📚 GCSE Edexcel Biology: Mendelian Genetics Exam Essentials | GCSE Edexcel 生物：孟德尔遗传 考点精讲
Mendelian genetics forms the backbone of inheritance topics in the Edexcel GCSE Biology specification. Mastering these principles enables you to predict offspring ratios, construct and interpret Punnett squares, analyse family pedigree charts, and understand how genetic disorders are passed on. This article revises every key concept you need, from Gregor Mendel’s original pea plant experiments to the inheritance of blood groups.
孟德尔遗传是 Edexcel GCSE 生物学遗传板块的核心。掌握这些原理能让你预测后代比例、绘制并解读庞纳特方格、分析家族系谱图并理解遗传病的传递方式。本文将复习你需要的所有关键概念，从孟德尔的豌豆实验到血型遗传。
1. Gregor Mendel and His Pea Plants | 格雷戈尔·孟德尔与他的豌豆实验
Gregor Mendel, an Austrian monk, conducted breeding experiments on garden pea plants (Pisum sativum) in the mid-1800s. He chose peas because they have several clearly contrasting traits, can self-pollinate naturally, and can also be cross-pollinated by hand. By carefully transferring pollen and counting the offspring, Mendel gathered numerical data that revealed patterns of inheritance.
格雷戈尔·孟德尔是19世纪中叶的一位奥地利修道士，他对豌豆植物进行了杂交实验。他选择豌豆是因为它们具有多对明显对比的性状、能自然自花传粉，并可人工异花传粉。通过细致地转移花粉并统计后代数量，孟德尔收集了大量数据，揭示了遗传的规律。
Mendel studied seven traits, each with two distinct forms: seed shape (round or wrinkled), seed colour (yellow or green), pod shape (inflated or constricted), pod colour (green or yellow), flower colour (purple or white), flower position (axial or terminal), and stem height (tall or dwarf). His choice to follow one trait at a time and to use thousands of plants enabled him to deduce fundamental laws of inheritance.
孟德尔研究了七个性状，每个性状都有两种截然不同的形式：种子形状（圆粒或皱粒）、种子颜色（黄色或绿色）、豆荚形状（饱满或缢缩）、豆荚颜色（绿色或黄色）、花色（紫色或白色）、花位（腋生或顶生）以及茎高（高秆或矮秆）。他每次只追踪一个性状，并使用了成千上万株植物，从而能够推断出遗传的基本定律。
2. Key Genetic Terms | 关键遗传术语
Before tackling inheritance problems, you must be confident with the following vocabulary. The table below provides clear English and Chinese explanations of every essential term.
在解决遗传问题之前，你必须熟练掌握以下术语。下面的表格为每个重要术语提供了清晰的英文和中文解释。
| Term | 中文术语 | Definition | 中文定义 |
|---|---|---|---|
| Gene | 基因 | A section of DNA that codes for a specific protein, determining a characteristic. | 一段编码特定蛋白质的DNA片段，决定某种性状。 |
| Allele | 等位基因 | An alternative form of a gene. For example, the gene for height has a tall allele and a dwarf allele. | 同一基因的不同形式。例如，控制株高的基因有高秆等位基因和矮秆等位基因。 |
| Dominant allele | 显性等位基因 | An allele that is always expressed in the phenotype, even if only one copy is present. Represented by a capital letter, e.g. T. | 只要存在一个拷贝就会在表型中表达的等位基因。常用大写字母表示，如T。 |
| Recessive allele | 隐性等位基因 | An allele that is only expressed when two copies are present (homozygous). Represented by a lowercase letter, e.g. t. | 只有当两个拷贝都存在（纯合状态）时才能在表型中表达的等位基因。常用小写字母表示，如t。 |
| Homozygous | 纯合子 | Having two identical alleles for a particular gene (e.g. TT or tt). | 某一基因具有两个相同的等位基因（如TT或tt）。 |
| Heterozygous | 杂合子 | Having two different alleles for a particular gene (e.g. Tt). | 某一基因具有两个不同的等位基因（如Tt）。 |
| Genotype | 基因型 | The combination of alleles an organism possesses for a gene (e.g. TT, Tt or tt). | 一个生物体所拥有的等位基因组合（如TT、Tt或tt）。 |
| Phenotype | 表型 | The observable characteristic expressed by the genotype (e.g. tall or short). | 由基因型表达的可观察特征（如高秆或矮秆）。 |
3. Monohybrid Inheritance | 单基因遗传
Monohybrid inheritance involves a single gene with two alleles. When pure-breeding homozygous parents with contrasting traits are crossed, the first filial generation (F1) are all heterozygous and display the dominant phenotype. If these F1 hybrids are self-pollinated or crossed with each other, the recessive phenotype reappears in the F2 generation in a predictable 3:1 ratio.
单基因遗传涉及一对等位基因控制的一个性状。当具有相对性状的纯合亲本杂交时，子一代（F1）全部为杂合子并表现显性性状。如果F1代杂合子自交或相互杂交，隐性性状会在子二代（F2）中再次出现，且比例为可预测的3:1。
For example, let the allele for tall stems be T (dominant) and the allele for dwarf stems be t (recessive). A cross between a true-breeding tall plant (TT) and a dwarf plant (tt) produces F1 offspring that are all Tt and tall. Crossing two F1 plants (Tt × Tt) produces gametes T and t from each parent. The resulting genotypic ratio is 1 TT : 2 Tt : 1 tt, which gives a phenotypic ratio of 3 tall : 1 short. This is Mendel’s law of segregation – the two alleles for each characteristic separate during gamete formation.
例如，设高秆等位基因为T（显性），矮秆等位基因为t（隐性）。纯合高秆（TT）与矮秆（tt）杂交产生的F1代全部为Tt，表现高秆。F1植株之间杂交（Tt × Tt），每个亲本产生配子T和t。后代基因型比例为1 TT : 2 Tt : 1 tt，表型比例为3高秆 : 1矮秆。这就是孟德尔的分离定律——控制每个性状的两个等位基因在形成配子时彼此分离。
4. Punnett Squares and Predicted Outcomes | 庞纳特方格与预期结果
A Punnett square is a grid used to predict the genotypes and phenotypes of offspring from a genetic cross. You place the possible gametes from one parent along the top and the gametes from the other parent down the side. The boxes inside represent the possible combinations in the zygotes.
庞纳特方格是一种用于预测杂交后代基因型和表型的网格。你将一个亲本可能产生的配子写在顶部横行，另一个亲本的配子写在左侧竖列。方格内的单元格代表合子中可能的等位基因组合。
Consider a cross between a heterozygous tall plant (Tt) and a homozygous recessive dwarf plant (tt). The Tt parent produces gametes T and t; the tt parent produces only t. The square yields genotypes Tt and tt in equal numbers, leading to a 1 tall : 1 dwarf phenotypic ratio. This type of cross, called a test cross, can reveal the genotype of an individual showing a dominant trait but of unknown genotype.
考虑杂合高秆植株（Tt）与隐性纯合矮秆植株（tt）的杂交。Tt亲本产生配子T和t；tt亲本只产生配子t。方格显示基因型Tt和tt数量相等，表型比例为1高秆 : 1矮秆。这种杂交称为测交，可以确定表现显性性状但基因型未知的个体的真实基因型。
Another common cross is between a homozygous dominant (TT) and a homozygous recessive (tt). All offspring are heterozygous (Tt) and show the dominant phenotype. You may be asked to construct these squares and extract ratios in the exam.
另一种常见杂交是显性纯合子（TT）与隐性纯合子（tt）杂交。所有后代均为杂合子（Tt），表现显性表型。考试中可能会要求你画出这些方格并得出比例。
5. Phenotypic Ratios: 3:1 and 1:1 | 表型比例：3:1 与 1:1
Two classic phenotypic ratios appear repeatedly in monohybrid crosses. When both parents are heterozygous (Aa × Aa), the expected offspring ratio is 3 dominant : 1 recessive. This ratio arises because of the way alleles segregate and recombine.
单基因杂交中会反复出现两种经典的表型比例。当双亲均为杂合子（Aa × Aa）时，预期的后代表型比例为3显性 : 1隐性。这一比例源于等位基因的分离与重组方式。
When one parent is heterozygous and the other is homozygous recessive (
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