Sex-linked Inheritance | 伴性遗传考点精讲

📚 Sex-linked Inheritance | 伴性遗传考点精讲

Sex-linked inheritance describes how genes located on the sex chromosomes (usually the X chromosome) are passed from one generation to the next. Because males and females have different combinations of sex chromosomes, the patterns of inheritance for these genes differ significantly from those on autosomes. Understanding sex-linked inheritance is a core topic in CIE A-Level Biology, requiring you to analyse pedigrees, calculate probabilities, and explain why certain conditions appear more frequently in one sex.

伴性遗传描述了位于性染色体（通常是 X 染色体）上的基因如何从一代传递到下一代。由于雄性和雌性具有不同的性染色体组合，这些基因的遗传模式与常染色体上的基因明显不同。理解伴性遗传是 CIE A-Level 生物学的核心内容，要求你分析家系图、计算概率，并解释为什么某些疾病更常见于某一性别。

1. What is Sex-linked Inheritance? | 什么是伴性遗传？

Genes located on the sex chromosomes are said to be sex-linked. In humans and many other organisms, the X chromosome carries many genes that are not directly involved in sex determination, while the Y chromosome is much smaller and carries few genes. As a result, most sex-linked traits are X-linked.

位于性染色体上的基因被称为伴性基因。在人类和许多其他生物中，X 染色体携带大量与性别决定无直接关系的基因，而 Y 染色体要小得多，只携带少数几个基因。因此，大多数伴性性状都是 X 连锁的。

Because females possess two X chromosomes (XX) and males one X and one Y (XY), the expression of X-linked alleles differs between the sexes. Males are hemizygous for X-linked genes – they have only one allele for each X-linked gene, so a recessive allele will always be expressed if it is present.

由于女性拥有两条 X 染色体 (XX)，男性拥有一条 X 和一条 Y (XY)，X 连锁等位基因的表达在两性间存在差异。男性对于 X 连锁基因是半合子——每个基因只有一个等位基因，因此如果存在隐性等位基因，它将一定会表达出来。

2. Sex Determination and Sex Chromosomes | 性别决定与性染色体

In humans, sex is determined by the presence of X and Y chromosomes. Females are the homogametic sex (XX), producing eggs that all contain one X chromosome. Males are the heterogametic sex (XY), producing sperm that carry either an X or a Y chromosome with equal probability.

在人类中，性别由 X 和 Y 染色体的组合决定。女性是同型配子性别 (XX)，产生的卵子都含有一条 X 染色体。男性是异型配子性别 (XY)，产生含有 X 或 Y 染色体的精子，且概率相等。

The Y chromosome carries the SRY gene (sex-determining region Y), which triggers the development of testes and thus male characteristics. X-linked genes are not related to sex determination but are carried on the X chromosome solely because of its size and gene content.

Y 染色体携带 SRY 基因（Y 染色体性别决定区），该基因触发睾丸的发育，从而产生男性特征。X 连锁基因与性别决定无关，它们仅因 X 染色体较大且基因丰富而位于其上。

3. X-linked Recessive Inheritance | X连锁隐性遗传

The most common type of sex-linked inheritance is X-linked recessive. In this mode, the mutant allele is recessive, and the normal allele is dominant. A female needs two copies of the recessive allele to show the trait, whereas a male only needs one copy because he has no second X chromosome to mask it.

最常见的伴性遗传类型是 X 连锁隐性遗传。在这种模式下，突变等位基因为隐性，正常等位基因为显性。女性需要两个隐性等位基因才能表现该性状，而男性只需要一个，因为他没有第二条 X 染色体来掩盖其效应。

Carrier females are heterozygous – they have one normal allele and one recessive allele. They do not usually express the trait but can pass the recessive allele to their offspring. Males cannot be carriers; they either show the trait or are completely free of the allele.

携带者女性是杂合子——她们有一个正常等位基因和一个隐性等位基因。她们通常不表现该性状，但可以将隐性等位基因传递给后代。男性不可能是携带者；他们要么表现出该性状，要么完全不含该等位基因。

4. Key Features of X-linked Recessive Traits | X连锁隐性性状的关键特征

When analysing pedigrees, certain patterns strongly suggest X-linked recessive inheritance:

在分析家系图时，某些模式强烈暗示 X 连锁隐性遗传：

More males are affected than females. 受累男性多于女性。
Trait often skips a generation – an affected male passes the allele to his daughters, who become carriers, and then to 50% of their sons. 性状常隔代出现——患病男性将等位基因传给女儿，女儿成为携带者，然后她们有 50% 的儿子患病。
Affected males cannot pass the trait to their sons (since they give their sons the Y chromosome, not the X). 患病男性不可能将性状传给儿子（因为他们传给儿子的是 Y 染色体，而非 X 染色体）。
All daughters of an affected male are carriers (if the mother is homozygous normal). 患病男性的所有女儿都是携带者（如果母亲是正常纯合子）。
A female can only be affected if her father is affected and her mother is at least a carrier. 女性患病的前提是她的父亲患病且母亲至少为携带者。

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

Red-green colour blindness is a classic X-linked recessive condition caused by a mutant allele that affects the photopigments in cone cells. The gene is located on the X chromosome. We use the symbol X^N for the normal allele and Xⁿ for the colour-blind allele.

红绿色盲是一种典型的 X 连锁隐性疾病，由影响视锥细胞光色素的突变等位基因引起。该基因位于 X 染色体上。我们用符号 X^N 表示正常等位基因，Xⁿ 表示色盲等位基因。

Possible genotypes and phenotypes are:

可能的基因型和表现型如下：

Genotype	Sex	Phenotype
X^NX^N	Female	Normal vision
X^NXⁿ	Female	Normal vision (carrier)
XⁿXⁿ	Female	Colour blind
X^NY	Male	Normal vision
XⁿY	Male	Colour blind

A cross between a carrier female (X^NXⁿ) and a normal male (X^NY) can be represented as:

携带者女性 (X^NXⁿ) 和正常男性 (X^NY) 的杂交可以表示为：

X^NXⁿ × X^NY → offspring: X^NX^N (normal female), X^NXⁿ (carrier female), X^NY (normal male), XⁿY (colour-blind male)

This cross produces 25% chance of a colour-blind male and no colour-blind females, perfectly illustrating X-linked recessive patterns.

该杂交产生 25% 的色盲男性概率，没有色盲女性，完美展示了 X 连锁隐性遗传模式。

6. Example: Haemophilia | 实例：血友病

Haemophilia A is another X-linked recessive disorder, caused by a lack of clotting factor VIII. The normal allele is represented as X^H and the haemophilia allele as X^h.

血友病 A 是另一种 X 连锁隐性疾病，由缺乏凝血因子 VIII 引起。正常等位基因表示为 X^H，血友病等位基因为 X^h。

In the famous pedigree of Queen Victoria’s family, the haemophilia allele was passed through carrier daughters to affected males in several royal houses of Europe. This historical example clearly demonstrates the ‘skipping generations’ pattern.

在著名的维多利亚女王家族家系图中，血友病等位基因通过携带者女儿传递至多个欧洲王室中的患病男性。这一历史实例清楚地展示了“隔代遗传”的模式。

A carrier female (X^HX^h) who marries a normal male (X^HY) will have a 50% chance that each son is affected. Her daughters have a 50% chance of being carriers. An affected male (X^hY) will pass the allele to all of his daughters (making them carriers) but to none of his sons.

携带者女性 (X^HX^h) 与正常男性 (X^HY) 结婚，每个儿子有 50% 的概率患病。她的女儿们有 50% 的概率成为携带者。患病男性 (X^hY) 会将等位基因传给所有女儿（使她们成为携带者），但不会传给任何一个儿子。

7. Pedigree Analysis for Sex-linked Recessive Disorders | 伴性隐性遗传病家系分析

To identify an X-linked recessive pattern in a pedigree, look for these clues:

要在系谱中识别 X 连锁隐性模式，寻找以下线索：

Predominantly males affected. 患者以男性为主。
Affected males often born to unaffected parents; the mother must be a carrier. 患病男性常出生于无病父母；母亲必定为携带者。
No male-to-male transmission. 无男性到男性的传递。
All daughters of an affected male are carriers (if the mother is normal). 患病男性的所有女儿皆为携带者（若母亲正常）。
Affected females have an affected father and a carrier or affected mother. 患病女性的父亲患病，且母亲为携带者或患者。

If any one of these features is violated (e.g., an affected male passes the trait to his son), the trait cannot be X-linked recessive – it must be autosomal or another mode.

如果上述任一特征被违背（例如，患病男性将性状传给儿子），则该性状不可能是 X 连锁隐性——它必定是常染色体遗传或其他方式。

8. Calculating Probabilities in Sex-linked Crosses | 伴性杂交中的概率计算

Probability problems often involve determining the chance that a child will be affected or be a carrier, given the parents’ genotypes or phenotypes. Always begin by writing the sex chromosomes and alleles clearly.

概率问题通常涉及根据父母的基因型或表现型确定孩子患病或成为携带者的几率。始终从清晰地写出性染色体和等位基因开始。

Worked example: A woman who is a carrier for red-green colour blindness (X^NXⁿ) marries a man with normal vision (X^NY). What is the probability that their first child will be a colour-blind son?

例题：一名红绿色盲携带者女性 (X^NXⁿ) 与一名色觉正常的男性 (X^NY) 结婚。他们的第一个孩子是色盲儿子的概率是多少？

Using a Punnett square or simple probability: the mother can produce gametes X^N or Xⁿ; the father produces X^N or Y. The possible male offspring are X^NY (normal) and XⁿY (colour blind). Therefore, the probability of a son being colour blind is 1/2. Since the probability of having a son is 1/2, the combined probability of a colour-blind son is 1/2 × 1/2 = 1/4.

使用庞纳特方格或简单概率：母亲可产生 X^N 或 Xⁿ 的配子；父亲产生 X^N 或 Y。可能的男性后代为 X^NY（正常）和 XⁿY（色盲）。因此，儿子色盲的概率是 1/2。由于生儿子的概率为 1/2，结合概率为色盲儿子的概率是 1/2 × 1/2 = 1/4。

A common mistake is to forget to consider the probability of the child being male. Always state clearly whether the question asks for the probability of an affected male among all children or specifically among sons.

常见的错误是忘记考虑孩子为男性的概率。务必清楚地说明题目问的是所有孩子中患病男性的概率，还是特指儿子中的患病概率。

9. X-linked Dominant Inheritance | X连锁显性遗传

X-linked dominant disorders are rarer. In this case, the mutant allele is dominant, so even one copy in females will cause the trait. Examples include hypophosphatemic rickets (vitamin D-resistant rickets).

X 连锁显性遗传病较为罕见。在这种情况下，突变等位基因为显性，因此女性即使是杂合子也会表现出该性状。例如，低磷血症性佝偻病（抗维生素 D 佝偻病）。

Key features of X-linked dominant inheritance:

X 连锁显性遗传的主要特征：

Both males and females are affected, but often more females are affected because females have two X chromosomes and thus twice the chance of inheriting the dominant allele. 男女均可患病，但通常女性患者更多，因为女性有两条 X 染色体，能获得显性等位基因的概率是男性的两倍。
Affected males will pass the trait to all of their daughters but to none of their sons. 患病男性会将性状传给所有女儿，但不会传给任何儿子。
An affected female (heterozygous) will pass the trait to half of her offspring, regardless of sex. 患病女性（杂合子）会将性状传递给一半的后代，不分性别。
The pedigree may show affected individuals in each generation without skipping. 系谱中可能每一代都有患者，无隔代现象。

Although less common in exam questions, recognising this pattern is useful when a pedigree contradicts X-linked recessive predictions.

尽管在考试题目中较少见，但当系谱与 X 连锁隐性预测相矛盾时，识别此模式很有用。

10. Y-linked Inheritance | Y连锁遗传

Y-linked traits are due to genes on the Y chromosome. They are passed only from father to son, and all males in the paternal line will express the trait. Females never inherit or transmit Y-linked traits. An example is the SRY gene itself, but in terms of visible traits, one often cited is the hairy ear pinna phenotype (though its Y-linkage is debated).

Y 连锁性状由 Y 染色体上的基因决定。它们只从父亲传递给儿子，父系所有男性都会表达该性状。女性从不遗传或传递 Y 连锁性状。例子如 SRY 基因本身，但就可见性状而言，常被引用的有耳廓多毛表型（尽管其 Y 连锁尚有争议）。

In exam answers, simply note that Y-linked traits appear only in males and every affected male must have an affected father, with vertical transmission through the male line. They are rarely asked in CIE papers beyond a brief identification.

在考试答案中，只需指出 Y 连锁性状仅出现在男性，且每个患病男性必定有一个患病的父亲，并且沿男性家族垂直传递。除简要识别外，CIE 试题中很少深入考查。

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Sex-linked Inheritance | 伴性遗传 考点精讲