Sex-linked Inheritance: IGCSE Biology Key Points | IGCSE 生物：伴性遗传考点精讲

📚 Sex-linked Inheritance: IGCSE Biology Key Points | IGCSE 生物：伴性遗传考点精讲

Sex-linked inheritance is a fascinating area of genetics where the pattern of trait transmission depends on the sex chromosomes. In IGCSE Biology, you must be able to explain how genes located on the X or Y chromosome lead to characteristic inheritance patterns, especially for conditions like red-green colour blindness and haemophilia. This article will help you master the key concepts, solve genetic cross problems, and analyse pedigree charts with confidence.

伴性遗传是遗传学中一个引人入胜的领域，其性状的传递模式取决于性染色体。在 IGCSE 生物学中，你需要能够解释位于 X 或 Y 染色体上的基因如何导致特征性的遗传模式，特别是红绿色盲和血友病等疾病。本文将帮助你掌握核心概念，解决遗传杂交问题，并自信地分析系谱图。

1. Sex Determination and Chromosomes | 性别决定与染色体

In humans, sex is determined by a pair of sex chromosomes, named X and Y. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). During fertilisation, the mother’s egg always carries an X chromosome, whereas the father’s sperm can carry either an X or a Y. Therefore, it is the father who determines the sex of the offspring: an X-bearing sperm produces a female, and a Y-bearing sperm produces a male.

人类性别由一对性染色体决定，即 X 和 Y 染色体。女性有两条 X 染色体（XX），男性有一条 X 和一条 Y 染色体（XY）。受精时，母亲的卵子总是携带一条 X 染色体，而父亲的精子可能携带 X 或 Y 染色体。因此，是父亲决定了后代的性别：携带 X 的精子产生女性，携带 Y 的精子产生男性。

The X chromosome is large and contains many genes unrelated to sex determination. The Y chromosome is much smaller and carries few genes, most notably the SRY gene that triggers male development. This difference in gene content is central to understanding sex-linked inheritance.

X 染色体较大，含有许多与性别决定无关的基因。Y 染色体小得多，携带的基因很少，其中最重要的是触发男性发育的 SRY 基因。这种基因含量的差异是理解伴性遗传的关键。

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. Because the X and Y chromosomes have different sets of genes, traits controlled by genes on the X chromosome do not follow simple Mendelian ratios when the male sex is involved. Most commonly, we study X-linked genes where the Y chromosome lacks a corresponding allele. This means males only have one copy of these genes (they are hemizygous), so a single recessive allele on their single X chromosome will be fully expressed.

伴性遗传是指位于性染色体上的基因的遗传模式。由于 X 和 Y 染色体上携带的基因不同，当涉及男性时，X 染色体上基因控制的性状并不遵循简单的孟德尔比例。最常见的例子是研究位于 X 染色体上的基因，而 Y 染色体上没有对应的等位基因。这意味着男性对这些基因只有一个拷贝（呈半合子状态），因此单独的一条 X 染色体上出现一个隐性等位基因就会充分表达相应性状。

In contrast, females have two X chromosomes, so they can be homozygous dominant, homozygous recessive, or heterozygous. A heterozygous female for an X-linked recessive allele does not show the trait but is a carrier. The special dynamics of X-linked recessive alleles make these traits much more common in males.

相反，女性有两条 X 染色体，因此她们可能是纯合显性、纯合隐性或杂合。对于伴 X 隐性等位基因呈杂合的女性不会表现出性状，但她是携带者。伴 X 隐性等位基因的这种特殊规律使这些性状在男性中更为常见。

3. X-linked Recessive Inheritance: Key Features | 伴 X 隐性遗传的主要特征

When a recessive allele causing a genetic disorder is located on the X chromosome, the following features are typical in a pedigree:

当导致遗传疾病的隐性等位基因位于 X 染色体上时，系谱图中通常表现出以下特征：

Males are much more frequently affected than females. This is because a male needs only one copy of the recessive allele (from his mother) to express the trait, as his Y chromosome does not carry a matching allele.

男性发病率远高于女性。这是因为男性只需一个隐性等位基因（从母亲处获得）就能表达性状，因为他的 Y 染色体上不带对应的等位基因。
Affected males cannot pass the trait to their sons, because they give their Y chromosome to sons, not their X. However, all their daughters will receive the affected X chromosome and become carriers.

患病男性不会将性状传给儿子，因为他们传给儿子的是 Y 染色体而非 X。然而，他们所有的女儿都会得到这条带病基因的 X 染色体，并成为携带者。
Carrier females (heterozygous) are typically unaffected, but they can pass the recessive allele to their children. There is a 50% chance that a carrier mother will pass the allele to each child; sons who inherit it will be affected, and daughters who inherit it will be carriers.

携带者女性（杂合子）通常不受影响，但可以将隐性等位基因传给子女。携带者母亲有 50% 的概率将致病等位基因传给每个孩子；继承该基因的儿子会患病，继承该基因的女儿将成为携带者。

4. Example: Red-Green Colour Blindness | 实例：红绿色盲

Red-green colour blindness is a classic X-linked recessive disorder. The allele for normal colour vision is dominant, and we can represent it as a superscript on the X chromosome. Let us use X^N for the normal allele and Xⁿ for the colour-blind allele. Because the Y chromosome does not carry this gene, it is simply written as Y.

红绿色盲是一种典型的伴 X 隐性遗传病。正常色觉的等位基因为显性，我们可以将其以角标形式标在 X 染色体上。我们用 X^N 表示正常等位基因，用 Xⁿ 表示色盲等位基因。由于 Y 染色体上不含此基因，直接写作 Y。

Genotype	Sex & Phenotype
X^N X^N	Female, normal colour vision
X^N Xⁿ	Female, normal vision (carrier)
Xⁿ Xⁿ	Female, colour blind
X^N Y	Male, normal colour vision
Xⁿ Y	Male, colour blind

Note that a female must inherit two copies of the recessive allele (one from each parent) to be colour blind, whereas a male need only inherit one copy from his mother. This explains why colour blindness is far more common in males (about 8% of males) than in females (about 0.5%).

请注意，女性必须从父母双方各继承一个隐性等位基因才会表现为色盲，而男性只需从母亲那里遗传一个即可。这就解释了为何色盲在男性中（约 8%）远比女性（约 0.5%）常见。

5. Genetic Crosses for Colour Blindness | 红绿色盲的遗传图解

Let us examine a cross between a carrier female (X^NXⁿ) and a normal male (X^NY).

我们来看一个携带者女性（X^NXⁿ）与正常男性（X^NY）的杂交。

Parental genotypes: X^NXⁿ × X^NY

Gametes from mother: X^N and Xⁿ. Gametes from father: X^N and Y. Using a Punnett square, we obtain:

母本配子：X^N 和 Xⁿ。父本配子：X^N 和 Y。使用庞纳特方格，我们得到：

Offspring: X^NX^N (normal female), X^NXⁿ (carrier female), X^NY (normal male), XⁿY (colour-blind male).

Probability results: each child has a 25% chance of being a normal female, 25% carrier female, 25% normal male, and 25% colour-blind male. In other words, half of the sons would be expected to be colour blind, and no daughters would be colour blind (though half would be carriers).

概率结果：每个孩子有 25% 的几率为正常女性，25% 为携带者女性，25% 为正常男性，25% 为色盲男性。换句话说，预期一半的儿子会是色盲，而女儿都不会是色盲（但一半会是携带者）。

Now consider a cross between a colour-blind male (XⁿY) and a homozygous normal female (X^NX^N).

现在考虑一个色盲男性（XⁿY）与纯合正常女性（X^NX^N）的杂交。

Offspring: all daughters X^NXⁿ (carriers), all sons X^NY (normal).

Thus, none of the children will be colour blind, but every daughter will be a carrier. This illustrates the skipping of a generation often seen in X-linked recessive pedigrees.

因此，所有子女都不会是色盲，但每一个女儿都是携带者。这体现了伴 X 隐性系谱中常见的一代跳越现象。

6. Example: Haemophilia | 实例：血友病

Haemophilia is another well-known X-linked recessive disorder. It is characterised by the inability of the blood to clot properly, leading to excessive bleeding even from minor injuries. There are two main types (haemophilia A and B), both caused by mutations in genes on the X chromosome. We can use the alleles X^H (normal clotting) and X^h (haemophilia).

血友病是另一种著名的伴 X 隐性遗传病。其特征是血液无法正常凝固，即使轻微受伤也会导致过度出血。主要有两种类型（A 型和 B 型），均是由 X 染色体上基因突变所致。我们可以使用等位基因 X^H（正常凝血）和 X^h（血友病）。

As with colour blindness, males with the genotype X^hY will have haemophilia, while females must be X^hX^h to be affected. Carrier females (X^HX^h) are asymptomatic. Queen Victoria of the United Kingdom was a famous carrier of haemophilia, and through her descendants the condition spread to several European royal families, earning it the nickname ‘the royal disease’.

与色盲类似，基因型为 X^hY 的男性会患血友病，而女性必须是 X^hX^h 才会患病。携带者女性（X^HX^h）没有症状。英国维多利亚女王是血友病的著名携带者，通过她的后代，该疾病传播到多个欧洲皇室家族，因此得名“王室病”。

7. Pedigree Analysis for X-linked Recessive Disorders | 伴 X 隐性遗传病的系谱分析

When you are given a pedigree chart in an IGCSE exam, you can identify X-linked recessive inheritance by looking for these clues:

在 IGCSE 考试中遇到系谱图时，你可以通过以下线索识别伴 X 隐性遗传：

More males than females are affected. Often all affected individuals are male.

患病男性多于女性，往往所有患者都是男性。
An affected male does not pass the condition to his sons; all his daughters are carriers.

患病男性不会将疾病传给儿子；他所有的女儿都是携带者。
The condition can skip a generation: a carrier daughter passes it to her son, so an affected grandson appears without an affected father.

疾病可能隔代出现：一位携带者女儿将致病基因传给儿子，于是出现患病的孙子，而他的父亲并不患病。
If a female is affected, her father must have the condition and her mother must be at least a carrier. This is rare.

若女性患病，她的父亲必定是患者，母亲至少是携带者。这种情况非常罕见。

Always eliminate autosomal inheritance first: if fathers can transmit the trait to sons, it is NOT X-linked recessive.

务必首先排除常染色体遗传：如果父亲能将性状传给儿子，则不是伴 X 隐性遗传。

8. Why Are Males More Often Affected? | 为什么男性更常患病？

The simple answer lies in hemizygosity. Males have only one X chromosome, so for any gene located on the X, there is no second allele on the Y to mask a recessive allele. Even if the recessive allele is rare, a single copy inherited from the mother will be expressed. Females, on the other hand, need two copies – one from each parent – which is statistically far less likely. This is why X-linked recessive conditions are sometimes called sex-linked recessive diseases, and why they appear predominantly in males.

答案很简单，在于半合子性。男性只有一条 X 染色体，因此对于任何位于 X 染色体上的基因，Y 染色体上都没有第二个等位基因来掩盖隐性等位基因。即使隐性等位基因很罕见，从母亲那里遗传的一个拷贝就会被表达。而女性则需要两个拷贝——分别来自父母双方——这在统计学上可能性低得多。这就是为什么伴 X 隐性遗传病有时被称为性连锁隐性遗传病，以及为何主要在男性中发病。

It is important to state this clearly in exams. Use the term ‘hemizygous’ if you want to show advanced understanding.

在考试中清晰地表述这一点很重要。如果想展示更深的理解，可以使用“半合子”这个术语。

9. Y-linked Inheritance | 伴 Y 遗传

Y-linked inheritance refers to genes located on the Y chromosome that have no counterpart on the X. Such genes can only be passed from father to son. A well-known example is the SRY gene that initiates male sex determination, but there are also genes linked to traits like hairy ears (hypertrichosis pinnae) in some families. In a pedigree, Y-linked traits appear in every male in a direct male line, and never in females.

伴 Y 遗传是指位于 Y 染色体上的、在 X 染色体上没有对应部分的基因。这类基因只能由父亲传给儿子。一个著名的例子是启动男性性别决定的 SRY 基因，但还有一些基因与某些家族中耳廓多毛等性状相关。在系谱图中，伴 Y 性状出现在所有直系男性后代中，而从不出现于女性。

For the IGCSE syllabus, you do not need to go into great depth on Y-linked traits; you are more likely to be examined on X-linked recessive examples. However, it is useful to recognise that such a pattern exists and is distinct from X-linked and autosomal patterns.

在 IGCSE 教学大纲中，你不需深入探究伴 Y 性状；更有可能考察的是伴 X 隐性的例子。不过，认识到这种模式的存在并区别于伴 X 及常染色体模式是有帮助的。

10. Summary and Exam Tips | 总结与考场贴士

To excel in IGCSE questions on sex-linked inheritance, remember these points:

要在 IGCSE 伴性遗传问题上取得高分，请牢记以下几点：

Always define your allele symbols clearly before you begin a genetic cross. Use superscripts on X to show alleles (e.g., X^N, Xⁿ). Never put a corresponding allele on the Y chromosome unless the gene is located there.

进行遗传杂交前，务必明确定义等位基因符号。用角标在 X 上标注等位基因（如 X^N、Xⁿ）。除非基因也位于 Y 上，否则绝不要在 Y 染色体上添加相应的等位基因。
When writing the outcome of a cross, present the genotypic ratio and then link it to the phenotypic ratio, making sure to state the sex of each phenotype. For example: ‘50% of the sons will be colour blind.’

书写杂交结果时，先给出基因型比例，然后将其与表现型比例联系起来，并确保说明每种表现型的性别。例如：“50% 的儿子会是色盲。”
In pedigree problems, rule out autosomal dominant and recessive patterns by checking for male-to-male transmission. If there is even one case of affected father to affected son, it cannot be X-linked recessive.

在系谱图问题中，通过检查男性向男性传递，排除常染色体显性和隐性模式。哪怕只有一例患病父亲传给病儿，就不能是伴 X 隐性。
Practise common crosses: carrier female × normal male, normal female × affected male, and carrier female × affected male. Be prepared to calculate probabilities and explain why males are more prone to the condition.

练习常见的杂交组合：携带者女性 × 正常男性，正常女性 × 患病男性，以及携带者女性 × 患病男性。准备好计算概率并解释为何男性更易患病。
Use the correct terminology: hemizygous, carrier, X-linked recessive, pedigree. These words can earn you marks for biological knowledge.

使用准确的术语：半合子、携带者、伴 X 隐性、系谱。这些词语会为你赢得生物学知识的分数。

By mastering these principles and practising with real past paper questions, you will be able to tackle any sex-linked inheritance question confidently.

通过掌握这些原则并结合历年真题练习，你将能自信地应对任何伴性遗传问题。

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Sex-linked Inheritance: IGCSE Biology Key Points | IGCSE 生物：伴性遗传 考点精讲