📚 Mendelian Genetics for CCEA IGCSE Biology: Key Concepts | IGCSE CCEA 生物：孟德尔遗传 考点精讲
Gregor Mendel's experiments on pea plants laid the foundation for our understanding of heredity. His work established key principles, including the segregation of alleles and independent assortment, which are essential for IGCSE CCEA Biology. This revision guide breaks down the core concepts, terminology, and problem-solving techniques you need to master Mendelian genetics.
格雷戈尔·孟德尔在豌豆植物上的实验为遗传学奠定了基础。他确立了等位基因分离和自由组合等关键原理，这对于 IGCSE CCEA 生物课程至关重要。本复习指南将分解你需要掌握的核心概念、术语和解题技巧，助你精通孟德尔遗传学。
1. Mendel's Experiments and the Birth of Genetics | 孟德尔的实验与遗传学的诞生
Gregor Mendel, a 19th-century monk, conducted breeding experiments with pea plants (Pisum sativum). He chose peas because they reproduce quickly, have several distinct traits, and can self-pollinate or cross-pollinate. By meticulously tracking seven characters, such as seed shape (round vs. wrinkled), seed colour (yellow vs. green) and stem height (tall vs. dwarf), Mendel discovered fundamental patterns of inheritance.
19世纪修道士格雷戈尔·孟德尔用豌豆（Pisum sativum）进行了繁殖实验。他选择豌豆是因为它们繁殖迅速、具有多个明显性状，并且可以自花传粉或异花传粉。通过仔细追踪七个特征，如种子形状（圆与皱）、种子颜色（黄与绿）和茎高（高茎与矮茎），孟德尔发现了遗传的基本模式。
He began with true-breeding (homozygous) plants and crossed them. For example, a pure-breeding tall plant (TT) crossed with a pure-breeding dwarf plant (tt) produced all tall offspring in the F₁ generation. When these F₁ plants were self-pollinated, the F₂ generation showed a ratio of approximately 3 tall : 1 dwarf.
他从纯育（纯合）植物开始并对其进行杂交。例如，纯种高茎植物（TT）与纯种矮茎植物（tt）杂交，F₁代全部为高茎。当这些F₁植物自花传粉时，F₂代表现出大约3高茎：1矮茎的比例。
P: TT × tt → F₁: all Tt (tall)
亲本：TT × tt → F₁: 全部 Tt（高茎）
F₂: 3 tall : 1 dwarf
F₂: 3高茎 : 1矮茎
2. Essential Genetic Terminology | 必备遗传学术语
Gene: A unit of heredity located on a chromosome; it is a segment of DNA that codes for a specific polypeptide, determining a characteristic.
基因：位于染色体上的遗传单位；是编码特定多肽的一段DNA，决定某一性状。
Allele: One of two or more alternative forms of a gene. Alleles arise by mutation and occupy the same locus on homologous chromosomes.
等位基因：一个基因的两种或多种不同形式之一。等位基因由突变产生，并占据同源染色体上的相同基因座。
Dominant allele: An allele that is always expressed in the phenotype, even if only one copy is present. Represented by a capital letter (e.g., T for tall).
显性等位基因：一个等位基因只要存在一份就会在表型中表达。通常用大写字母表示，如 T（高茎）。
Recessive allele: An allele that is only expressed in the phenotype when two copies are present (homozygous recessive). Represented by a lowercase letter (e.g., t for dwarf).
隐性等位基因：只有当两个拷贝都存在（隐性纯合）时才能在表型中表达的等位基因。用小写字母表示，如 t（矮茎）。
Homozygous: Having two identical alleles for a gene (e.g., TT or tt).
纯合子：某一基因的两个等位基因完全相同（如 TT 或 tt）。
Heterozygous: Having two different alleles for a gene (e.g., Tt).
杂合子：某一基因的两个等位基因不同（如 Tt）。
Genotype: The genetic makeup of an organism, describing the alleles present (e.g., TT, Tt, tt).
基因型：生物体的遗传组成，描述其所具有的等位基因（如 TT、Tt、tt）。
Phenotype: The observable characteristics of an organism, resulting from the interaction of genotype with the environment (e.g., tall, dwarf).
表型：生物体可观察的特征，由基因型与环境相互作用产生（如高茎、矮茎）。
3. The Law of Segregation | 分离定律
The Law of Segregation states that during gamete formation, the two alleles for each gene separate so that each gamete carries only one allele for each gene. This occurs because homologous chromosomes segregate during meiosis I. When a gamete combines with another during fertilisation, the offspring receives one allele from each parent, restoring the pair.
分离定律指出，在配子形成过程中，每个基因的两个等位基因会分开，因此每个配子只携带每个基因的一个等位基因。这是因为同源染色体在减数第一次分裂时分离。当配子与另一个配子在受精过程中结合时，子代从每个亲本获得一个等位基因，从而恢复成对状态。
For a heterozygous parent (Tt), meiosis produces gametes carrying either T or t with equal probability. This equal segregation is the physical basis of the 3:1 ratio observed in F₂ generations.
对于一个杂合亲本（Tt），减数分裂会产生携带 T 或 t 的配子，且概率相等。这种均等分离是F₂代观察到3:1比例的物理基础。
4. Monohybrid Crosses: Predicting Outcomes | 单杂交：预测结果
notypes of offspring. Consider a cross between two heterozygous tall pea plants (Tt × Tt).
单杂交追踪单个基因的遗传。使用庞尼特方格我们可以预测子代的基因型和表型。考虑两个杂合高茎豌豆植物（Tt × Tt）之间的杂交。
The Punnett square below shows the combination of gametes:
下面的庞尼特方格显示了配子的组合：
| T | t | |
| T | TT | Tt |
| t | Tt | tt |
Genotypic ratio: 1 TT : 2 Tt : 1 tt
基因型比：1 TT : 2 Tt : 1 tt
Phenotypic ratio: 3 tall (TT and Tt) : 1 dwarf (tt)
表型比：3 高茎（TT 和 Tt）：1 矮茎（tt）
5. 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 partner. The phenotypic ratio of the offspring reveals the unknown genotype.
测交用于确定表现出显性性状的生物体是显性纯合子还是杂合子。将该个体与隐性纯合个体杂交。后代的表型比例可揭示未知的基因型。
wn is homozygous dominant (TT × tt), all offspring will be Tt and show the dominant trait.
如果未知个体是显性纯合子（TT × tt），所有后代都将为 Tt 并表现出显性性状。
t), half the offspring will be Tt (dominant) and half tt (recessive), giving a 1 : 1 ratio.
如果未知个体是杂合子（Tt × tt），一半后代为 Tt（显性），一半为 tt（隐性），呈现出 1 : 1 的比例。
Test cross outcome: heterozygous → 1 dominant : 1 recessive
测交结果：杂合子 → 1 显性 : 1 隐性
6. The Law of Independent Assortment | 自由组合定律
The Law of Independent Assortment states that alleles for different genes segregate independently of one another during gamete formation, provided the genes are located on different chromosomes (or are far apart on the same chromosome). Mendel discovered this principle through dihybrid crosses.
自由组合定律指出，在配子形成过程中，不同基因的等位基因彼此独立分配，只要这些基因位于不同的染色体上（或者在同一染色体上相距很远）。孟德尔通过双杂交发现了这一原理。
For a plant heterozygous for seed shape (Rr) and seed colour (Yy), the allele for shape (R or r) separates independently of the allele for colour (Y or y). Thus, four types of gametes are produced in equal numbers: RY, Ry, rY, ry.
对于一个种子形状（Rr）和种子颜色（Yy）均为杂合的植物，形状等位基因（R 或 r）的分离与颜色等位基因（Y 或 y）的分离相互独立。因此，会产生四种等比例的配子：RY、Ry
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