📚 Sex-linked Inheritance | 伴性遗传 考点精讲
Sex-linked inheritance refers to the pattern of inheritance for genes located on the sex chromosomes (X and Y). In GCSE CCEA Biology, understanding how these genes are passed on and why some genetic disorders affect males and females differently is essential. This revision guide covers the key concepts, worked examples, and common exam pitfalls.
伴性遗传是指位于性染色体（X 和 Y）上的基因的遗传模式。在 GCSE CCEA 生物学中，理解这些基因如何传递以及为什么某些遗传疾病对男性和女性的影响不同至关重要。本复习指南将涵盖关键概念、实例分析和常见考试陷阱。
1. Sex Determination | 性别决定
Human cells contain 23 pairs of chromosomes. One pair, the sex chromosomes, determines biological sex. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). The Y chromosome is much smaller and carries fewer genes, including the SRY gene that triggers male development.
人类细胞含有 23 对染色体。其中一对是性染色体，决定生物学性别。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）。Y 染色体要小得多，携带的基因也较少，其中包括触发男性发育的 SRY 基因。
During gamete formation, females produce eggs all carrying an X chromosome. Males produce sperm, half carrying an X and half carrying a Y. Therefore, the sex of the offspring is determined by the sperm that fertilises the egg.
在配子形成过程中，女性产生的卵子全部携带一条 X 染色体。男性产生的精子一半携带 X 染色体，一半携带 Y 染色体。因此，后代的性别由使卵子受精的精子决定。
2. What Are Sex-linked Genes? | 什么是伴性基因？
Genes located on the sex chromosomes are said to be sex-linked. Most sex-linked genes are found on the X chromosome because it is larger and has many more genes than the Y chromosome. The Y chromosome mainly carries genes for male sex determination and sperm production; very few other traits are Y-linked.
位于性染色体上的基因被称为伴性基因。大多数伴性基因存在于 X 染色体上，因为它比 Y 染色体大得多，携带的基因也多得多。Y 染色体主要携带与男性性别决定和精子产生有关的基因；极少有其他性状是 Y 染色体连锁的。
In GCSE, we focus on X-linked genes, particularly those that cause recessive disorders such as red-green colour blindness and haemophilia. Because males have only one X chromosome, they express whatever allele is present, even if it is recessive.
在 GCSE 中，我们重点关注 X 连锁基因，尤其是那些导致隐性遗传病的基因，例如红绿色盲和血友病。由于男性只有一条 X 染色体，他们会表达该染色体上存在的任何等位基因，即使是隐性的也会表达。
3. X-linked Recessive Inheritance: Red-Green Colour Blindness | X 连锁隐性遗传：红绿色盲
Red-green colour blindness is a common X-linked recessive condition. The gene for normal colour vision is dominant (let’s represent it as Xᴴ), while the allele for colour blindness is recessive (Xʰ). Because the gene is on the X chromosome, the genotype and phenotype depend on sex.
红绿色盲是一种常见的 X 连锁隐性遗传病。正常色觉的基因是显性（我们可表示为 Xᴴ），而色盲的等位基因是隐性（Xʰ）。由于该基因位于 X 染色体上，基因型和表现型取决于性别。
Possible genotypes and phenotypes:
可能的基因型和表现型：
- XᴴXᴴ – normal female (homozygous dominant)
- XᴴXᴴ – 正常女性（显性纯合子）
- XᴴXʰ – carrier female (heterozygous); normal vision but carries the recessive allele
- XᴴXʰ – 携带者女性（杂合子）；视力正常但携带隐性等位基因
- XʰXʰ – colour-blind female (must inherit recessive allele from both parents)
- XʰXʰ – 色盲女性（必须从父母双方获得隐性等位基因）
- XᴴY – normal male
- XᴴY – 正常男性
- XʰY – colour-blind male (expresses the recessive allele from his single X)
- XʰY – 色盲男性（来自其唯一 X 染色体的隐性等位基因会表达）
Note: A female needs two recessive alleles to be colour-blind, whereas a male needs only one. This explains why colour blindness is much more common in males (about 8% of males vs 0.5% of females).
注意：女性需要两个隐性等位基因才会色盲，而男性只需一个。这就解释了为什么色盲在男性中更为常见（约 8% 的男性，而女性仅约 0.5%）。
4. Genetic Cross: Carrier Mother × Normal Father | 遗传杂交：携带者母亲 × 正常父亲
Consider a cross between a carrier female (XᴴXʰ) and a normal male (XᴴY). We can use a Punnett square to show the expected offspring genotypes.
考虑一个携带者女性（XᴴXʰ）与正常男性（XᴴY）之间的杂交。我们可以用庞纳特方格来展示预期的后代基因型。
| Xᴴ (egg) | Xʰ (egg) | |
|---|---|---|
| Xᴴ (sperm) | XᴴXᴴ | XᴴXʰ |
| Y (sperm) | XᴴY | XʰY |
Offspring outcomes: 25% normal female (XᴴXᴴ), 25% carrier female (XᴴXʰ), 25% normal male (XᴴY), 25% colour-blind male (XʰY). There are no colour-blind females in this cross, but half the sons are affected.
后代结果：25% 正常女性（XᴴXᴴ），25% 携带者女性（XᴴXʰ），25% 正常男性（XᴴY），25% 色盲男性（XʰY）。在这个杂交中没有色盲女性，但一半的儿子会患病。
5. Genetic Cross: Affected Father × Normal Mother | 遗传杂交：患病父亲 × 正常母亲
Cross between a colour-blind male (XʰY) and a homozygous normal female (XᴴXᴴ). Sperm: Xʰ or Y; eggs: all Xᴴ.
色盲男性（XʰY）与纯合正常女性（XᴴXᴴ）杂交。精子：Xʰ 或 Y；卵子：全部 Xᴴ。
| Xᴴ (egg) | |
|---|---|
| Xʰ (sperm) | XᴴXʰ |
| Y (sperm) | XᴴY |
All daughters are carriers (XᴴXʰ) but have normal vision. All sons are normal (XᴴY) because they inherit the X chromosome from the mother. Note: An affected father cannot pass the X-linked recessive allele to his sons; he passes his Y chromosome to sons.
所有女儿都是携带者（XᴴXʰ）但视力正常。所有儿子都正常（XᴴY），因为他们从母亲那里获得了 X 染色体。注意：患病的父亲不能将 X 连锁隐性等位基因传给儿子；他把 Y 染色体传给儿子。
6. Why Are Males More Often Affected? | 为什么男性更常患病？
Males are hemizygous for X-linked genes – they have only one allele. If that allele is recessive and causes a disorder, the male will express the trait because there is no corresponding allele on the Y chromosome to mask it. Females, however, require two recessive alleles to express the disorder; if they are heterozygous, they are carriers and are typically unaffected.
男性对 X 连锁基因是半合子——他们只有一个等位基因。如果该等位基因是隐性且引起疾病，男性就会表现该性状，因为 Y 染色体上没有对应的等位基因来掩盖它。而女性需要两个隐性等位基因才能表现出疾病；如果是杂合子，她们就是携带者，通常不受影响。
This is why X-linked recessive conditions are more prevalent in males. In a pedigree, you will often see affected males in every generation, with carrier females passing the allele to their sons.
这就是为什么 X 连锁隐性遗传病在男性中更常见。在系谱
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