Mendelian Genetics for IGCSE WJEC Biology | 孟德尔遗传考点精讲

📚 Mendelian Genetics for IGCSE WJEC Biology | 孟德尔遗传考点精讲

Mendelian genetics is the foundation of inheritance, explaining how characteristics are passed from parents to offspring through discrete units called genes. This topic is central to the WJEC IGCSE Biology specification and builds the skills needed to predict offspring traits using Punnett squares and understand the ratios that arise from monohybrid crosses.

孟德尔遗传是遗传学的基础，解释特征如何通过称为基因的离散单位从亲代传递给子代。该主题是 WJEC IGCSE 生物考试大纲的核心部分，可帮助学生掌握使用旁氏表预测子代性状的技巧，并理解单因子杂交中出现的比例。

1. Gregor Mendel and His Pea Plants | 孟德尔与他的豌豆实验

Gregor Mendel, an Austrian monk, performed breeding experiments on pea plants in the mid‑19th century. He chose pea plants because they have several distinct traits (e.g. tall vs. short, round vs. wrinkled seeds), they can self‑pollinate or be cross‑pollinated, and they produce many offspring rapidly.

格雷戈尔·孟德尔是奥地利修道士，于 19 世纪中期用豌豆进行了交配实验。他选择豌豆是因为它们具有多对明显不同的性状（例如高与矮、圆粒与皱粒），可以自花授粉或人工异花授粉，并且能快速产生大量后代。

Mendel’s approach was quantitative – he counted large numbers of offspring and analysed the ratios, rather than looking at a blended mixture. This allowed him to deduce that inheritance is particulate, not blending.

孟德尔采用定量方法——他统计大量后代并分析比例，而不是观察融合的混合性状。这使他推断出遗传是颗粒性的，而非融合遗传。

2. Key Genetic Terms You Must Know | 必须掌握的遗传学术语

The WJEC exam expects you to use precise terminology. Here are the core definitions:

WJEC 考试要求使用准确的术语。以下是核心定义：

Gene – a section of DNA that codes for a specific protein, determining a trait.

基因 – 一段编码特定蛋白质的 DNA，决定某一性状。
Allele – an alternative version of a gene. For example, the gene for plant height has alleles for tall (T) and short (t).

等位基因 – 一个基因的替代形式。例如，控制株高的基因有高（T）和矮（t）两种等位基因。
Dominant allele – an allele that is always expressed in the phenotype when present, even if only one copy exists. Represented by a capital letter (e.g. T).

显性等位基因 – 只要有该等位基因存在（哪怕只有一个拷贝），其性状就能在表现型中表达。用大写字母表示（如 T）。
Recessive allele – an allele that is only expressed in the phenotype if two copies are present (homozygous condition). Represented by a lowercase letter (e.g. t).

隐性等位基因 – 只有当存在两个拷贝时（纯合状态），才能在表现型中表达的等位基因。用小写字母表示（如 t）。
Homozygous – having two identical alleles for a trait (e.g. TT or tt). Also called pure‑breeding.

纯合 – 同一性状的两个等位基因相同（如 TT 或 tt），也称为纯系。
Heterozygous – having two different alleles for a trait (e.g. Tt).

杂合 – 同一性状的两个等位基因不同（如 Tt）。
Genotype – the genetic makeup of an organism, consisting of the alleles present (e.g. TT, Tt, or tt).

基因型 – 生物体的遗传组成，即存在的等位基因（如 TT、Tt 或 tt）。
Phenotype – the observable characteristics of an organism, resulting from the genotype and environmental influences (e.g. tall or short).

表现型 – 生物体可观察的特征，由基因型和环境共同影响（如高或矮）。

3. Monohybrid Cross – The Classic Experiment | 单因子杂交——经典实验

Mendel’s first experiment crossed true‑breeding tall plants (TT) with true‑breeding short plants (tt). This is a monohybrid cross because it follows the inheritance of one single characteristic.

孟德尔的第一个实验将纯系高茎豌豆（TT）与纯系矮茎豌豆（tt）杂交。这是一种单因子杂交，因为它只追踪单一性状的遗传。

The first generation (F₁) were all tall. This showed that the tall allele is dominant over the short allele. The F₁ plants were heterozygous (Tt) and displayed the dominant phenotype.

子一代（F₁）全部为高茎。这证明高茎等位基因对矮茎等位基因为显性。F₁ 植株均为杂合子（Tt），并表现出显性表现型。

Mendel then allowed the F₁ plants to self‑pollinate. The second generation (F₂) produced both tall and short plants in a ratio close to 3 : 1. This reappearance of the recessive trait was crucial evidence for the particulate theory of inheritance.

随后孟德尔让 F₁ 植株自花授粉。子二代（F₂）同时出现高茎和矮茎植株，比例接近 3 : 1。隐性性状的再次出现为颗粒遗传理论提供了关键证据。

4. Constructing Punnett Squares | 构建旁氏表

A Punnett square is a grid used to predict the genotypes and phenotypes of offspring from a genetic cross. Always write the parental alleles along the top and side, then combine them inside.

旁氏表是一个用于预测杂交后代基因型和表现型的方格图。始终将亲代等位基因写在顶部和左侧，然后在方格内进行组合。

For a cross between two heterozygous tall plants (Tt × Tt):

以两个杂合高茎植株（Tt × Tt）杂交为例：

	T	t
T	TT	Tt
t	Tt	tt

The resulting genotypic ratio is 1 TT : 2 Tt : 1 tt. The phenotypic ratio is 3 tall : 1 short, because both TT and Tt give tall plants.

产生的基因型比例为 1 TT : 2 Tt : 1 tt。表现型比例为 3 高 : 1 矮，因为 TT 和 Tt 均表现为高茎。

Always link the Punnett square back to probability – each box represents a 25% chance if all combinations are equally likely.

请务必将旁氏表与概率联系起来——如果所有组合可能性相等，每个方格代表 25% 的几率。

5. Phenotypic and Genotypic Ratios | 表现型与基因型比例

In a monohybrid cross between two heterozygous individuals (F₁ × F₁), the expected phenotypic ratio is 3 : 1 (dominant : recessive). The typical genotypic ratio is 1 : 2 : 1 (homozygous dominant : heterozygous : homozygous recessive).

在两个杂合个体（F₁ × F₁）的单因子杂交中，预期的表现型比例为 3 : 1（显性 : 隐性）。典型的基因型比例为 1 : 2 : 1（显性纯合 : 杂合 : 隐性纯合）。

However, if a homozygous dominant individual is crossed with a homozygous recessive (TT × tt), all F₁ offspring are heterozygous (Tt) and show the dominant phenotype – a ratio of 100% dominant. The F₂ generation then shows the 3 : 1 ratio.

但是，如果纯合显性与纯合隐性杂交（TT × tt），则所有 F₁ 后代均为杂合（Tt）并表现显性性状——比例为 100% 显性。随后 F₂ 代会表现出 3 : 1 的比例。

WJEC may ask you to predict these ratios from a given cross or to explain why actual results might differ slightly from expected ratios due to chance in small sample sizes.

WJEC 考试可能要求你根据给定杂交预测这些比例，或解释为何实际结果可能因样本量小而略有偏差。

6. The Test Cross | 测交

A test cross is used to determine whether an organism showing the dominant phenotype is homozygous (TT) or heterozygous (Tt). The organism is crossed with a homozygous recessive individual (tt).

测交用于确定一个表现出显性性状的生物体是纯合（TT）还是杂合（Tt）。将该生物体与隐性纯合（tt）个体杂交。

If any offspring show the recessive trait, the unknown parent must be heterozygous (Tt). If all offspring show the dominant trait, the unknown parent is likely homozygous (TT).

若任一后代表现出隐性性状，则未知亲本必定是杂合（Tt）。若所有后代均表现显性性状，则未知亲本很可能是纯合（TT）。

For example, crossing a tall plant of unknown genotype with a short plant (tt): if half the offspring are short, the tall parent is Tt.

例如，将基因型未知的高茎植株与矮茎（tt）植株杂交：若一半后代为矮茎，则高茎亲本为 Tt。

7. Mendel’s Law of Segregation | 孟德尔分离定律

Mendel’s first law states that each organism carries two alleles for each trait, and these alleles separate (segregate) during gamete formation so that each gamete carries only one allele for each gene.

孟德尔第一定律指出，每个生物体针对每一性状携带两个等位基因，这些等位基因在配子形成时分离，使得每个配子只携带每个基因的一个等位基因。

This explains why crosses between two heterozygous parents produce the 3 : 1 ratio – the alleles segregate randomly and re‑combine at fertilisation.

这解释了为什么两个杂合亲本杂交会产生 3 : 1 的比例——等位基因随机分离，并在受精时重新组合。

The law is beautifully demonstrated by monohybrid crosses and is fundamental to understanding all Mendelian inheritance patterns.

该定律通过单因子杂交得到了完美诠释，是理解所有孟德尔遗传模式的基础。

8. Codominance – A Variation on Mendel’s Pattern | 共显性——孟德尔模式的变体

Not all alleles follow the simple dominant‑recessive relationship. In codominance, both alleles are expressed equally in the heterozygous phenotype. A classic example is the AB blood group in humans.

并非所有等位基因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状态下均能平等表达。经典例子是人类的 AB 血型。

If we represent the A and B alleles as Iᴬ and Iᴮ, a heterozygous genotype IᴬIᴮ produces blood type AB, where both antigens are present on red blood cells.

若用 Iᴬ 和 Iᴮ 表示 A 和 B 等位基因，杂合基因型 IᴬIᴮ 表现为 AB 血型，红细胞表面同时存在两种抗原。

When solving codominance problems, use superscript notation and Punnett squares as usual. The phenotypic ratio often becomes 1 : 2 : 1 if two heterozygous individuals are crossed (e.g. IᴬIᴮ × IᴬIᴮ gives 1 IᴬIᴬ : 2 IᴬIᴮ : 1 IᴮIᴮ).

解决共显性问题时，使用上标记号和旁氏表即可。若两个杂合个体杂交（例如 IᴬIᴮ × IᴬIᴮ），表现型比例通常变为 1 : 2 : 1（1 IᴬIᴬ : 2 IᴬIᴮ : 1 IᴮIᴮ）。

9. Incomplete Dominance | 不完全显性

Incomplete dominance occurs when the heterozygous phenotype is a blend of the two homozygous phenotypes. For example, in snapdragon flowers, a cross between red (RR) and white (rr) produces pink (Rr) offspring.

不完全显性指的是杂合表现型是两种纯合表现型的中间体。例如金鱼草花色中，红花（RR）与白花（rr）杂交产生粉花（Rr）后代。

Note that neither allele is completely dominant; the heterozygous phenotype is distinct. The genotypic ratio 1 : 2 : 1 in the F₂ generation matches the phenotypic ratio 1 red : 2 pink : 1 white.

注意，这里没有任何一个等位基因为完全显性；杂合表现型是独特的。F₂ 代基因型比例 1 : 2 : 1 与表现型比例 1 红 : 2 粉 : 1 白完全吻合。

Both codominance and incomplete dominance can appear in WJEC questions, so be able to distinguish them: codominance shows both traits fully, while incomplete dominance shows an intermediate blend.

共显性和不完全显性都可能出现在 WJEC 试题中，因此要学会区分：共显性同时表现出两种性状，而不完全显性表现为中间混合性状。

10. Interpreting Family Pedigrees | 解读家族系谱图

WJEC often includes pedigree charts (family trees) to test your understanding of Mendelian inheritance. In such diagrams, squares represent males, circles represent females, and shaded symbols indicate individuals expressing the recessive trait.

WJEC 常通过系谱图（家族树）考察你对孟德尔遗传的理解。此类图中，方块代表男性，圆圈代表女性，涂色的符号代表表现出隐性性状的个体。

To determine the genotypes of individuals, work backwards from the recessive individuals – they must be homozygous recessive. Then use parental and offspring relationships to deduce other genotypes.

要确定个体的基因型，可从隐性个体逆向推导——他们必定是隐性纯合。然后利用亲代与子代关系推知其他基因型。

For a recessive trait, unaffected parents can have affected children only if both are heterozygous. This is a key clue when analysing pedigrees.

对于隐性性状，表现正常的双亲只有在双方均为杂合时才可能生出患病后代。这是分析系谱图的关键线索。

11. Common Misconceptions and Exam Pitfalls | 常见误解与考试易错点

Students often confuse “dominant” with “common”. A dominant trait is not necessarily the most frequent in a population; it simply masks a recessive allele when present. For example, Huntington’s disease is caused by a dominant allele but is rare.

学生常将“显性”与“常见”混淆。显性性状不一定在种群中最为普遍，它只是在存在时能掩盖隐性等位基因。例如，亨廷顿病由显性等位基因引起，但极为罕见。

Another pitfall is forgetting to use the same letter for alleles of the same gene. Never mix letters: use T and t, not T and s.

另一个易错点是忘记用同一字母的两种形式表示同一基因的等位基因。切勿混用不同字母：应使用 T 和 t，而不是 T 和 s。

When linking ratios to real data, always mention that small sample sizes may cause deviation from the expected Mendelian ratio. Use words like ‘chance’ and ‘probability’ rather than certainty.

当将比例与实际数据联系起来时，务必提及小样本量可能导致与预期孟德尔比例发生偏差。使用“几率”和“概率”等词语，而非确定性表述。

12. Summary and Key Takeaways | 总结与核心要点

Mendelian genetics is a predictive toolkit. Master the Punnett square, understand the 3 : 1 and 1 : 2 : 1 ratios for monohybrid crosses, and know how to apply test crosses and pedigree analysis. Remember that alleles segregate during gamete formation, and that heterozygous individuals carry a hidden recessive allele.

孟德尔遗传是一个预测工具。掌握旁氏表，理解单因子杂交中的 3 : 1 和 1 : 2 : 1 比例，并知道如何运用测交和系谱分析。牢记等位基因在配子形成时分离，杂合个体携带有隐藏的隐性等位基因。

For the WJEC exam, practice writing clear genetic diagrams with parent genotypes, gametes, a Punnett square, and then stating the genotypic and phenotypic ratios. Always show your working step by step.

针对 WJEC 考试，请练习清楚地书写遗传图解，包括亲代基因型、配子、旁氏表，然后说明基因型和表现型比例。务必逐步展示你的推理过程。

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Mendelian Genetics for IGCSE WJEC Biology | 孟德尔遗传考点精讲