是由显性等位基因（P）引起的显性遗传病。只要携带至少一个P等位基因就会表现该性状。因此，如果患病亲本是杂合子，则每个孩子有50%的概率遗传此病症。
12. Sex Determination and Sex-Linked Traits | 性别决定与伴性性状
In humans, sex is determined by the 23rd pair of chromosomes: XX for females and XY for males. Females produce eggs containing a single X chromosome; males produce sperm containing either an X or a Y chromosome with equal probability.
人类的性别由第23对染色体决定：XX为女性，XY为男性。女性产出的卵子含有一条X染色体；男性产生的精子含有X或Y染色体的概率各为一半。
The sex of offspring is therefore determined by the sperm. The Punnett square for sex determination predicts a 1:1 ratio of male to female offspring. While not strictly Mendelian, the inheritance of X and Y chromosomes follows the same principles of segregation.
因此子代性别由精子决定。性别决定的庞纳特方格预测男女比例为1:1。虽然并非严格遵循孟德尔方式，但X和Y染色体的遗传同样遵循分离定律。
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