📚 GCSE Edexcel Biology: Sex-linked Inheritance | GCSE Edexcel 生物：伴性遗传 考点精讲
Sex-linked inheritance is a fascinating topic in genetics that explains why certain traits, like red-green colour blindness and haemophilia, are more common in males than females. This article will break down the core concepts, genetic diagrams, and common exam pitfalls so you can confidently tackle any question on sex-linked characteristics in your Edexcel GCSE Biology exam.
伴性遗传是遗传学中一个引人入胜的话题，它解释了为什么某些性状（如红绿色盲和血友病）在男性中比女性中更常见。本文将拆解核心概念、遗传图解和常见考试陷阱，使你在 Edexcel GCSE 生物考试中自信应对任何关于伴性性状的问题。
1. Chromosomes and Sex Determination | 染色体与性别决定
Human body cells contain 23 pairs of chromosomes. Twenty-two of these pairs are autosomes, which control most body characteristics. The 23rd pair determines sex. This pair consists of two sex chromosomes, known as X and Y. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
人体体细胞含有 23 对染色体。其中 22 对是常染色体，控制大多数身体特征。第 23 对决定性别。这一对由两条性染色体组成，称为 X 和 Y。女性有两条 X 染色体 (XX)，而男性有一条 X 和一条 Y 染色体 (XY)。
During fertilisation, the mother always contributes an X chromosome (since her eggs contain an X). The father's sperm can carry either an X or a Y chromosome. Therefore, the father determines the sex of the offspring: an X sperm produces a female (XX), and a Y sperm produces a male (XY).
受精时，母亲总是提供一条 X 染色体（因为她的卵细胞含有 X）。父亲的精子可以携带 X 或 Y 染色体。因此，父亲决定了后代的性别：X 精子产生女性 (XX)，Y 精子产生男性 (XY)。
2. What Are Sex-linked Genes? | 什么是伴性基因？
Sex-linked genes are those located on the sex chromosomes, and in GCSE Biology, we focus almost exclusively on genes found on the X chromosome. The Y chromosome is much smaller and carries very few genes, mainly those for male sex determination. Because males have only one X chromosome, any recessive allele on the X will be expressed in the male phenotype, even if it is recessive. This is because there is no homologous allele on the Y chromosome to mask it.
伴性基因是位于性染色体上的基因，在 GCSE 生物中，我们几乎只关注位于 X 染色体上的基因。Y 染色体小得多，携带的基因很少，主要是决定男性性别的基因。由于男性只有一条 X 染色体，X 染色体上的任何隐性等位基因都会在男性表型中表达，即使是隐性的。这是因为 Y 染色体上没有同源等位基因可将其掩盖。
Females, having two X chromosomes, will only express a recessive X-linked trait if they are homozygous recessive (have the recessive allele on both X chromosomes). If they are heterozygous, they are carriers – they possess the allele but do not show the trait. This is why X-linked recessive disorders are much more common in males.
女性因为有两条 X 染色体，只有在纯合隐性（两条 X 染色体上都有隐性等位基因）时才会表现隐性 X 连锁性状。如果她是杂合子，就是携带者——她具有该等位基因但不表现出性状。这就是 X 连锁隐性遗传病在男性中更为常见的原因。
3. Red-Green Colour Blindness as an Example | 以红绿色盲为例
Red-green colour blindness is a classic example of an X-linked recessive disorder. The gene controlling red-green colour vision is located on the X chromosome. The normal allele (let's represent it as XN) is dominant over the colour-blind allele (Xn). Since males have only one X chromosome, a male with XnY will be colour blind. A male with XNY will have normal vision.
红绿色盲是 X 连锁隐性遗传病的典型例子。控制红绿色视觉的基因位于 X 染色体上。正常等位基因（我们记为 XN）对色盲等位基因 (Xn) 为显性。由于男性只有一条 X 染色体，具有 XnY 的男性将是色盲。具有 XNY 的男性视力正常。
Females can have three possible genotypes: XNXN (normal, non-carrier), XNXn (normal vision but carrier), or XnXn (colour blind). Colour blind females are rare because they must inherit the recessive allele from both an affected father and a carrier (or affected) mother.
女性可能有三种基因型：XNXN（正常，非携带者）、XNXn（正常视力但为携带者）或 XnXn（色盲）。色盲女性很罕见，因为她们必须从患病的父亲和携带者（或患病）母亲那里各继承一个隐性等位基因。
4. Haemophilia – Another Key Example | 血友病——另一个重要例子
Haemophilia is a genetic disorder where blood does not clot properly due to a lack of clotting factor VIII or IX. It is also an X-linked recessive condition, governed by genes on the X chromosome. The inheritance pattern is identical to that of colour blindness. Males with the defective allele (XhY) will suffer from haemophilia, while females must be homozygous recessive (XhXh) to show the condition, making it extremely rare in females.
血友病是一种遗传病，由于缺乏凝血因子 VIII 或 IX，血液无法正常凝固。它也是一种 X 连锁隐性遗传病，由 X 染色体上的基因控制。其遗传模式与色盲完全相同。具有缺陷等位基因 (XhY) 的男性将患血友病，而女性必须纯合隐性 (XhXh) 才会表现出病症，因此血友病在女性中极为罕见。
Royal pedigree analysis often highlights haemophilia as it affected several members of European royal families, descended from Queen Victoria, who was a carrier. Pedigree charts are common exam questions.
王室家谱分析经常强调血友病，因为它影响了欧洲多个皇室成员，他们是维多利亚女王（携带者）的后裔。家谱图是常见的考试题型。
5. Genetic Diagrams for Sex-linked Inheritance | 伴性遗传的遗传图解
When drawing a genetic cross for sex-linked traits, you must include the sex chromosomes in the genotype. A Punnett square is used, showing the mother's gametes (each containing an X chromosome with either the normal or recessive allele) and the father's gametes (sperm containing X or Y chromosome, with the X possibly carrying the allele).
绘制伴性性状的遗传杂交图时，基因型中必须包含性染色体。使用庞纳特方格，展示母亲的配子（每个含有一条 X 染色体，带有正常或隐性等位基因）和父亲的配子（含 X 或 Y 染色体的精子，其中 X 染色体可能携带该等位基因）。
For example, cross a carrier female (XNXn) with a normal male (XNY). The gametes are: mother XN and Xn; father XN and Y. The Punnett square shows offspring: XNXN (normal female), XNXn (carrier female), XNY (normal male), XnY (colour-blind male). The phenotype ratio is 1 normal female : 1 carrier female : 1 normal male : 1 affected male. Note that carrier females are phenotypically normal.
例如，让携带者女性 (XNXn) 与正常男性 (XNY) 杂交。配子为：母亲 XN 和 Xn；父亲 XN 和 Y。庞纳特方格显示后代：XNXN（正常女性）、XNXn（携带者女性）、XNY（正常男性）、XnY（色盲男性）。表型比例为 1 正常女性 : 1 携带者女性 : 1 正常男性 : 1 患病男性。注意携带者女性表型正常。
6. Why Males Are More Commonly Affected | 为什么男性更常受影响
The fundamental reason is hemizygosity in males. Males are said to be hemizygous for X-linked genes because they possess only one copy. A single recessive allele on the X chromosome will cause the trait to appear, as there is no corresponding allele on the Y chromosome to provide a dominant normal version. In contrast, females have two copies, so a recessive allele can be masked by a dominant normal allele on the other X chromosome.
根本原因在于男性的半合子状态。男性因只有一份拷贝而被称为 X 连锁基因的半合子。X 染色体上的单个隐性等位基因就会导致性状出现，因为 Y 染色体上没有对应的等位基因提供显性的正常版本。相比之下，女性有两份拷贝，因此隐性等位基因可以被另一条 X 染色体上的显性正常等位基因所掩盖。
This leads to the typical pattern: an affected female must have an affected father and a mother who is at least a carrier. An affected male can arise from a carrier mother and an unaffected father. Thus, more males than females exhibit X-linked recessive conditions.
这导致了典型的模式：患病的女性必须有一个患病的父亲和一个至少是携带者的母亲。患病男性则可由携带者母亲和正常父亲所生。因此，表现出 X 连锁隐性遗传病的男性比女性多。
7. Pedigree Analysis for Sex-linked Traits | 伴性性状的家谱分析
In pedigree charts, squares represent males and circles represent females. Shaded symbols indicate affected individuals. A key clue for X-linked recessive inheritance is that mostly males are affected, and affected females must have an affected father. Often, a carrier female passes the trait to half her sons. There is no male-to-male transmission because a father passes his Y chromosome to his sons, not his X chromosome.
在家谱图中，方块代表男性，圆圈代表女性。阴影符号表示患病个体。X 连锁隐性遗传的一个关键线索是：患者主要为男性，且患病女性必须有一个患病的父亲。通常，携带者女性将性状传给一半的儿子。不存在男传男现象，因为父亲将 Y 染色体而不是 X 染色体传给儿子。
Exam questions may ask you to deduce genotypes from a pedigree. You must be able to justify whether a female is homozygous normal or heterozygous based on her parents and offspring. If a normal-vision mother has a colour-blind son, she must be a carrier (XNXn), because she passed the Xn to her son.
考试问题可能要求你根据家谱推断基因型。你必须能够根据父母和后代的情况，论证一位女性是纯合正常还是杂合子。如果一位视力正常的母亲有一个色盲儿子，那么她一定是携带者 (XNXn)，因为她把 Xn 传给了儿子。
8. Co-dominance and Sex Linkage – Separate Concepts | 共显性与伴性遗传——分开的概念
Students often confuse co-dominance with sex linkage. Co-dominance occurs when both alleles in a heterozygote contribute to the phenotype, like in ABO blood groups. Sex linkage refers to the location of a gene on a sex chromosome. These are independent concepts. However, a gene on the X chromosome could theoretically show co-dominance, but at GCSE level, X-linked traits are typically recessive.
学生经常将共显性与伴性遗传混淆。共显性是指杂合子中两个等位基因都对表型有贡献，例如 ABO 血型。伴性遗传是指基因位于性染色体上。这些是独立的概念。然而，理论上 X 染色体上的基因可能表现共显性，但在 GCSE 水平，X 连锁性状通常是隐性的。
Always read the question carefully: if a disease is said to be caused by a recessive allele on the X chromosome, treat it as sex-linked recessive, not co-dominance. If both alleles are expressed equally in a heterozygote, then it is co-dominance, regardless of which chromosome it is on.
仔细阅读题目：如果一种疾病被描述为由 X 染色体上的隐性等位基因引起，则按 X 连锁隐性处理，而不是共显性。如果杂合子中两个等位基因同等表达，则无论位于哪条染色体上，都是共显性。
9. Common Types of Exam Questions | 常见的考试题型
Typical questions include completing a Punnett square for a sex-linked cross, interpreting the results, and giving the probability of an affected child. You may be asked to calculate the percentage chance of a son being colour blind, or the probability that a daughter is a carrier. Remember to specify sex when probabilities differ between male and female offspring.
典型问题包括完成伴性杂交的庞纳特方格、解释结果以及给出患病孩子的概率。你可能需要计算儿子是色盲的概率百分比，或女儿是携带者的概率。当雄性和雌性后代的概率不同时，记得指明性别。
rier female × normal male cross, what is the probability that a child will be colour blind? Overall, 1 in 4 offspring are affected (the XnY male). But if asked 'What is the probability that a son will be colour blind?', you must look only at the male offspring: 1 out of 2 sons (50%) will be affected, as the other son is normal.
例如，携带者女性 × 正常男性杂交，孩子是色盲的概率是多少？总体而言，4 个后代中有 1 个患病 (XnY 男性)。但如果问“儿子是色盲的概率是多少？”，你必须只看男性后代：2 个儿子中有 1 个 (50%) 会患病，另一个儿子正常。
10. Key Terminology to Use Precisely | 需准确使用的关键术语
- Allele: A version of a gene. / 等位基因：基因的一种形式。
- Dominant: An allele that is always expressed if present. / 显性：一旦存在就会表达的等位基因。
- Recessive: An allele only expressed if two copies are present (or one on X in males). / 隐性：只有在存在两个拷贝时（或男性 X 上有一个时）才表达的等位基因。
- Homozygous: Having two identical alleles of a gene. / 纯合子：一个基因有两个相同的等位基因。
- Heterozygous: Having two different alleles of a gene. / 杂合子：一个基因有两个不同的等位基因。
- Carrier: A heterozygous individual who does not show a recessive disorder but can pass the allele to offspring. / 携带者：不表现隐性遗传病但可将等位基因传给后代的杂合个体。
- Hemizygous: Having only one copy of a gene (e.g., X-linked genes in males). / 半合子：只有一个基因拷贝（例如男性中的 X 连锁基因）。
Examiners expect you to use these terms correctly and apply them to sex-linked scenarios. Avoid vague language like 'the gene is carried on the X' without specifying 'X chromosome'.
考官期望你正确使用这些术语并将其应用于伴性遗传情境。避免模糊的语言，比如只是说“基因携带在 X 上”而不指明“X 染色体”。
11. Tackling Extended Response Questions | 应对拓展回答题
An extended answer might ask you to explain why a particular disorder is more common in males. Plan your response: 1) State that the gene is located on the X chromosome. 2) Mention that males have only one X chromosome (XY). 3) Explain that a single recessive allele on the X causes the disorder because there is no corresponding allele on the Y to mask it. 4) Contrast with females who need two recessive alleles to be affected, so they are often carriers instead. Use a Punnett square as evidence if helpful.
拓展回答可能要求你解释为什么某种疾病在男性中更常见。规划你的回答：1) 指出该基因位于 X 染色体上。2) 提到男性只有一条 X 染色体 (XY)。3) 解释 X 染色体上的单个隐性等位基因就能导致发病，因为 Y 染色体上没有对应的等位基因来掩盖它。4) 与女性对比，女性需要两个隐性等位基因才会患病，因此她们往往是携带者而不是患者。如果对回答问题有帮助，可以使用庞纳特方格作为证据。
Marks are allocated for logical structure and correct use of genetic terminology. Always distinguish between 'affected' and 'carrier', and between 'offspring', 'sons', and 'daughters'.
分数根据逻辑结构和正确使用遗传术语来分配。务必区分“患者”与“携带者”，以及“后代”“儿子”和“女儿”。
12. Exam Tips and Common Mistakes | 考试技巧与常见错误
Common mistakes include forgetting to put the sex chromosomes in the genotype (e.g., writing 'Nn' instead of XNXn), confusing carriers with affected individuals, and failing to consider that a father cannot pass an X-linked trait to his son. Also, students sometimes misread the word 'child' and calculate probability for all offspring, whereas the question asks specifically for the chance that a son is affected.
常见错误包括基因型中遗漏性染色体（例如写 'Nn' 而不是 XNXn）、混淆携带者与患者，以及没有考虑到父亲不能将 X 连锁性状传给儿子。此外，学生有时会误读“孩子”一词，针对所有后代计算概率，而题目具体要求的是儿子患病的概率。
Always double-check your Punnett squares by ensuring gametes are correctly formed. Label each offspring with its genotype and phenotype. In 6-mark quality of written communication questions, give a clear, step-by-step explanation.
始终通过确保配子正确形成来再次检查你的庞纳特方格。标记每个后代的基因型和表型。在 6 分的书面表达质量题中，给出清晰、逐步的解释。
Sex-linked inheritance is a high-yield topic in Edexcel GCSE Biology. Mastering these concepts, genetic diagrams, and precise language will ensure you achieve top marks.
伴性遗传是 Edexcel GCSE 生物中的高频考点。掌握这些概念、遗传图解和精确的语言将确保你获得高分。
Published by TutorHao | Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图