Mendelian Genetics for IB and WJEC Biology | 孟德尔遗传考点精讲 (IB/WJEC)

📚 Mendelian Genetics for IB and WJEC Biology | 孟德尔遗传考点精讲 (IB/WJEC)

Mendelian genetics forms the bedrock of modern inheritance studies. Whether you are preparing for IB Biology or WJEC specifications, a clear grasp of Mendel’s laws, monohybrid and dihybrid crosses, test crosses, and extensions such as codominance and sex‑linkage is essential. This guide revisits each concept with carefully paired English–Chinese explanations, worked examples, and exam‑focused tips to help you master the topic.

孟德尔遗传是现代遗传学的基础。无论你正在为IB生物还是WJEC考试做准备，清晰掌握孟德尔定律、单基因杂交与双基因杂交、测交以及共显性、伴性遗传等扩展内容都至关重要。本指南以英中对照的讲解、计算示例和考点技巧，助你精通这一主题。

1. Introduction and Historical Context | 引言与历史背景

Gregor Mendel, an Augustinian friar working in the 19th century, systematically cross‑bred pea plants (Pisum sativum) and recorded the inheritance patterns of seven discrete traits. His work remained unrecognised until the early 1900s, when it was rediscovered and integrated with chromosome theory. The power of Mendel’s approach lay in his choice of true‑breeding lines and his quantitative analysis of offspring ratios.

19世纪的奥古斯丁会修士格雷戈尔·孟德尔系统地杂交了豌豆植株，记录了七种不连续性状的遗传模式。他的工作在20世纪初被重新发现并与染色体理论结合。孟德尔方法的关键在于选择了纯合品系并对子代比例进行定量分析。

Mendel’s success can be attributed to three key factors: (1) focusing on traits with two clearly contrasting forms, (2) ensuring large sample sizes, and (3) carrying out reciprocal crosses to rule out parental sex effects. Modern IB and WJEC syllabi expect students to appreciate why Mendel’s experimental design was so robust.

孟德尔的成功归因于三个关键因素：(1) 选择具有两种明显对比形式的性状，(2) 确保大样本量，(3) 进行正反交以排除亲本性别效应。现代IB和WJEC大纲要求学生理解孟德尔实验设计为何如此严谨。

2. Key Terminology | 关键术语

Before tackling crosses, you must be confident with the vocabulary. Below is a quick reference table; all terms are used throughout the article. For each entry, the English definition is given first, followed by the Chinese equivalent.

在处理杂交之前，你必须熟练掌握相关术语。下面提供一个快速参考表；本文全程使用这些术语。每个条目先给出英文定义，再给出中文对应。

Term	Definition / 定义
Gene	A length of DNA that codes for a polypeptide. / 编码多肽的DNA片段。
Allele	An alternative form of a gene. / 基因的一种可变形式。
Genotype	The combination of alleles an organism possesses. / 一个生物体所拥有的等位基因组合。
Phenotype	The observable characteristics resulting from the genotype and environment. / 由基因型和环境共同决定的可见特征。
Homozygous	Having two identical alleles for a gene (e.g., AA or aa). / 对于某基因具有两个相同等位基因（如AA或aa）。
Heterozygous	Having two different alleles for a gene (e.g., Aa). / 对于某基因具有两个不同等位基因（如Aa）。
Dominant	An allele that is expressed in the phenotype even when only one copy is present. / 只需一个拷贝就能在表型中表达的等位基因。
Recessive	An allele that is only expressed when two copies are present (homozygous). / 只有在纯合状态下才表达的等位基因。

These definitions are examined directly in both IB and WJEC multiple‑choice questions, so commit them to memory.

IB和WJEC的选择题会直接考查这些定义，请牢记。

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

The Law of Segregation states that during gamete formation, the two alleles for each gene separate so that each gamete carries only one allele. When Mendel crossed true‑breeding purple‑flowered plants (PP) with white‑flowered plants (pp), all F₁ offspring were purple (Pp). Allowing the F₁ to self‑pollinate produced an F₂ generation with a ratio of approximately 3 purple : 1 white.

分离定律指出，在配子形成时，每个基因的两个等位基因彼此分离，使每个配子只携带一个等位基因。当孟德尔将纯合紫花植株(PP)与白花植株(pp)杂交时，所有F₁子代均为紫色(Pp)。让F₁自花授粉，产生的F₂代呈现约3紫 : 1白的比例。

This 3:1 phenotypic ratio is the hallmark of a monohybrid cross involving a dominant–recessive allele pair. The underlying genotypic ratio is 1 PP : 2 Pp : 1 pp. At the molecular level, segregation reflects the separation of homologous chromosomes during anaphase I of meiosis.

3:1的表型比例是涉及显隐性等位基因对的单基因杂交的标志。其基因型比例为1 PP : 2 Pp : 1 pp。在分子水平上，分离反映了减数第一次分裂后期同源染色体的分开。

Sample Punnett square for F₂ of PP × pp (parents provide gametes P and p):

PP × pp的F₂庞纳特方格示例（亲本提供配子P和p）：

	P	p
P	PP	Pp
p	Pp	pp

Thus, the F₁ heterozygous cross (Pp × Pp) yields PP, Pp, pP, pp – a classic Mendelian segregation.

因此，F₁杂合子杂交(Pp × Pp)产生PP、Pp、pP、pp——经典的孟德尔分离。

4. Monohybrid Crosses and Probability | 单基因杂交与概率

Monohybrid crosses track the inheritance of a single gene. The expected genotypic and phenotypic ratios assume random fertilisation. In IB and WJEC exams, you may be asked to calculate the probability that an offspring will exhibit a particular trait. Use the product rule: the probability of independent events occurring together is the product of their individual probabilities.

单基因杂交追踪单一基因的遗传。预期的基因型比例和表型比例基于随机受精。IB和WJEC考试可能要求计算后代出现某一性状的概率。使用乘法法则：独立事件共同发生的概率是各自概率的乘积。

For example, crossing Aa × Aa: the probability of obtaining an aa offspring equals (½ chance of receiving a from mother) × (½ from father) = ¼. Similarly, the probability of at least one dominant A allele (phenotype dominant) is 1 − ¼ = ¾.

例如，Aa × Aa杂交：获得aa后代的概率等于（从母亲得到a的½概率）×（从父亲得到a的½概率）= ¼。类似地，至少有一个显性A等位基因（表现为显性表型）的概率是1 − ¼ = ¾。

Be careful with conditional probability questions. If a question states that a healthy sibling (in the case of an autosomal recessive disease) has phenotypically normal parents, the probability that the sibling is a carrier is ⅔, not ½, because the homozygous recessive possibility has been eliminated.

注意条件概率题。如果题目表明患有常染色体隐性遗传病的正常兄弟姐妹的双亲表型正常，那么该兄弟姐妹是携带者的概率是⅔，而不是½，因为纯合隐性的可能性已被排除。

5. Dihybrid Crosses and the Law of Independent Assortment | 双基因杂交与自由组合定律

Mendel’s Law of Independent Assortment states that alleles for different genes segregate independently of one another during gamete formation. This holds true for genes located on different chromosomes or far apart on the same chromosome. In a classic dihybrid cross (e.g., seed colour Y/y and seed shape R/r), crossing F₁ heterozygotes (YyRr × YyRr) produces a 9:3:3:1 phenotypic ratio.

孟德尔的自由组合定律指出，不同基因的等位基因在配子形成时彼此独立地分离。这适用于位于不同染色体上或同一条染色体上相距很远的基因。在经典的双基因杂交中（例如种子颜色Y/y和种子形状R/r），F₁杂合子杂交(YyRr × YyRr)产生9:3:3:1的表型比例。

Gametes from YyRr: YR, Yr, yR, yr – each with equal probability (¼). The 16-box Punnett square reveals:

YyRr产生的配子：YR、Yr、yR、yr——每种概率相等（¼）。16格庞纳特方格显示：

9/16 show both dominant traits (yellow, round) / 显示双显性性状（黄色、圆粒）
3/16 show dominant for colour, recessive for shape (yellow, wrinkled) / 颜色显性、形状隐性（黄色、皱粒）
3/16 show recessive for colour, dominant for shape (green, round) / 颜色隐性、形状显性（绿色、圆粒）
1/16 show both recessive traits (green, wrinkled) / 显示双隐性性状（绿色、皱粒）

The IB and WJEC specifications often ask you to derive gamete genotypes from a dihybrid parent using the FOIL (First, Outside, Inside, Last) method. Practice is key.

IB和WJEC大纲经常要求使用FOIL法从双因子杂合亲本推导配子基因型。练习是关键。

6. Test Crosses and Back Crosses | 测交与回交

A test cross is performed to determine the genotype of an individual showing a dominant phenotype. The individual is crossed with a homozygous recessive (e.g., pp). If all offspring display the dominant trait, the unknown parent was homozygous dominant (PP). If a 1:1 ratio of dominant to recessive appears, the parent was heterozygous (Pp).

测交用于确定显性表型个体的基因型。将该个体与隐性纯合子（如pp）杂交。若所有后代表现显性性状，则待测亲本为显性纯合子(PP)。若出现1:1的显隐性比例，则亲本为杂合子(Pp)。

A back cross, by contrast, involves crossing an offspring with one of its parents (or a genetically equivalent individual). While test crosses are the most direct method to reveal hidden recessive alleles, back crosses are useful in plant and animal breeding to reinforce desirable traits.

相反，回交是指将子代与亲本之一（或基因型相同的个体）杂交。虽然测交是揭示隐藏隐性等位基因的最直接方法，但回交在动植物育种中用于强化优良性状。

You may encounter exam questions that provide the offspring ratios and ask you to deduce the parental genotypes. Always work backwards:

你可能会遇到给出后代表型比例并让你推导亲本基因型的考题。反向推导方法如下：

1:1 ratio → one parent heterozygous, one homozygous recessive. / 1:1比例 → 一个亲本杂合，一个为隐性纯合。
3:1 ratio → both parents heterozygous. / 3:1比例 → 两个亲本均杂合。
All dominant → at least one parent homozygous dominant. / 全部显性 → 至少一个亲本为显性纯合。

7. Incomplete Dominance and Codominance | 不完全显性与共显性

Not all traits follow strict dominant–recessive patterns. In incomplete dominance, the heterozygous phenotype is an intermediate blend. For example, crossing red (RR) and white (rr) snapdragons yields pink (Rr) flowers in the F₁. Self‑pollinating the F₁ gives a 1:2:1 phenotypic ratio (red : pink : white), which mirrors the genotypic ratio.

并非所有性状都遵循严格的显隐性模式。在不完全显性中，杂合表型为中间混合型。例如，红花(RR)与白花(rr)金鱼草杂交，产生粉色(Rr)花的F₁。F₁自交产生1:2:1的表型比例（红 : 粉 : 白），与基因型比例一致。

In codominance, both alleles are fully expressed in the heterozygote. The ABO blood group system is a classic example: IA and IB are codominant, while i is recessive. Hence, genotype IAIB produces blood type AB, with both A and B antigens present.

在共显性中，杂合状态下两个等位基因都完全表达。ABO血型系统是经典例子：IA与IB为共显性，i为隐性。因此，基因型IAIB产生AB型血，同时存在A和B抗原。

Multiple alleles – when more than two allelic forms exist in the population – also generate interesting patterns. The human ABO system has three alleles (IA, IB, i), producing four phenotypes and six genotypes. Understanding these patterns is crucial for IB and WJEC blood‑type pedigree questions.

复等位基因——种群中存在两个以上等位基因形式——也会产生有趣的模式。人类ABO系统有三个等位基因(IA、IB、i)，产生四种表型和六种基因型。理解这些模式对于IB和WJEC中血型系谱题至关重要。

8. Sex‑Linked Inheritance | 伴性遗传

Genes located on the sex chromosomes (X in mammals, Z in birds) show distinctive inheritance patterns. In humans, X‑linked recessive traits, such as red‑green colour blindness and haemophilia, appear more frequently in males. This is because males are hemizygous – they possess only one X chromosome and thus a single recessive allele will be expressed.

位于性染色体上的基因（哺乳动物为X染色体，鸟类为Z染色体）表现出独特的遗传模式。在人类中，X连锁隐性性状，如红绿色盲和血友病，在男性中更为常见。这是因为男性是半合子——他们只有一条X染色体，因此单个隐性等位基因就会表达。

For an X‑linked recessive condition:

Affected father (XʰY) and unaffected mother (XᴴXᴴ) → all daughters are carriers (XᴴXʰ), all sons are unaffected (XᴴY). / 患病父亲(XʰY)与正常母亲(XᴴXᴴ) → 所有女儿为携带者(XᴴXʰ)，所有儿子正常(XᴴY)。
Carrier mother (XᴴXʰ) and unaffected father (XᴴY) → ½ sons affected, ½ daughters carriers (no daughters affected). / 携带者母亲(XᴴXʰ)与正常父亲(XᴴY) → ½儿子患病，½女儿为携带者（无女儿患病）。

Pedigree symbols and patterns often indicate sex‑linkage: predominantly affected males, no father‑to‑son transmission. IB questions frequently require you to assign genotypes across three generations.

系谱符号和模式常提示伴性遗传：患者主要为男性，无双亲父传子的现象。IB题目经常要求你为三代人分配基因型。

9. Pedigree Analysis and Problem Solving | 系谱分析与解题策略

Pedigree charts are a staple of genetics exams. You must learn to recognise autosomal dominant, autosomal recessive, X‑linked recessive, and X‑linked dominant patterns. Key clues:

系谱图是遗传学考试的基本内容。你必须学会识别常染色体显性、常染色体隐性、X连锁隐性和X连锁显性模式。关键线索：

Autosomal recessive: affected individuals can appear in offspring of unaffected parents; often skips generations. / 常染色体隐性：患者的父母可能正常；常隔代出现。
Autosomal dominant: every affected individual has at least one affected parent; does not skip generations. / 常染色体显性：每个患者至少有一个患病亲本；不隔代。
X‑linked recessive: more males affected; affected males cannot pass the trait to sons. / X连锁隐性：男性患者更多；患病父亲不传给儿子。

Once the pattern is identified, assign symbols (e.g., A/a, Xᴴ/Xʰ) and calculate probabilities. When answering, always link your reasoning to the specific individuals in the pedigree – examiners look for evidence of logical deduction, not just a final number.

一旦确定遗传模式，指定符号（如A/a、Xᴴ/Xʰ）并计算概率。回答时，始终将推理与系谱中的具体个体联系起来——考官期望看到逻辑推导的证据，而不仅仅是最终数字。

10. Common Misconceptions and Exam Tips | 常见误区与考试技巧

Many students mistakenly believe that dominant alleles are “stronger” or more common in the population. In reality, dominance refers to the effect on phenotype in a heterozygote, not frequency. Polydactyly (extra fingers) is caused by a dominant allele, yet it is rare in the population.

许多学生错误地认为显性等位基因“更强”或在人群中更常见。实际上，显性指的是在杂合子中对表型的影响，而非频率。多指症（多指）由显性等位基因引起，却在人群中罕见。

Another common error is confusing the 9:3:3:1 dihybrid ratio with linked genes. If the observed ratio deviates significantly, genes may be linked (located close together on the same chromosome). The WJEC specification introduces the χ² test to assess such deviations; IB often incorporates this into data‑analysis questions.

另一个常见错误是将9:3:3:1的双因子比例与连锁基因混淆。如果观察到的比例显著偏离，基因可能连锁（位于同一条染色体上且距离很近）。WJEC大纲引入卡方检验来评估这种偏离；IB常将其纳入数据分析题。

Finally, always show your working in Punnett squares or probability calculations. In multi‑mark questions, the process carries as much weight as the answer. Use consistent notation – define alleles at the start (e.g., B = brown eyes, b = blue eyes) – and double‑check gamete combinations.

最后，在庞纳特方格或概率计算中始终展示过程。在多分值的题目中，解题步骤与答案同样重要。使用一致的符号——开头定义等位基因（如B=棕色眼，b=蓝色眼）——并仔细核对配子组合。

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Mendelian Genetics for IB and WJEC Biology | 孟德尔遗传考点精讲 (IB/WJEC)