📚 IGCSE Edexcel Biology: Sex-linked Inheritance | IGCSE Edexcel 生物：伴性遗传考点精讲
Welcome to TutorHao’s revision guide on sex-linked inheritance for IGCSE Edexcel Biology. This article breaks down the core concepts you need to master: how genes located on the sex chromosomes are inherited, why males are more frequently affected by certain recessive disorders, and how to solve genetic cross problems confidently. We will use clear diagrams, Punnett squares, and worked examples to ensure you are fully prepared for the exam.
欢迎来到 TutorHao 的 IGCSE Edexcel 生物伴性遗传复习指南。本文拆解你需要掌握的核心概念：位于性染色体上的基因如何遗传、为什么男性更易患上某些隐性遗传病，以及如何自信地解答遗传杂交题。我们将使用清晰的图解、庞纳特方格和实例讲解，确保你为考试做好充分准备。
1. Chromosomes, Sex Determination and the 23rd Pair | 染色体、性别决定与第 23 对染色体
In humans, each body cell contains 23 pairs of chromosomes. The first 22 pairs are called autosomes and are identical in both males and females. The 23rd pair are the sex chromosomes, which determine the sex of the individual. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). It is the father’s sperm that determines the sex of the child because he can contribute either an X or a Y chromosome.
人类每个体细胞含有 23 对染色体。前 22 对称为常染色体，在男女中相同。第 23 对是性染色体，决定个体的性别。女性拥有两条 X 染色体（XX），而男性拥有一条 X 和一条 Y 染色体（XY）。正是父亲的精子决定了孩子的性别，因为他可以提供 X 或 Y 染色体。
During gamete formation, meiosis ensures that each egg cell carries one X chromosome. Sperm cells, however, can carry either an X or a Y chromosome. A Punnett square for sex determination always shows a 50% chance of having a male child and a 50% chance of having a female child.
在配子形成过程中，减数分裂确保每个卵细胞携带一条 X 染色体。然而，精子细胞可以携带 X 或 Y 染色体。性别决定的庞纳特方格总是显示生男孩的概率为 50%，生女孩的概率也为 50%。
2. What is Sex-linked Inheritance? | 什么是伴性遗传？
Sex-linked inheritance refers to the pattern of inheritance for genes located on the sex chromosomes. In IGCSE Edexcel, we focus on genes found on the X chromosome. Because the Y chromosome is much smaller and carries fewer genes, many alleles on the X chromosome have no corresponding allele on the Y. This means that males, having only one X chromosome, will express the trait determined by any allele on their single X, even if it is recessive. Females, with two X chromosomes, can be carriers of a recessive allele without showing the trait.
伴性遗传指的是位于性染色体上的基因的遗传模式。在 IGCSE Edexcel 大纲中，我们重点学习位于 X 染色体上的基因。由于 Y 染色体小得多且携带的基因较少，X 染色体上的许多等位基因在 Y 染色体上没有对应的等位基因。这意味着，只有一条 X 染色体的男性，即使其上的等位基因是隐性的，也会表现该性状。而拥有两条 X 染色体的女性可以携带隐性等位基因而不表现该性状。
Thus, recessive sex-linked disorders are much more common in males than in females. A female would need two copies of the recessive allele (one on each X chromosome) to exhibit the disorder, which is far rarer.
因此，隐性伴性遗传病在男性中比在女性中常见得多。女性需要两个隐性等位基因拷贝（每条 X 染色体上一个）才会患病，这要罕见得多。
3. Key Genetic Terminology | 关键遗传学术语
Before tackling problems, make sure you are comfortable with these terms:
- Allele – a version of a gene.
- Dominant – an allele that is always expressed if present.
- Recessive – an allele only expressed if the dominant allele is absent (or if only one X is present in males).
- Genotype – the combination of alleles an organism has.
- Phenotype – the observable characteristic.
- Homozygous – having two identical alleles of a gene.
- Heterozygous – having two different alleles of a gene.
- Carrier – in sex-linked contexts, a female who has one recessive allele on one X chromosome but does not show the disorder.
在解决问题之前，请确保你熟悉以下术语：
- 等位基因 – 基因的一种形式。
- 显性 – 如果存在，总会被表达的等位基因。
- 隐性 – 只有在显性等位基因不存在时才表达的等位基因（或男性只有一条 X 染色体时）。
- 基因型 – 生物体所拥有的等位基因组合。
- 表型 – 可观察到的特征。
- 纯合 – 具有某个基因的两个相同等位基因。
- 杂合 – 具有某个基因的两个不同等位基因。
- 携带者 – 在伴性遗传中，指一条 X 染色体上带有一个隐性等位基因但不表现该疾病的女性。
4. Standard Notation for Sex-linked Genes | 伴性基因的标准表示法
IGCSE Edexcel expects you to use clear symbols to show sex-linked alleles. The chromosomes are written with the allele as a superscript. For example, consider the allele for colour blindness. Let the normal vision allele be represented by N and the colour blindness allele by n. A female with normal vision could be written as XNXN or XNXn (carrier). A female with colour blindness would be XnXn. A male with normal vision is XNY, and a colour-blind male is XnY. Notice that the Y chromosome does not carry an allele for these X-linked traits.
IGCSE Edexcel 要求你使用清晰的符号来表示伴性等位基因。染色体用上标等位基因表示。例如，考虑色盲的等位基因。设正常视觉等位基因为N，色盲等位基因为n。正常视觉的女性可写作XNXN或XNXn（携带者）。色盲女性的基因型则为XnXn。正常视觉的男性为XNY，色盲男性为XnY。请注意，Y 染色体不携带这些 X 连锁性状的等位基因。
This notation is vital for constructing correct Punnett squares and calculating the probability of offspring being affected.
这种表示法对于构建正确的庞纳特方格和计算后代患病的概率至关重要。
5. Red-Green Colour Blindness: A Classic Example | 红绿色盲：经典实例
Red-green colour blindness is a common X-linked recessive disorder. Affected individuals cannot distinguish between red and green hues. The allele for normal colour vision (N) is dominant over the allele for colour blindness (n). A mother who is a carrier (XNXn) and a father with normal vision (XNY) can have children with the following possible genotypes:
红绿色盲是一种常见的 X 连锁隐性遗传病。患者无法区分红色和绿色调。正常色觉的等位基因（N）对色盲等位基因（n）为显性。一位携带者母亲（XNXn）和一位正常视觉父亲（XNY）所生的孩子可能的基因型如下：
| Egg \ Sperm | XN | Y |
|---|---|---|
| XN | XNXN (normal female) | XNY (normal male) |
| Xn | XNXn (carrier female) | XnY (colour-blind male) |
From this cross, there is a 25% chance of having a colour-blind son, a 25% chance of a normal son, a 25% chance of a carrier daughter, and a 25% chance of a normal homozygous daughter. No female child will be colour-blind in this cross.
从该杂交来看，有 25% 的概率生下一个色盲儿子，25% 的概率生下一个正常儿子，25% 的概率生下一个携带者女儿，以及 25% 的概率生下一个正常纯合女儿。在此杂交中，没有任何女儿会是色盲。
6. Haemophilia: Another X-linked Recessive Disorder | 血友病：另一种 X 连锁隐性遗传病
Haemophilia is a well-known blood clotting disorder caused by a recessive allele on the X chromosome. Let the normal clotting allele be H and the haemophilia allele be h. A carrier female (XHXh) and a haemophiliac male (XhY) can produce offspring with the following genotypes:
血友病是一种众所周知的凝血障碍，由 X 染色体上的隐性等位基因引起。设正常凝血等位基因为 H，血友病等位基因为 h。一位携带者女性（XHXh）和一位血友病男性（XhY）可以产生以下基因型的后代：
| Xh | Y | |
|---|---|---|
| XH | XHXh (carrier female) | XHY (normal male) |
| Xh | XhXh (haemophiliac female) | XhY (haemophiliac male) |
Notice that in this cross, a daughter can be haemophiliac if she inherits the recessive allele from both parents. This illustrates that while sex-linked recessive disorders are rarer in females, they can occur when an affected male and a carrier female have children.
请注意，在此杂交中，如果女儿从父母双方都遗传了隐性等位基因，则她可能成为血友病患者。这说明虽然隐性伴性遗传病在女性中较为罕见，但当事关一位患病男性和一位携带者女性时，他们的孩子中可能出现女性患者。
7. Why Are Males More Likely to Be Affected? | 为什么男性更容易患病？
Males have only one X chromosome, inherited from their mother. The Y chromosome does not carry a matching allele for most X-linked genes. Therefore, if a male inherits a recessive disease-causing allele on his X chromosome, there is no dominant normal allele on the Y to mask its effect. The phenotype is directly expressed. In contrast, females have two X chromosomes and would need to inherit two recessive alleles (one from each parent) to show the trait. If they inherit only one, they become carriers but remain phenotypically normal.
男性只有一条 X 染色体，遗传自母亲。Y 染色体对大多数 X 连锁基因不携带匹配的等位基因。因此，如果男性从他的 X 染色体上继承了一个隐性致病等位基因，Y 染色体上没有显性的正常等位基因来掩盖其影响。表型即直接表达。相反，女性有两条 X 染色体，需要继承两个隐性等位基因（父母各一）才会表现该性状。如果只继承一个，她们会成为携带者但仍表现正常。
This is why in a typical family tree, you will see more affected males than females, and affected males are often born to carrier mothers and normal fathers.
这就是为什么在典型的家系图中，你会看到男性患者多于女性患者，并且患病的男性通常是由携带者母亲和正常父亲所生。
8. Interpreting Pedigree Charts for Sex-linked Traits | 解读伴性性状的家系图
Exam questions often provide a pedigree chart and ask you to work out genotypes. For a recessive X-linked trait:
- An affected male mates with a normal non-carrier female — all daughters will be carriers (they get his affected X), and all sons will be normal (they get his Y and a normal X from their mother).
- A carrier female and a normal male — half of their sons may be affected, half of the daughters may be carriers.
- An affected female is rare and must have an affected father and a mother who is at least a carrier.
考试题目常提供家系图并要求你推断基因型。对于隐性 X 连锁性状：
- 患病男性与非携带者正常女性婚配——所有女儿将成为携带者（她们得到他患病的 X），所有儿子正常（他们得到他的 Y 以及母亲的一条正常 X）。
- 携带者女性与正常男性——他们的儿子中有一半可能患病，女儿中有一半可能是携带者。
- 患病女性罕见，她的父亲必定是患者，母亲至少是携带者。
Recognising that the trait skips generations through carrier females and predominantly appears in males is a strong clue of X-linked recessive inheritance.
识别该性状通过女性携带者代代相传，并主要在男性中出现，是 X 连锁隐性遗传的强烈提示。
9. Solving Genetic Cross Problems Step-by-Step | 逐步解答遗传杂交题
Follow these steps to answer any IGCSE sex-linked cross question:
Step 1: Determine the parents’ phenotypes and assign alleles. Choose a letter and use superscript notation.
Step 2: Write down the parental genotypes using the X and Y chromosome format.
Step 3: Identify the possible gametes each parent can produce.
Step 4: Draw a Punnett square, putting the female gametes across the top and male gametes down the side.
Step 5: Fill in the offspring genotypes and determine their phenotypes.
Step 6: State the phenotypic ratio or probability as requested by the question.
按照以下步骤解答任何 IGCSE 伴性杂交题：
步骤 1：确定亲代表型并分配等位基因。选择一个字母并使用上标表示法。
步骤 2：使用 X 和 Y 染色体格式写出亲代基因型。
步骤 3：确定每个亲本可能产生的配子。
步骤 4：绘制庞纳特方格，将雌性配子置于顶行，雄性配子置于侧列。
步骤 5：填写后代基因型并确定其表型。
步骤 6：按照题目要求陈述表型比例或概率。
Always include the Punnett square in your answer and label the phenotypes clearly.
答案中始终要包含庞纳特方格，并清楚地标注表型。
10. Common Exam Mistakes and How to Avoid Them | 常见考试错误及对策
Mistake 1: Using the wrong notation, such as writing alleles on the Y chromosome for X-linked traits. The Y chromosome is largely genetically inert for these genes; do not place a colour blindness allele on the Y.
错误 1：使用错误的表示法，例如将等位基因写在 Y 染色体上用于 X 连锁性状。Y 染色体对这些基因来说基本上是没有遗传活性的；不要把色盲等位基因放在 Y 染色体上。
Mistake 2: Forgetting that males always inherit their X chromosome from their mother. In a cross, a son’s X always comes from his mother, and his Y from his father. This explains why an affected male cannot pass the trait to his sons (he gives them his Y).
错误 2：忘记男性总是从母亲那里遗传 X 染色体。在杂交中，儿子的 X 总是来自母亲，Y 来自父亲。这解释了为什么患病男性不能将该性状传递给他的儿子（他给了儿子 Y 染色体）。
Mistake 3: Confusing carrier and affected females. Only a female with two recessive alleles (e.g., XnXn) is affected; XNXn is a carrier and has normal vision. Stating that a carrier female is colour-blind will lose marks.
错误 3：混淆携带者女性和患病女性。只有拥有两个隐性等位基因的女性（如 XnXn）才是患者；XNXn 是携带者，视觉正常。说携带者女性是色盲会失分。
11. Practice Question with Model Answer | 练习题与示例答案
Question: Haemophilia is an X-linked recessive condition. A woman who is a carrier for haemophilia marries a man who does not have haemophilia. Determine the probability that their child will be a son with haemophilia. Use the alleles H and h.
问题：血友病是一种 X 连锁隐性遗传病。一位血友病携带者女性与一位非血友病男性结婚。计算他们的孩子是患血友病儿子的概率。使用等位基因 H 和 h。
Model answer:
Parental genotypes: woman = XHXh, man = XHY.
Gametes: woman produces XH and Xh; man produces XH and Y.
示例答案：
亲代基因型：女性 = XHXh，男性 = XHY。
配子：女性产生 XH 和 Xh；男性产生 XH 和 Y。
| XH | Y | |
|---|---|---|
| XH | XHXH (normal female) | XHY (normal male) |
| Xh | XHXh (carrier female) | XhY (haemophiliac male) |
According to the Punnett square, there are four equally likely outcomes. Only one of the four is a son with haemophilia (XhY). Therefore, the probability is 1/4 or 25%.
根据庞纳特方格，有四种等可能的结果。其中只有一个是患血友病的儿子（XhY）。因此，概率为 1/4 或 25%。
12. Quick Comparison: Autosomal vs. Sex-linked Inheritance | 快速比较：常染色体遗传与伴性遗传
| Feature | Autosomal Recessive | X-linked Recessive |
|---|---|---|
| Affected males and females | Equal frequency | Males much more commonly affected |
| Carrier state | Both males and females can be carriers | Only females can be carriers |
| Inheritance pattern | Can skip generations regardless of sex | Often passes from carrier mother to affected son |
| Affected father | Can pass allele to sons and daughters | Will pass affected X to all daughters (carriers), but not to sons |
Understanding these differences is crucial for exam questions that ask you to identify the type of inheritance from a pedigree.
理解这些差异对于考试中要求你通过家系图识别遗传类型的问题至关重要。
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