📚 A-Level WJEC Biology: Mendelian Genetics Key Points | 孟德尔遗传 考点精讲
Mendelian genetics provides the fundamental framework for understanding how traits are passed from parents to offspring. For WJEC A-Level Biology, a thorough grasp of Mendel’s laws, monohybrid and dihybrid crosses, test crosses, and extensions such as codominance and multiple alleles is essential. This article highlights the core concepts, worked examples, and common misconceptions to help you excel in the exam.
孟德尔遗传是理解性状如何从亲代传递给子代的基本框架。对于 WJEC A-Level 生物学考试，透彻掌握孟德尔定律、单杂交与双杂交、测交以及共显性、复等位基因等延伸概念至关重要。本文将着重讲解核心概念、典型例子和常见误区，助你考试顺利。
1. Gregor Mendel and His Pea Plant Experiments | 格雷戈尔·孟德尔与他的豌豆实验
Gregor Mendel, an Austrian monk, conducted groundbreaking experiments on the garden pea Pisum sativum in the mid‑19th century. He chose pea plants because they have several distinct contrasting traits, can self‑pollinate, and are easy to cross‑pollinate manually. Mendel focused on seven characters, each with two clear variants (e.g., tall/dwarf, round/wrinkled seeds).
奥地利修道士格雷戈尔·孟德尔在19世纪中期对豌豆（Pisum sativum）进行了开创性实验。他选择豌豆是因为其具有多对明显的相对性状、能够自花传粉且易于人工异花授粉。孟德尔专注于七个性状，每个性状都有两个截然不同的表型（如高茎/矮茎、圆粒/皱粒）。
Mendel’s success stemmed from his quantitative approach. He counted large numbers of offspring, analysed the ratios mathematically, and began with true‑breeding (homozygous) lines. He crossed contrasting true‑breeders to produce the F₁ generation, then allowed F₁ plants to self‑pollinate to obtain the F₂ generation. The consistent phenotypic ratios he observed led to the formulation of fundamental laws of inheritance.
孟德尔成功的秘诀在于定量分析。他统计了大量后代，用数学方法分析比率，并从纯种（纯合）品系入手。他将具有相对性状的纯种亲本杂交获得 F₁ 代，再让 F₁ 自交得到 F₂ 代。他所观察到的稳定的表型比率，为基本遗传规律的提出奠定了基础。
2. Monohybrid Cross and the Law of Segregation | 单杂交与分离定律
A monohybrid cross examines the inheritance of a single gene with two alleles. When Mendel crossed a homozygous tall plant (TT) with a homozygous dwarf plant (tt), all F₁ offspring were tall (Tt). The dwarf phenotype disappeared, indicating that the tall allele is dominant over the recessive dwarf allele. When F₁ plants were selfed, the F₂ generation showed a 3:1 ratio of tall to dwarf (genotypic ratio 1 TT : 2 Tt : 1 tt).
单杂交研究的是由一对等位基因控制的单一性状的遗传。孟德尔将纯合高茎植株（TT）与纯合矮茎植株（tt）杂交，F₁ 代全部为高茎（Tt）。矮茎表型消失，表明高茎等位基因为显性，矮茎为隐性。F₁ 自交后，F₂ 代表现出高茎与矮茎 3:1 的比率（基因型比率为 1 TT : 2 Tt : 1 tt）。
The reappearance of the dwarf trait in F₂ can only be explained if the two alleles segregate during gamete formation, so each gamete carries only one allele. This is the Law of Segregation: the two alleles for a trait separate during meiosis, and offspring inherit one allele from each parent. The 3:1 phenotypic ratio is characteristic of a monohybrid cross between two heterozygous individuals where one allele is completely dominant.
矮茎性状在 F₂ 中重新出现，唯一合理的解释是：在配子形成过程中，两个等位基因相互分离，使每个配子仅含有一个等位基因。这就是分离定律：控制一对性状的两个等位基因在减数分裂时彼此分开，子代从每个亲本各获得一个等位基因。3:1 的表型比率是两个杂合体单杂交的典型结果，前提是显性完全。
3. Dihybrid Cross and the Law of Independent Assortment | 双杂交与自由组合定律
Mendel extended his work to follow two genes simultaneously. A classic dihybrid cross investigated seed shape (round R dominant over wrinkled r) and seed colour (yellow Y dominant over green y). He crossed homozygous round yellow (RRYY) with homozygous wrinkled green (rryy). All F₁ were di‑heterozygous (RrYy) and showed both dominant phenotypes: round and yellow.
孟德尔进一步同时对两对基因进行了追踪。经典的双杂交实验关注种子形状（圆粒 R 对皱粒 r 为显性）和种子颜色（黄粒 Y 对绿粒 y 为显性）。他将纯合圆黄（RRYY）与纯合皱绿（rryy）杂交，F₁ 全为双杂合体（RrYy），均表现两种显性性状：圆粒和黄粒。
Self‑pollination of F₁ produced an F₂ generation with four phenotypic classes in a 9:3:3:1 ratio (9 round yellow, 3 round green, 3 wrinkled yellow, 1 wrinkled green). This ratio is observed only when the two genes are located on different chromosomes (or are far apart on the same chromosome) and thus assort independently. The Law of Independent Assortment states that alleles of different genes separate independently of one another into gametes, leading to all possible combinations.
F₁ 自交得到的 F₂ 代出现四种表型，其比率为 9:3:3:1（9 圆黄，3 圆绿，3 皱黄，1 皱绿）。该比率只有当两对基因位于不同染色体上（或位于同一染色体但相距甚远）并独立分配时才会出现。自由组合定律指出，不同基因的等位基因在形成配子时独立分离，彼此互不干扰，从而产生所有可能的组合。
4. Key Genetic Terminology | 遗传学核心术语
Genotype is the genetic makeup of an organism, e.g. Tt. Phenotype is the observable characteristic, e.g. tall stature. An allele is one of the alternative forms of a gene. The dominant allele masks the effect of the recessive allele in a heterozygous condition. Homozygous individuals have two identical alleles for a trait (TT or tt), while heterozygous individuals have two different alleles (Tt).
基因型是生物体的遗传组成，例如 Tt。表现型是观察到的特征，例如高茎。等位基因是基因的替代形式之一。显性等位基因在杂合状态下会掩盖隐性等位基因的效应。纯合个体拥有一对控制该性状的相同等位基因（TT 或 tt），而杂合个体则拥有一对不同等位基因（Tt）。
The P generation refers to the parental generation, F₁ is the first filial generation, and F₂ is the second filial generation. A monohybrid cross involves one gene, and a dihybrid cross involves two genes. True‑breeding (or pure‑breeding) organisms consistently produce offspring with the same phenotype when self‑fertilised. A Punnett square is a grid used to predict the genotypes of offspring from a cross.
P 代指亲代，F₁ 是子一代，F₂ 是子二代。单杂交涉及一对基因，双杂交涉及两对基因。纯种（或纯育）生物在自交时能够稳定产生具有相同表型的后代。庞尼特方格是用来预测杂交后代基因型的棋盘格工具。
5. Using Punnett Squares | 使用庞尼特方格
To construct a Punnett square for a monohybrid cross Aa × Aa, write the possible gametes from one parent along the top (A and a) and those from the other parent along the side. Fill in the boxes by combining alleles. The resulting genotypic ratio is 1 AA : 2 Aa : 1 aa. When A is dominant, the phenotypic ratio becomes 3:1.
制作单杂交 Aa × Aa 的庞尼特方格，将一方亲本可能产生的配子（A 和 a）写在顶端，另一方写在左侧。合并等位基因填入方格。得到的基因型比率为 1 AA : 2 Aa : 1 aa。当 A 为显性时，表型比率即为 3:1。
For a dihybrid cross between two di‑heterozygotes (RrYy × RrYy), each parent produces four types of gametes (RY, Ry, rY, ry). A 4 × 4 Punnett square reveals the 9:3:3:1 phenotypic ratio. This systematic approach helps prevent errors in counting and is highly favoured in WJEC exams. Always show your gametes clearly and label genotypes and phenotypes.
对于双杂合体杂交（RrYy × RrYy），每个亲本产生四种配子（RY, Ry, rY, ry）。使用 4×4 方格可得 9:3:3:1 表型比率。这种系统方法能有效避免计数错误，WJEC 考试中极为推崇。务必清楚注明配子类型，并标注基因型与表现型。
6. The Test Cross | 测交
A test cross is used to determine the genotype of an individual exhibiting a dominant trait. The organism in question is crossed with a homozygous recessive individual for the same trait. If any recessive offspring appear, the tested parent must be heterozygous; if all offspring show the dominant trait, the parent is likely homozygous dominant.
测交用于确定表现出显性性状个体究竟是纯合还是杂合。将该个体与相应隐性纯合体杂交。若后代出现隐性性状，则待测亲本必为杂合体；若所有后代均表现显性性状，则待测亲本很可能为显性纯合体。
For example, a tall pea plant of unknown genotype (T_) is test‑crossed with a dwarf plant (tt). If the tall plant is TT, all offspring are Tt (tall). If it is Tt, the offspring show a 1:1 ratio of tall (Tt) to dwarf (tt). This cross clearly demonstrates Mendel’s law of segregation and is a common exam question.
例如，将一株高茎豌豆但其基因型未知（T_）与矮茎植株（tt）测交。若待测植株为 TT，则所有后代均为 Tt（高茎）。若为 Tt，后代将出现 1:1 的高茎（Tt）与矮茎（tt）。测交鲜明地展现了分离定律，是常见考题。
7. Codominance and Incomplete Dominance | 共显性与不完全显性
Not all traits follow complete dominance. In incomplete dominance, the heterozygous phenotype is a blend of the two homozygous phenotypes. For example, in snapdragons, a cross between red (CᴿCᴿ) and white (CᵂCᵂ) produces pink (CᴿCᵂ) offspring. An F₂ cross yields a 1 red : 2 pink : 1 white ratio, matching the genotypic ratio. Note that the phenotypic ratio is no longer 3:1.
并非所有性状都遵循完全显性。不完全显性中，杂合体的表型是两个纯合体表型的中间型。例如金鱼草，红花（CᴿCᴿ）与白花（CᵂCᵂ）杂交产生粉红花（CᴿCᵂ）。F₂ 杂交结果为 1 红 : 2 粉红 : 1 白，与其基因型比率完全吻合。注意此时表型比率不再是 3:1。
Codominance occurs when both alleles are expressed equally in the heterozygote. A classic WJEC example is the ABO blood group system. The Iᴬ and Iᴮ alleles are codominant, and both are dominant over i. A person with genotype Iᴬ Iᴮ has blood type AB, expressing both A and B antigens on red blood cells. This pattern gives multiple heterozygous phenotypes.
共显性中，杂合体同时表达两个等位基因的性状，互不遮盖。WJEC 中典型例子是 ABO 血型系统。Iᴬ 与 Iᴮ 等位基因为共显性，且两者对 i 均为显性。基因型为 Iᴬ Iᴮ 的人血型为 AB，红细胞上同时表达 A 抗原和 B 抗原。这种遗传模式能产生多种杂合表型。
8. Multiple Alleles and Blood Groups | 复等位基因与血型
Although any individual carries only two alleles, a gene may have more than two alleles in the population; this is known as multiple alleles. The ABO system is controlled by three alleles: Iᴬ, Iᴮ, and i. The Iᴬ allele codes for A antigen, Iᴮ for B antigen, and i produces no antigen. The six possible genotypes yield four blood types: type A (IᴬIᴬ or Iᴬi), type B (IᴮIᴮ or Iᴮi), type AB (IᴬIᴮ), and type O (ii).
虽然每个个体只携带两个等位基因，但群体中一个基因可能存在两个以上的等位基因，称为复等位基因。ABO 血型由三个等位基因控制：Iᴬ、Iᴮ 和 i。Iᴬ 负责 coding A 抗原，Iᴮ 负责 B 抗原，i 不产生抗原。六种可能的基因型对应四种血型：A 型（IᴬIᴬ 或 Iᴬi）、B 型（IᴮIᴮ 或 Iᴮi）、AB 型（IᴬIᴮ）和 O 型（ii）。
Solving blood group problems often requires test crosses or pedigree logic. For example, if a mother with blood type O (ii) has a child with blood type AB, the father must have contributed both Iᴬ and Iᴮ alleles, which is only possible if the father is genotype IᴬIᴮ. Such questions test your understanding of codominance and multiple alleles simultaneously.
解决血型问题常常需要测交或系谱推理。例如，一位 O 型血母亲（ii）生下了 AB 型血的孩子，父亲必然同时提供了 Iᴬ 和 Iᴮ 等位基因，只有基因型为 IᴬIᴮ 才可能。这类题目同时考查共显性和复等位基因的理解。
9. Pedigree Analysis | 系谱分析
Pedigree charts are graphical representations of family inheritance patterns. They are used to deduce whether a trait is dominant or recessive, autosomal or sex‑linked. In WJEC, you will mainly interpret autosomal dominant and autosomal recessive pedigrees. For autosomal recessive conditions, affected individuals often have unaffected parents who are carriers; the trait may skip generations. For autosomal dominant conditions, every affected individual usually has an affected parent, and the trait appears in every generation.
系谱图是家族遗传模式的图解表示。可用于推断某性状是显性还是隐性、常染色体遗传还是性连锁。WJEC 考试主要考查常染色体显性和常染色体隐性系谱的解读。常染色体隐性疾病中，患病个体其双亲往往不患病而是携带者，性状可能隔代出现。常染色体显性疾病中，患者通常至少有一位亲本患病，性状代代出现。
When interpreting a pedigree, assign genotypes logically using the dominant/recessive relationships. For a recessive trait, use A for the normal allele and a for the disease allele. Unaffected individuals married into the family are assumed to be homozygous normal unless evidence suggests otherwise. Calculate probabilities for future offspring by drawing Punnett squares from the determined parental genotypes.
解读系谱时，要根据显隐性关系合理推定基因型。以隐性性状为例，设正常等位基因为 A，致病基因为 a。嫁入家族的正常个体若无额外证据，可假定为纯合正常。根据已推断的亲本基因型，通过庞尼特方格计算未来子女患病概率。
10. Mendel’s Laws and Meiosis | 孟德尔定律与减数分裂
Mendel’s laws can be explained by the behaviour of chromosomes during meiosis. The law of segregation reflects the separation of homologous chromosomes in anaphase I. Each gamete receives one copy of each chromosome, hence one allele of each gene. The law of independent assortment is based on the random alignment of different homologous pairs at the metaphase I plate. The orientation of one pair does not influence another, provided the genes are on different chromosomes.
孟德尔定律可用减数分裂中染色体的行为来解释。分离定律对应减数第一次分裂后期同源染色体的分离。每个配子获得各对染色体中的一条，因而每个基因只得到一个等位基因。自由组合定律源于减数第一次分裂中期各同源染色体对的随机排列。只要基因位于不同染色体上，一对染色体的取向不会影响另一对。
This chromosomal basis reinforces why dihybrid ratios deviate from 9:3:3:1 when genes are linked (located on the same chromosome). Linked genes tend to be inherited together unless crossing over during prophase I creates new combinations. Such linkage is a direct exception to independent assortment but is entirely consistent with chromosome theory. WJEC expects you to recognise how Mendel’s
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